Human Body Systems 3,4,5,6,7, 8, 13,14, 15

FIGURE 3-1 The Human Digestive System

Brushing helps reduce cavities by removing food particles and plaque. Most toothpastes also contain small amounts of fluoride that hardens the enamel, making them more resistant to acids. In many cities and towns, fluoride is added to drinking water in an attempt to provide additional protection. Flossing your teeth also helps reduce cavities by removing food particles lodged between your teeth where toothbrushes can’t reach.

After food is chewed, it must be swallowed. The tongue plays a key role in swallowing by pushing food to the back of the oral cavity where it passes into the esophagus (ee-SOFF-ah-guss). The esophagus is a long muscular tube that leads to the stomach (Figure 3-1).

The tongue also aids in speech and contains taste receptors, or taste buds, on its upper surface (FIGURE 3-2). Taste buds are oval structures located in tiny bumps on the upper surface of the tongue. Taste buds are stimulated by five basic flavors: sweet, sour, salty, bitter, and umami. (Umami is a meaty flavor associated with the flavorant monosodium glutamate sometimes added to Chinese food.) Various combinations of these flavors (combined with the odors we smell) give us a rich assortment of tastes.

FIGURE 3-2 Taste Buds A photomicrograph of a taste bud.

Food propelled into the esophagus is prevented from entering the nearby trachea (TRAY-key-ah), or windpipe, which carries air to the lungs, by a flap of tissue known as the epiglottis (e-pah-GLOT-tiss) (FIGURE 3-3). The epiglottis acts like a trapdoor, closing off the trachea during swallowing.

The Esophagus and Stomach

Swallowing is a voluntary action caused when the tongue pushes food into the back of the oral cavity. From this point on, however, movement of the food to the stomach is involuntary. That is, the muscles in the wall of the esophagus push the food along automatically, without any conscious control (FIGURE 3-4).

In humans, the stomach shown in FIGURES 3-1 and 3-5 lies on the left side of the abdominal cavity. When food reaches the stomach from the esophagus, a ring of muscles located at its attachment to the esophagus relaxes, allowing food to enter. The muscle then contracts after the food enters the stomach, preventing stomach acid from percolating upward. If the muscle fails to close, acid rising in the esophagus causes irritation, a condition known as heartburn.

Inside the stomach, food is liquefied by acids produced by tiny glands located in the wall of the stomach. The food is churned by muscle contraction in the walls of the organ and is mixed with acid from the glands.

The liquids from salivary glands and the glands of the stomach now convert the food into a thin, watery paste known as chyme. The stomach can hold 2–4 liters (2 quarts to 1 gallon) of liquefied food.

FIGURE 3-3 The Epiglottis This trapdoor prevents food from entering the trachea during swallowing. As illustrated, the trachea is lifted during swallowing, pushing against the epiglottis, which bends downward.

FIGURE 3-4 Peristalsis (a) Peristaltic contractions in the esophagus propel food into the stomach. (b) When food reaches the stomach, the gastroesophageal sphincter opens, allowing food to enter.

Contrary to what many think, very little digestion occurs in the stomach. The stomach’s role is largely to prepare food for digestion by enzymes encountered in the next part of the digestive system, the small intestine. There are some exceptions to this rule, however. Protein is one of them.

Proteins are coagulated by stomach acid. Some protein-digesting enzyme is also present in the stomach. Produced by the acid-producing glands, this enzyme breaks the proteins down, but only slightly. The protein fragments must be broken down further in the small intestine.

The stomach lining is protected from the acid by an alkaline secretion known as mucus (MEW-kuss). It is produced by cells in the lining of the stomach. Mucus also protects the lining of the stomach from protein-digesting enzymes.

The Small Intestine

Liquefied and partially digested food is ejected into the small intestine. This occurs when another ring of smooth muscle relaxes at the junction of the stomach and small intestine.

The stomach contents empty in 2–6 hours, depending on the size of the meal and the type of food. The larger the meal, the longer it takes to empty. Solid foods (meat) empty slower than liquid foods (milk shakes). After the stomach empties, contractions in the walls of the organ continue. These contractions are felt as hunger pangs.

The small intestine is a coiled tube in the abdominal cavity about 6 meters (20 feet) long in adults (Figure 3-1). Inside the small intestine, large food molecules are broken into smaller ones by digestive enzymes from two sources, discussed shortly. Starch, for instance, is broken down into glucose molecules. Smaller food molecules can now pass through the lining of the small intestine into the blood stream where they are distributed throughout the body to nourish body cells. Some food molecules also pass into the lymphatic system. The lymphatic system is a network of vessels that runs throughout the body. Many of these vessels carry excess tissue fluid from the tissues of the body to the circulatory system. Those found in the wall of the small intestine absorb fats and transport them to the blood stream.

As just noted, the digestion of food molecules inside the small intestine requires enzymes produced from two sources: the pancreas and the lining of the small intestine.

The pancreas is nestled in a loop formed by the first portion of the small intestine (FIGURE 3-6). The digestive enzymes of the pancreas flow through a duct into the small intestine. The pancreas also produces sodium bicarbonate, a chemical that neutralizes the acids in the food produced by the glands of the stomach. Sodium bicarbonate protects the small intestine from stomach acid and creates an optimal environment for the function of pancreatic enzymes.

FIGURE 3-5 The Stomach The stomach lies in the abdominal cavity. In its wall are three layers of smooth muscle that mix the food and force it into the small intestine, where most digestion occurs. The gastroesophageal and pyloric sphincters control the inflow and outflow of food, respectively.

Pancreatic enzymes begin the breakdown of large molecules in food (proteins, starches, and so on). Digestion is then completed by enzymes released by cells lining the small intestine. The small food molecules can now pass through the lining of the intestine where they enter the blood stream.

The Liver

Digestion of food also requires the liver. Situated on the right side of the abdomen under the protection of the ribs, the liver is one of the largest and most versatile organs in the body. It performs as many as 500 different functions. The liver, for example, is one of the body’s storage depots for glucose and fats. By storing glucose and fats and releasing them as they are needed, the liver helps ensure a constant supply of energy-rich molecules required by body cells. The liver also synthesizes some key proteins involved in blood clotting, and detoxifies many potentially harmful chemicals such as nicotine, barbiturates, and alcohol.

The liver also plays a key role in the digestion of fats through the production of a fluid called bile. Bile is released into the small intestine when food is present. Bile contains bile salts. Bile salts are steroid molecules that break fat globules into smaller ones so they can be broken down by fat-digesting enzymes in the intestine.

Bile is produced by the cells of the liver and then stored in a small sac, the gallbladder attached to the underside of the organ (Figure 3-1). The gallbladder concentrates the bile by removing water from it. When food enters the small intestine, the gallbladder contracts. Bile flows out through a duct into the small intestine (Figure 3-6).

FIGURE 3-6 The Organs of Digestion The liver, gallbladder, and pancreas all play key roles in digestion. All empty by the common bile duct into the small intestine, in which digestion takes place.

Bile flow to the small intestine may be blocked by gallstones, which are deposits of cholesterol and other materials in the gallbladder of some in dividuals. Gallstones may lodge in the ducts draining the organ, reducing—even blocking—the flow of bile. The lack of bile salts in the small intestine greatly reduces lipid digestion and absorption.

Gallstones occur more frequently in older, overweight individuals. When they cause problems, gallstones are usually surgically removed. This procedure requires that the entire gallbladder be removed.

Food Absorption by the Small Intestine

Once food molecules are digested, they can pass through the lining of the small intestine where they enter the blood or lymphatic vessels located beneath the epithelium, as noted earlier. This process is referred to as absorption. Virtually all food digestion and absorption occurs in the small intestine.

Numerous mechanisms are involved in absorption. Most nutrients pass through the lining of the intestine into the blood capillaries, tiny blood vessels, by diffusion. Diffusion is the movement of a molecule from a region of high concentration to low concentration.

The Large Intestine

After most of the nutrients have been absorbed, what’s left of the food passes into the large intestine. The large intestine is about 1.5 meters (5 feet) long (FIGURE 3-7).

In the large intestine, water, sodium, and potassium ions in the waste are absorbed, passing into the blood stream. After water and salt have been removed, the contents of the large intestine are known as the feces. The feces consist primarily of undigested food and materials, such as water-insoluble fiber (cellulose). The feces also contain lots of intestinal bacteria, which account for about one-third of its dry weight.

The feces are propelled along the large intestine by contractions of smooth muscle in the walls of the organ. The fecal matter accumulates in the rectum, the last part of the large intestine. This causes the rectum to expand. This, in turn, stimulates nerve receptors in the wall of the rectum. Nerve impulses from the receptors in the rectum travel via nerves to the spinal cord. In babies and very young children (not yet toilet trained), the incoming nerve impulses stimulate nerve cells that supply the smooth muscle in the wall of the rectum. Impulses carried along these nerves cause the walls of the rectum to contract, expelling the feces automatically.

In adults and older children, defecation does not occur until the external anal sphincter relaxes. This ring of muscles at the end of the anal canal is composed of skeletal muscle and is under conscious control in all individuals, except babies and young children. If the time and place are appropriate, the external anal sphincter is relaxed, and defecation can occur. Defecation is usually assisted by voluntary contractions of the abdominal muscles.

If circumstances are inappropriate, voluntary tightening of the external anal sphincter prevents defecation. When defecation is delayed, the muscle in the wall of the rectum relaxes, and the urge to defecate fades—until the next movement of feces into the rectum occurs. If defecation is delayed too long, however, additional water may be removed from the feces, making them hard and dry and difficult to pass. This condition is called constipation.

Besides being uncomfortable, constipation may result in a dull headache, loss of appetite, and depression. Constipation can also be caused by decreases in contractions of the large intestine, which occur with age, emotional stress, or low-fiber, high meat diets and other factors.

Constipation can also result in serious problems. Hardened fecal material, for instance, may become lodged in the appendix, a small wormlike organ attached to the very first part of the large intestine. This, in turn, may lead to inflammation of the organ, a condition called appendicitis (ah-PEN-deh-SI-tiss). When this occurs, the appendix becomes swollen and filled with pus and must be surgically removed to prevent this otherwise useless organ from bursting and spilling its contents into the abdominal cavity. Fecal matter leaking into the abdominal cavity introduces billions of bacteria and can result in a deadly infection.

FIGURE 3-7 The Large Intestine This organ consists of four basic parts: the cecum, appendix, colon, and rectum.

Controlling Digestion

Digestion is a complex process controlled by the nervous system and the endocrine system. This section discusses some of the key events involved in the control of digestion.

Digestion begins in the oral cavity, as noted earlier. The sight, smell, taste, and sometimes even the thought of food stimulates the release of saliva. Chewing has a similar effect. The secretion of saliva is largely a reflex response controlled by the nervous system.

Besides activating the release of saliva, the stimuli listed above also cause the brain to send nerve impulses to the stomach. These nerve impulses initiate the secretion of stomach acid (hydrochloric acid or HCl) and protein-digesting enzymes from the glands in the stomach’s lining. The most potent stimulus for the release of these substances, however, is the presence of protein in the stomach.

Food entering the small intestine also stimulates the release of two hormones by the small intestine. They circulate in the blood and enter the pancreas. Here, they stimulate the release of pancreatic juice, containing food-digesting enzymes and sodium bicarbonate.

The digestion and absorption of food requires the actions of many organs belonging to the digestive system. The processes involved are intricately coordinated by the nervous system and endocrine system (hormone-producing glands) to ensure that food is broken down into molecules that the cells of our bodies can use.

he human circulatory system is a marvel of structure and function. It consists of a powerful muscular pump, the heart, which beats approximately 100,000 times per day—or a million times in 10 days (FIGURE 4-1). This strong muscular pump propels blood through an extensive branching network of blood vessels that, placed end to end, extends 50,000 miles.

Blood circulating throughout the body carries oxygen from the lungs and nutrients absorbed by the digestive system to the cells, tissues, and organs of the body. In addition, the circulatory system picks up waste from cells in the body’s tissues and transports it to organs that get rid of it for us, primarily the kidneys and liver. Blood that travels through the circulatory system also helps to distribute heat throughout the body.

The Heart

The heart is located in the chest between the lungs. The walls of the heart, shown in FIGURE 4-2, are composed of three layers: the pericardium, the myocardium, and the endocardium. The pericardium (pear-ah-CAR-dee-um) is a thin, closed sac that surrounds the heart and the bases of large vessels that enter and leave this organ. It is filled with a clear, slippery fluid that reduces friction produced when the heart contracts. The middle layer, the myocardium (my-oh-CAR-dee-um), is the thickest part of the wall. It is composed chiefly of cardiac muscle cells. The inner layer, the endocardium (end-oh-CAR-dee-um), is the thin lining of the heart chambers.

The heart pumps blood through two distinct but connected circuits, shown in Figure 4-1B. They are the pulmonary circuit, which carries blood to and from the lungs, and the systemic circuit, which transports blood to and from the rest of the body. As you shall soon see, the pulmonary circuit delivers oxygen-rich blood to the heart, which can then be pumped throughout the body to nourish body cells via the systemic circuit. The systemic circuit not only delivers oxygen to the cells of the body, it also picks up waste materials, such as carbon dioxide, which is produced during the breakdown of glucose in cells. Glucose is broken down to produce energy. The systemic circuit transports carbon dioxide rich blood to the heart where it is pumped to the lungs, via the pulmonary circuit. It is in the lungs that carbon dioxide escapes.

FIGURE 4-1 The Circulatory System (a) The circulatory system consists of a series of vessels that transport blood to and from the heart, the pump. (b) The circulatory system has two major circuits, the pulmonary circuit, which transports blood to and from the lungs, and the systemic circuit, which transports blood to and from the body (excluding the lungs).

As shown in Figure 4-1b, the heart consists of four hollow chambers—two on the right side of the heart and two on the left. The right side of the heart pumps blood through the pulmonary circuit. The left side of the heart pumps blood through the systemic circuit.

Figure 4-2 illustrates the course that blood takes through the heart. Let’s start with blood returning from the body in the systemic circulation. Drawn in blue, blood low in oxygen (and rich in carbon dioxide) enters the right side of the heart in two large veins draining the body, the superior and inferior vena cavae (VEE-nah CA-vee). These veins empty directly into the right atrium (A-tree-um), the uppermost chamber of the heart. The blood is then pumped from here into the right ventricle (VEN-trick-el), the lower chamber on the right side. When the right ventricle is full, the muscles in its wall contract, forcing blood into the pulmonary arteries, which lead to the lungs.

FIGURE 4-2 Blood Flow Through the Heart Deoxygenated (carbon dioxide-enriched) blood (blue arrows) flows into the right atrium from the systemic circulation and is pumped into the right ventricle. The blood is then pumped from the right ventricle into the pulmonary artery, which delivers it to the lungs. In the lungs, the blood releases its carbon dioxide and absorbs oxygen. Reoxygenated blood (red arrows) is returned to the left atrium, and then flows into the left ventricle, which pumps it to the rest of the body through the systemic circuit.

In the lungs, this blood is oxygenated, then returned to the heart via the pulmonary veins. The pulmonary veins, in turn, empty directly into the left atrium, the upper chamber on the left side of the heart. It’s the first part of the systemic circuit.

Next, the oxygen-rich blood is pumped to the left ventricle. When it’s full, the left ventricle’s thick, muscular walls contract and propel the blood into the aorta (ay-OR-tah). The aorta is the largest artery in the body. It carries the oxygenated blood away from the heart, delivering it to the body via the systemic circuit.

The flow of blood just described presents a slightly misleading view of the way the heart works. As shown in FIGURE 4-3, both atria fill and contract simultaneously, delivering blood to their respective ventricles. The right and left ventricles also fill simultaneously, and when both ventricles are full, they too contract in unison. Blood is then pumped into the systemic and pulmonary circuits. The coordinated contraction of heart muscle is brought about by an internal timing device, or pacemaker (described later).

Heart Valves. The human heart contains four valves that control the direction of blood flow (FIGURE 4-4). The valves between the atria and ventricles are known as atrioventricular valves (AY-tree-oh-ven-TRICK-u-ler). Each valve consists of flaps of tissue anchored to the inner walls of the ventricles by slender cords (Figure 4-4). Two additional valves are located between the right and left ventricles and the arteries into which they pump blood, that is, the pulmonary artery and aorta.

FIGURE 4-3 Blood Flow Through the Heart (a) Blood enters both atria simultaneously from the systemic and pulmonary circuits. When full, the atria pump their blood into the ventricles. (b) When the ventricles are full, they contract simultaneously, (c) delivering the blood to the pulmonary and systemic circuits.

FIGURE 4-4 Heart Valves A cross section of the heart showing the four chambers and the location of the major vessels and valves.

The valves of the heart are one-way valves. That is, they permit the one-way flow of blood. When the ventricles contract, for example, blood forces the valves in the large arteries connecting to them to open. Blood flows out of the ventricles into the arteries. The backflow of blood causes the valve to shut, preventing blood from draining back into the ventricles.

Heart Sounds. When a doctor listens to your heart, he or she is listening to sounds of the heart valves closing. The noises she hears are called the heart sounds and are often described as “LUB-dupp.” The first heart sound (LUB) results from the closure of the atrioventricular valves. It is longer and louder than the second heart sound (dupp), produced when the valves in the connecting arteries shut. Because the valves do not close at the same time, careful placement of the stethoscope allows a doctor to listen to each valve individually to determine whether it is working properly.

For most of us, our heart valves function flawlessly throughout life. However, certain diseases can alter the valves. This, in turn, can decrease the efficiency of the heart and the circulation of blood. Rheumatic (RUE-ma-tick) fever, for example, is caused by a bacterial infection and affects many parts of the body, including the heart. Although it is relatively rare in wealthier countries, rheumatic fever is still a significant problem in less developed countries. Rheumatic fever begins as a sore throat caused by certain types of streptococcus (STREP -toe-COCK-iss) bacteria. The sore throat—known as strep throat—is usually followed by general illness. During this infection, the body forms antibodies (proteins made by cells of the immune system) to the bacteria. These antibodies can damage the heart valves, preventing them from closing completely. This causes blood to leak back into the atria and ventricles after contraction and results in a distinct “sloshing” sound, called a heart murmur. This condition reduces the efficiency of the heart and causes the organ to work harder to make up for the inefficient pumping. In severe cases, increased activity can result in heart failure.

The Heart’s Pacemaker. As we all know, the heart functions at different rates depending on activity level. At rest, it generally beats slowly. When we are working hard or running, it beats much faster. This change in heart rate helps the body adjust for differences in oxygen requirements of the cells of the body.

Heart rate is controlled by a number of mechanisms. One of the most important is an internal pacemaker, the sinoatrial (SI-noh-AY-tree-il) (SA) node. Located in the wall of the right atrium, the SA node is composed of a clump of specialized cardiac muscle cells. These cells produce a tiny electric impulse, like those produced by nerve cells. This impulse spreads rapidly from the node to the rest of heart muscle, first in the atria and then into the ventricles. Without the pacemaker the muscle cells would all contract independently, and the heart would be ineffective.

The electrical impulse created by the SA node is slowed briefly as it passes from the atria to the ventricles. This allows the blood-filled atria to contract and empty their contents into the ventricles. This delay provides the ventricles plenty of time to fill before they are stimulated to contract.

The SA node of the human heart produces a steady rhythm of about 100 beats per minute, much too fast for most human activities. To bring the heart rate in line with body demand, the SA node must be slowed down by impulses transmitted by nerves from a heart control center in the brain. These impulses slow the heart to about 70 beats per minute at rest. During exercise or stress or hard physical labor, the impulses from the brain are reduced, allowing the heart to beat faster to meet body demands.

Other nerves also influence heart rate. These nerves carry impulses that accelerate the heart rate, allowing the heart to attain rates of 180 beats or more when the cells’ demand for oxygen is great.

Several hormones also play a role in controlling heart rate. One of these is adrenalin. This hormone is secreted during stress or exercise by the adrenal glands located on top of the kidneys. Adrenalin accelerates the heart, increasing the flow of blood through the body.

Electrical Activity in the Heart. Electrical changes in the cardiac muscle cell occur when the muscles of the atria and ventricles contract. These changes can be detected by placing electrodes—small metal plates connected to wires that run to a voltage meter—on the chest. The resulting reading on the voltage meter is called an electrocardiogram (ECG or EKG). Diseases of the heart may result in noticeable changes in the EKG. As a result, an EKG is often a valuable diagnostic tool for heart doctors, called cardiologists.

Heart Attacks: Causes, Cures, and Treatments

Many Americans die each year from heart attacks. Those who survive must often undergo surgery and make dramatic changes in their diets to prevent another attack.

The most common type of heart attack is known as a myocardial infarction (my-oh-CAR-dee-al in-FARK-shun) or MI. MIs are caused by blood clots that block one or more of the arteries that supply the heart muscle. Blood clots can become lodged in arteries narrowed by plaque (discussed in more detail below). A blood clot lodged in a heart artery restricts the flow of blood to the heart muscle the artery serves, thus cutting off the vital supply of oxygen and nutrients. The lack of oxygen can damage and kill the heart muscle cells. The damaged region is called an infarct (in-FARKT)—hence the name myocardial infarction.

The formation of plaque results from a combination of several factors: smoking, poor diet, lack of exercise, heredity, stress, and others. Note, however, that narrowing of a coronary artery by plaque will usually not block the flow of blood enough to cause a heart attack unless it is quite severe—around 80% or 90% blockage. Less severe narrowing, however, can cause blood clots to form in the vessel at the site of narrowing. They can block the artery, causing a heart attack. Clots can also form in other parts of the body and travel in the blood stream. When they reach a narrowing in a heart artery, they can become lodged in the artery. This blocks blood flow, depriving heart muscle of much needed oxygen. If the size of the area damaged by lack of oxygen is small, a heart attack is usually not life-threatening. If the damage is great, a heart attack can prove fatal.

Getting help quickly lessens the chances of severe damage to the heart. Giving a person an aspirin during a heart attack also reduces the damage because aspirin reduces blood clotting.

Heart attacks can occur quite suddenly, without warning, or may be preceded by several weeks by a type of pain called angina (an-GINE-ah). Angina occurs when the supply of oxygen to the heart muscle is reduced. The pain appears in the center of the chest and can spread to a person’s throat, upper jaw, back, and arms (usually just the left one). Angina is a dull, heavy, constricting pain that appears when an individual is active, then disappears when he or she ceases the activity.

Angina may also be caused by stress and exposure to carbon monoxide, a pollutant that reduces the oxygen-carrying capacity of the blood. Angina begins to show up in men at age 30 and is nearly always caused by coronary artery disease. In women, angina tends to occur at a much later age.

Another type of heart attack occurs when the SA node loses control of the heart muscle. When this occurs, the cardiac muscle cells beat independently. The result is that the heart reduces, even stops, pumping blood. If the heart stops beating altogether, the condition is known as cardiac arrest.

Physicians treat this type of heart attack known as fibrillation by applying a strong electrical current to the chest. It is often sufficient to restore normal electrical activity and heartbeat. A normal heartbeat can also be restored by cardiopulmo-nary resuscitation (CPR), in which the heart is “massaged” externally by applying pressure to the breastbone.

Prevention Is the Best Cure. Heart disease is not inevitable. It can be avoided by a healthy diet, exercise, and stress management. Starting young before problems begin is the best preventative measure. Older adults can take a half an aspirin a day. When taken over long periods, it can help reduce one’s risk of a heart attack. Quitting smoking also greatly reduces one’s risk of a heart attack.

FIGURE 4-5 Stent.

Heart attacks are treated by injecting one of several blood clot-dissolving agents. When given within a few hours of the onset of a heart attack, they can greatly reduce the damage to heart muscle and accelerate a patient’s recovery.

In mild heart attacks, physicians can open clogged blood vessels by inserting a small device called a catheter into the heart artery. A tiny balloon attached to its tip is then inflated. Chemical clot dissolvers are then injected and the balloon is inflated. This procedure forces the artery open and loosens the plaque from the wall and is known as balloon angioplasty (AN-gee-oh-PLAS-tee). Scientists are also experimenting with lasers that burn away plaque in artery walls. Unfortunately, as in other techniques, cholesterol often builds up again in the walls of arteries within a few months.

To avoid plaque clogging an artery opened by balloon angioplasty, surgeons often insert a tiny device called a stent into the artery after balloon angioplasty (FIGURE 4-5). After the balloon is removed, the stent holds the artery open.

In severe cases, coronary arteries may be completely blocked by plaque. To restore blood flow to the heart, surgeons often perform coronary bypass surgery. In this procedure, doctors transplant small segments of veins from the leg into the heart (FIGURE 4-6). The veins are connected to the artery on either side of the clogged area so blood can bypass it. Unfortunately, the veins often fill—and often fairly quickly—with plaque. To avoid this problem, surgeons may use small sections of artery to bypass the clogged blood vessels in the heart. Strict control of diet is also helpful.

The Blood Vessels

While the heart serves as a central pump, it is the blood vessels that transport blood throughout the body. The blood vessels form an extensive network of ducts in the body. Three types of blood vessels are present: arteries, capillaries, and veins.

Arteries carry blood away from the heart. As they travel through the body, arteries branch from time to time to supply various organs. Within each organ, arteries branch again, forming smaller and smaller vessels. The smallest of all arteries is the arteriole (ar-TEAR-ee-ole).

FIGURE 4-6 Coronary Bypass Surgery (a) Atherosclerotic plaque in coronary arteries can block the flow of blood to heart muscle. (b) Venous grafts bypass coronary arteries blocked by atherosclerotic plaque.

FIGURE 4-7 Capillary Bed A network of capillaries between the arteriole and the venule delivers blood to the cells of body tissues (not shown).

As shown in FIGURE 4-7, arterioles empty into capillaries (CAP-ill-air-ees), tiny, thin-walled vessels that permit wastes and nutrients to pass through with relative ease. Capillaries form extensive, branching networks in body tissues, referred to as capillary beds. Capillaries have very thin walls that permit water and various nutrients to pass from the blood into the surrounding tissues and allow waste to move in the opposite direction.

Blood flows out of the capillaries into the smallest of all veins, the venules. Venules, in turn, converge to form small veins, which unite with other small veins, forming larger veins. Blood in veins flows toward the heart.

Blood Pressure. As blood is pumped throughout the circulatory system, it creates a force against the walls of the blood vessels. Created by the pumping of the heart, this force is known as the blood pressure. Blood pressure varies throughout the cardiovascular system, being the highest in the arteries and the lowest in the capillaries and veins.

Like many other physical conditions in the human body, blood pressure varies from time to time. For example, it changes in relation to stress levels. When someone makes you angry or stresses you out, your blood pressure rises! Blood pressure also increases with age and when arteries become hardened by plaque. High blood pressure is an indication of cardiovascular disease.

Blood pressure is measured with a device called a blood pressure cuff (FIGURE 4-8). The blood pressure cuff is wrapped around the upper arm. A stethoscope is placed over an artery just below the cuff. Air is pumped into the cuff until the pressure stops the flow of blood through the artery. The pressure in the cuff is then gradually reduced as air is released. When the blood pressure in the artery exceeds the pressure of the cuff, the blood starts flowing through the vessel once again. This point represents the systolic pressure (sis-TOL-ick), the pressure at the moment the ventricles contract. Systolic pressure is the higher of the two numbers in a blood pressure reading (120/70, for example). The pressure at the moment the heart relaxes to let the ventricles fill again is the diastolic pressure (DI-ah-STOL-ick) and is the lower of the two readings. It is determined by continuing to release air from the cuff until no arterial pulsation can be heard. At this point, blood is flowing continuously through the artery.

A typical reading for a young, healthy adult is about 120/70, although readings vary considerably from one person to the next. So, what is normal for one person may be abnormal for another. As a person ages, blood pressure tends to rise. Thus, a healthy 65-year-old might have a blood pressure reading of 140/90.

FIGURE 4-8 Blood Pressure Reading A sphygmomanometer (blood pressure cuff) is used to determine blood pressure.

Hypertension is a prolonged elevation in blood pressure. Like other diseases of the heart and blood vessels, it has many causes, including obesity.

The problem with hypertension is that blood pressure increases gradually over time. A person may feel fine for years. Symptoms, such as headaches, rapid, forceful beating of the heart (palpitations), and a general feeling of ill health usually occur only when blood pressure is dangerously high. Consequently, early detection and treatment are essential to prevent serious problems, including heart attacks.

How the Capillaries Function. The heart, arteries, and veins form an elaborate system that transports blood to and from the capillaries. It is here that wastes and nutrients are exchanged between the cells of the body and the blood.

As blood flows into a capillary bed, nutrients like glucose, gases like oxygen, water, and hormones that are transported in the blood begin to diffuse out of the capillaries. At the same time, water-dissolved wastes in the tissues, such as carbon dioxide, diffuse into the capillaries.

Rapid exchange is possible because blood pressure is very low and thus blood slows down considerably as it passes through the capillaries. Rapid exchange is also possible because the walls of capillaries are very thin, consisting of only a single layer of flattened cells. These cells permit dissolved substances to pass through them with ease.

The constriction and dilation (expansion) of the arterioles that “feed” the capillaries help to regulate blood flow through the capillaries. They also help to regulate body temperature. On a cold winter day, for example, the arterioles in the skin close down, restricting blood flow through the capillaries. This conserves body heat. On a warm day, the flow of blood through the skin increases. This releases heat and often creates a pink flush.

How Veins Function. Blood leaves the capillary beds stripped of oxygen and various nutrients and loaded with waste. As it drains from the capillaries, the blood enters the smallest of all veins. They unite to form larger veins. Eventually, all of the blood returning to the heart in the systemic circuit enters the superior and inferior vena cavae, two large veins that drain into the right atrium of the heart (Figure 4-2). These vessels drain the upper and lower parts of the body, respectively.

As noted earlier, blood pressure in the veins is low, and veins have relatively thin walls. Because the veins’ walls are so thin, obstructions can cause blood to pool in them, creating bluish bulges called varicose veins (VEAR-uh-cose) that can be quite painful. See FIGURE 4-9.

Some people inherit a tendency to develop varicose veins. However, most cases can be attributed to one of several factors that reduce the flow of blood back to the heart: pregnancy, obesity, sedentary lifestyles, and rarely an abdominal tumor.

Varicose veins may also form in the wall of the anal canal. The veins in this region are known as the internal hemorrhoidal veins (heh-meh-ROID-il). A swelling of the internal hemorrhoidal veins results in a condition known as hemorrhoids (heh-meh-ROIDS). Because the internal hemorrhoidal veins are supplied by numerous pain fibers, this condition can be quite painful.

Blood flows through arteries because of blood pressure, but in veins it flows because of other forces. In veins located above the heart, gravity draws blood down to the heart. In veins below the heart, blood is propelled by the movement of body parts. As you walk, for example, the contraction of muscles forces the blood upward, causing it to move against the force of gravity.

Valves in the veins also help in this process. Valves are flaps of tissue that span the veins and prevent the backflow of blood (FIGURE 4-10). Just as in the valves of the heart, blood pressure, however slight, pushes the flaps open. This allows the blood to move forward. As the blood fills the segment of the vein in front of the valve, it pushes back on the valve flaps and forces them shut. Blood is then propelled through the next set of valves and so on and so on until it empties into the vena cavae.

FIGURE 4-9 Varicose veins Any restriction of venous blood flow to the heart causes veins to balloon out, creating bulges commonly known as varicose veins.

FIGURE 4-10 Valves in Veins The slight blood pressure in the veins and the contraction of skeletal muscles propel the blood along the veins back toward the heart. The one-way valves stop the blood from flowing backward.

The Lymphatic System

The lymphatic system is an extensive network of vessels and glands (FIGURE 4-11). It is functionally related to two systems: the circulatory system and the immune system. This module examines the circulatory role of the lymphatic system.

The cells of the body are bathed in a watery liquid called tissue fluid. It provides a medium through which nutrients, gases, and wastes can diffuse between the capillaries and the cells.

Tissue fluid is replenished by water from capillaries. The flow of water out of the capillaries, however, normally exceeds the return flow back into the capillaries by about 3 liters (nearly 3 quarts) per day. The excess is picked up by small lymph vessels, called lymph capillaries, found in all of the tissues and organs of the body. These vessels have thin walls through which water and other substances easily pass.

Lymph drains from the capillaries into larger ducts. These vessels, in turn, merge with others, creating larger and larger ducts. They eventually empty into the large veins at the base of the neck.

FIGURE 4-11 The Lymphatic System The lymphatic system consists of vessels that transport lymph, or excess tissue fluid, back to the circulatory system.

Lymph moves through the vessels of the lymphatic system in much the same way that blood is transported in veins. In the upper parts of the body, it flows by gravity. In regions below the heart, it is propelled largely by muscle contraction. Lymph flow is also assisted by valves.

The lymphatic system also consists of several lymphatic organs. Because they help protect us from infections, we will discuss them in the module on the immune system.

Normally, lymph is removed from tissues at a rate equal to its production. In some instances, however, lymph production exceeds its uptake. A burn, for example, may cause extensive damage to capillaries, which causes them to release excess water into tissues. This flood overwhelms the lymph vessels, causing a buildup of fluid in tissues and swelling.

The heart and blood vessels are essential to all cells and organs of the body—and to our health. Unfortunately, many people engage in lifestyles and eating habits that damage these vital organs. Studies show that proper diet, exercise, rest, and stress control protect our hearts and blood vessels from damage, allowing us to live long, healthy, and productive lives.

circulatory system, described in the previous module, has one purpose: to transport blood throughout the body. The blood, in turn, supplies oxygen and other nutrients to body cells while removing potentially harmful wastes. Blood also performs a number of other extremely important functions, which you will learn about in this module. First, let’s look at what’s in blood.

Composition of Blood

Blood consists of a fluid known as plasma. Plasma is mostly water that contains numerous dissolved substances such as sodium, glucose, and blood proteins. Blood also contains cells, including white blood cells, red blood cells, and platelets, described shortly.

Blood accounts for about 8% of our body weight. A man weighing 70 kilograms (150 pounds) has about 5–6 liters (1.3–1.5 gallons) of blood. On average, women have about a liter less.

Plasma makes up about 55% of the blood volume. White blood cells, red blood cells, and platelets make up about 45% (FIGURE 5-1). Most of the blood cells consist of red blood cells. The concentration of red blood cells, however, varies with altitude. Residents of Denver, Colorado, a city located a mile above sea level, for example, have a slightly higher concentration of red blood cells than individuals living at sea level. The increase in red blood cells compensates for the lower oxygen levels in the atmosphere in higher altitudes. In these individuals, blood cells and platelets typically constitute about 50% of the blood volume, compared to 45% at sea level. Higher concentrations of red blood cells help residents cope with lower oxygen levels, but also give athletes an advantage when competing at lower altitudes. As a result, many athletes train at higher altitudes, like the U.S. Summer Olympic team, which is headquartered in Colorado Springs, Colorado.

With this information in mind, let’s take a deeper look at each of the main components of blood to learn more about what they do.

FIGURE 5-1 Blood Composition Blood removed from a person can be centrifuged to separate plasma from the cellular component. Red blood cells constitute about 45% of the blood volume, except at higher altitudes where they make up about 50% of the volume to compensate for the lower oxygen levels.

Plasma

Plasma performs many vital functions. For example, plasma transports macronutrients such as glucose, lipids, and amino acids and micronutrients such as sodium, chloride, and potassium from the digestive system to the cells of the body. Waste products are also transported in the plasma to excretory organs such as the lungs and kidneys. These organs remove the wastes, keeping us safe.

Besides nourishing cells and removing harmful waste products, the blood plasma transports hormones, or chemical messengers, from their site of production to their site of action. Hormones help regulate many body functions. Plasma also contains proteins that form blood clots, protecting us from bleeding to death when we cut ourselves. Other plasma proteins, such as antibodies, discussed in the next module, provide protection from disease.

Red Blood Cells

One of the main components of blood is the red blood cell. The red blood cell (RBC) is a highly flexible disk, concave on both sides. As most readers probably already know, RBCs transport oxygen and some carbon dioxide in the blood (FIGURE 5-2). (Most carbon dioxide is dissolved in blood plasma.)

RBCs have no organelles, so they mostly consist of a cell membrane enclosing large amounts of hemoglobin. Hemoglobin (HEME-oh-GLOBE-in) is a large protein found only in RBCs. Hemoglobin contains iron, which binds to oxygen. Carbon dioxide, a waste product of cellular respiration, also binds to hemoglobin, but to a much lesser degree.

Swept along in the bloodstream, RBCs travel throughout the circulatory system, traveling from arteries to capillaries to veins, and then back again. As they pass through the capillaries, the RBCs bend and twist. This permits them to squeeze through the many miles of tiny capillaries without getting trapped and clogging the vessels. This flexibility is important for our health and survival. Without it, RBCs could easily logjam in capillaries, blocking blood flow to the vital organs of the body.

Unfortunately, approximately one of every 500–1,000 African Americans suffers from a disease called sickle-cell anemia (ah-NEE-mee-ah). This disease caused by a minor genetic mutation results in a decrease in the flexibility of RBCs. As a result, RBCs become sickle-shaped cells and, more important, inflexible when they encounter low levels of oxygen in capillaries (FIGURE 5-3). Sickle-shaped cells collect at branch points in capillary beds. This blocks blood flow, disrupting the supply of nutrients and oxygen to tissues and organs. The lack of oxygen causes considerable pain, but can also be fatal to body cells. Blockages in the heart and brain often lead to heart attacks and serious brain damage. Many people who have sickle-cell anemia die in their late twenties and thirties; some die even earlier.

FIGURE 5-2 Red Blood Cells (a) Transmission electron micrograph of human RBCs showing their flexibility and their lack of organelles. (b) Scanning electron micrograph of human RBCs.

FIGURE 5-3 Sickle-Cell Anemia Scanning electron micrograph of a sickle cell.

On average, RBCs live about 120 days. At the end of their life span, the liver and spleen remove them from circulation. When the aged RBCs are removed from the blood, the iron contained in the hemoglobin is recycled and used to produce new RBCs in the red bone marrow found inside many bones. Because the recycling of iron is not 100% efficient, small amounts of iron must be ingested each day in the diet to make up for the loss.

Human RBCs are highly specialized cells that lose their nuclei and organelles during cellular differentiation (Figure 5-2a). Because of this, RBCs cannot divide to replace themselves as they age. In humans, new RBCs are produced in red bone marrow found in certain bones of the body. Red bone marrow produces about 2 million RBCs per second! Because of this, the number of RBCs in the blood remains more or less constant. Maintaining a constant concentration of RBCs is essential to homeostasis. RBC production is stimulated by a hormone produced by the kidney. This hormone, known as erythropoietin (eh-RI-throw-poe-EAT-in) or EPO, is also one of a dozen or more performance-enhancing chemical substances being used illegally by some athletes.

Human health depends on the ability of blood to transport oxygen throughout the body. A reduction in the oxygen-carrying capacity of the blood is known as anemia. Anemia results in weakness and fatigue. Individuals are often pale and tend to faint or become short of breath easily.

Anemia may result from excessive bleeding, which reduces the number of RBCs in the blood stream. It can also be caused by tumors in the red marrow that reduce RBC production. Several infectious diseases (such as malaria) also reduce RBC production, resulting in anemia. In addition, anemia can result from a reduction in the amount of hemoglobin in RBCs. This, in turn, may be caused by iron deficiency.

White Blood Cells

White blood cells are part of the body’s protective mechanism, a system that combats harmful bacteria and viruses and helps us maintain homeostasis. White blood cells circulate in the bloodstream, but like RBCs are produced in the bone marrow.

The body contains five types of white blood cells. Two of them are responsible for engulfing harmful bacteria. When they arrive at the “scene” of an infection, they escape through the walls of the capillaries, then migrate into the infected tissue where they gobble up bacteria and debris.

One type of white blood cell, known as the lymphocyte, produces antibodies. Antibodies are proteins that bind to and eliminate foreign organisms such as bacteria. Another type of lymphocyte attacks foreign cells, such as parasites and tumor cells, directly. You’ll learn more about these two types of lymphocytes in the module on the immune system.

Like red blood cells, WBCs are involved in homeostasis—that is, in maintaining internal constancy. Their numbers increase greatly during infections and other diseases. Increases and decreases in various types of WBCs can be used to diagnose many diseases. For example, a dramatic increase in lymphocytes and lower abdominal pain are usually signs of appendicitis, an infection of the appendix, a small, long-thought-to-be useless organ attached to the large intestine.

Diseases Involving WBCs

Like many other cells in the body, WBCs can malfunction. Some white blood cells, for example, can divide uncontrollably in the bone marrow. In other words, they become cancerous. The cancerous WBCs then enter the blood stream.

A cancer of WBCs is called leukemia (lew-KEE-mee-ah). It is dangerous because WBCs fill the bone marrow, crowding out the cells that produce RBCs and platelets. This results in a decline in RBC production, which leads to anemia. It also results in a reduction in platelet production. Because platelets are involved in blood clotting, leukemia reduces normal clotting and increases internal bleeding. In addition, WBCs produced in leukemia are often incapable of fighting infection. Leukemia patients typically die from infections and internal bleeding.

The most serious type of leukemia is acute leukemia, so named because it kills victims quickly. Children are the primary victims of this disease. Leukemia can be treated by irradiating the bone marrow (radiation kills the cancer cells) and by administering drugs that halt the uncontrollable cell division of the cancerous white blood cells.

Another common disorder of WBCs is infectious mononucleosis (MOH-no-NU-clee-OH-siss), commonly called mono. This disease is caused by a virus transmitted through saliva and may be spread by kissing or by sharing silverware, plates, and drinking glasses. Common complaints include fatigue, aches, sore throats, and low-grade fever. Rest and drinking plenty of liquids is required to recover.

Platelets

Platelets are tiny cell fragments produced in the bone marrow from large platelet-producing cells (FIGURE 5-4). Like RBCs, platelets lack nuclei and organelles and, therefore, are not true cells. Also like RBCs, platelets are unable to divide. Platelets are coated by a layer of sticky material, which causes them to adhere to irregular surfaces, such as tears in blood vessels or plaque in arteries.

Blood clotting is a chain reaction involving several steps. When a blood vessel is torn, long, branching fibers made of a protein called fibrin collect at the damage point. They produce a weblike network in the wall of the damaged blood vessel (FIGURE 5-5). The fibrin web traps RBCs and platelets, forming a plug that stops the flow of blood to the tissue.

FIGURE 5-4 Megakaryocyte A light micrograph of a megakaryocyte, a large, multinucleated cell found in bone marrow; the megakaryocyte fragments, giving rise to platelets.

FIGURE 5-5 Blood Clot A scanning electron micrograph of a fibrin clot that has already trapped platelets and RBCs, plugging a leak in a vessel. The RBCs are red, and the fibrin network is blue.

Blood clotting occurs fairly quickly. In most cases, a damaged blood vessel is sealed by a clot within 3–6 minutes of an injury. After the blood vessel is repaired, the clot is dissolved by a naturally occurring enzyme in the blood.

Blood clotting is a homeostatic mechanism vital to our survival, but in some individuals, blood clotting is impaired as a result of one of several problems such as an insufficient number of platelets. (A reduced platelet count may result from leukemia.) In some individuals, the liver may not produce a sufficient amount of blood-clotting factors, substances required for normal blood clotting.

The most common cause of this problem is a genetic defect known as hemophilia (hee-moe-FEE-lee-ah). Problems begin early in life, so even tiny cuts or bruises can bleed uncontrollably, threatening one’s life. Because of repeated bleeding into the joints, victims suffer great pain and often become disabled; they often die at a young age. Fortunately, hemophiliacs can be treated by transfusions of blood-clotting factors.

The blood and circulatory system operate tirelessly, transporting nutrients to body cells, removing wastes, sending hormonal messages to the cells of the body, repairing damaged vessels, and more. Although things can go wrong, for those who eat well and live a healthy lifestyle, this system performs admirably, keeping our bodies running smoothly and efficiently.

loating in the air we breathe and water we drink are billions upon billions of microorganisms—bacteria and viruses. Fortunately, most bacteria and viruses in our environment are harmless, incapable of causing disease. Those that are potentially harmful must evade our body’s defenses, the topic of this module. Before we look at the ways the human body protects itself from such invasions, let’s begin by looking at viruses and bacteria.

Viruses and Bacteria

Viruses consist of a strand of DNA or RNA surrounded by a protein coat, although some viruses possess an additional protective layer known as the viral envelope (FIGURE 6-1). On their own, viruses are pretty harmless. They are incapable of reproducing and are therefore not even considered to be living things.

FIGURE 6-1 General Structure of a Virus (a) The virus consists of a nucleic acid core of either RNA or DNA. Surrounding the viral core is a layer of protein known as the capsid. Each protein molecule in the capsid is known as a capsomere. (b) Some viruses have an additional protective coat known as the envelope. (c) Electron micrograph of the human immunodeficiency virus (HIV).

Viruses cause problems when they invade the body. Once inside the body, they can invade cells, where they take over the cells’ metabolic machinery and divert it to making new viruses. Because of this, some scientists liken viruses to cellular pirates. In very short time, infected cells produce hundreds of thousands of new viruses. They are then released into the bloodstream and can therefore spread to other cells, infecting them.

Viruses most often enter the body through the respiratory and digestive systems. However, other avenues of entry are also possible—for example, the reproductive system.

Fortunately for us, the immune system kills many viruses, but usually not until we’ve undergone considerable suffering. The many strains of influenza or flu viruses, for example, cause fevers, chills, headaches, and muscle pains that can last up to 2 weeks. During this time, the immune system mounts an attack, eliminating the virus in 10–14 days. Colds are also caused by approximately 200 different “cold” viruses (TABLE 6-1).

Although many viruses are eliminated from our bodies by the immune system, some, such as those that cause fever blisters and genital herpes, take refuge in certain body cells. When an individual is stressed, the viruses emerge, causing problems. The genital herpes virus, for example, produces tiny, painful sores on the genitals, thighs, and buttocks.

Bacteria are single-celled organisms that lack the organelles of more complex cells like those in humans and other animals. Bacteria contain a circular strand of DNA (FIGURE 6-2) but unlike viruses they are capable of growing and reproducing on their own outside of cells. Outside the plasma membrane of bacteria is a thick, rigid cell wall.

TABLE 6-1

How Do You Know If You Have a Cold or the Flu?

Symptoms Cold Gradual Flu Sudden

Fever Rare Common, may reach 101°F; may last 3 to 4 days

Headache Rare Common and can be severe

Cough Hacking Dry cough

Muscle aches and pains Slight Typical, often severe

Tiredness and weakness Mild Common, often severe

Chest discomfort Mild to moderate Common

Stuffy nose Common Sometimes

Sneezing Usual Sometimes

Sore throat Common Sometimes; may last 3 to 4 days

Caused by Any of 200 viruses Influenza virus

FIGURE 6-2 General Structure of a Bacterium (a) Bacteria come in many shapes and sizes, but all have a circular strand of DNA, cytoplasm, and a plasma membrane. Surrounding the membrane of many bacteria is a cell wall. (b) Electron micrograph of Salmonella bacteria.

Like viruses, bacteria enter the body through the respiratory tract, urinary system, and GI tract. Bacteria penetrate the epithelium of these systems, but also enter through cuts and abrasions in the skin. Inside the body, bacteria proliferate, using body nutrients to make more of their kind. Some bacteria produce toxins—substances that cause illness and, in some cases—death.

Bacterial infections are treated with drugs called antibiotics. Antibiotics inhibit protein synthesis in bacteria. Although antibiotics are highly effective, heavy use of these drugs throughout the world has resulted in the formation of numerous antibiotic-resistant strains of bacteria that kill many thousands of people each year. Nationwide, one out of every four bacterial infections today involves a resistant strain. Most of these infections occur in children under the age of 5. To treat them, doctors must use stronger doses or newer antibiotics. In some cases, the treatments are ineffective and the patients die.

To counteract antibiotic resistant strains of bacteria, many doctors are now being more careful about treating patients with antibiotics. They try to be sure that the infections they are treating are caused by bacteria and not by viruses, on which these drugs have no effect. For viral infections, the best remedy is usually rest, which gives the immune system time to eliminate the virus. Pharmaceutical companies have produced a number of potentially useful antiviral drugs.

Now that you understand a little bit about microorganisms, let’s turn our attention to the ways the body protects us from harmful viruses and bacteria.

The First Line of Defense

The first line of defense against bacteria, viruses, and other microbes is the skin and the linings of the respiratory, digestive, and urinary systems. They form a physical barrier that does a decent job of preventing potentially harmful microorganisms from invading the underlying tissues. A break in these linings, however, may permit microorganisms to enter.

The first line of defense also involves a number of chemical deterrents. The skin, for instance, produces a slightly acidic secretion that impairs bacterial growth. Saliva contains an enzyme that dissolves the cell wall of bacteria, killing them. The stomach lining releases hydrochloric acid that successfully destroys many ingested bacteria. Cells in the lining of the respiratory tract produce mucus with antimicrobial properties.

The Second Line of Defense

The first line of defense is not perfect. Tiny breaks can occur, allowing viruses and bacteria to enter the body. Fortunately, there are several chemical and cellular agents that take up the battle at this point.

A cut or abrasion, for instance, results in a kind of chemical and biological warfare waged against harmful microorganisms, known as the inflammatory response, shown in FIGURE 6-3. Characterized by redness, swelling, pain, and heat, the inflammatory response begins with the release of a variety of chemical substances by the injured tissue. Some chemicals attract cells in the tissues known as macrophages and certain white blood cells to the site. These cells engulf or phagocytize bacteria that have entered the wound. Soon after these cells begin to work, a yellowish fluid called pus begins to exude from the wound. It contains dead white blood cells, microorganisms, and cellular debris.

During the inflammatory response, other chemical substances released by injured tissues stimulate repair. Histamine (HISS-tah-meen) is one such chemical. Released by injured tissue, histamine causes arterioles in the damaged area to expand or dilate. Dilation allows more blood to flow into the region (Figure 6-3). Increased blood flow is responsible for the heat and redness in the wounded area. Heat accelerates healing. Increased blood flow ensures an adequate supply of nutrients to the site needed by cells to fight the infection.

Other chemical substances released by injured tissues cause the capillaries in the wound to become more leaky. This, in turn, increases the flow of plasma into a wounded region. Oxygen and nutrients in the plasma accelerate healing.

Other cells release chemicals that cause the brain to raise body temperature, creating fever. Mild fevers cause the spleen and liver to remove iron from the blood. Because many disease-causing bacteria require iron to reproduce, fever fights the bacterial infection.

Another chemical safeguard, not part of the inflammatory response, is a group of small proteins known as the interferons (in-ter-FEER-ons). Interferons are released from cells infected by viruses. These chemicals bind to the cell membranes of noninfected body cells. This, in turn, causes the uninfected cells to produce enzymes that inhibit viral replication. If a virus enters these cells, they cannot replicate. Interferons therefore help to halt the spread of viruses from one cell to another.

A group of blood proteins also helps fight bacterial infections. These proteins bind together to form a large structure that embeds itself in the membrane of bacteria, creating an opening into which water flows. The influx of water causes bacterial cells to swell, burst, and die.

The Third Line of Defense

The immune system is the third line of defense. Unlike the respiratory or digestive systems, the immune system is rather diffuse—dispersed throughout the body. Lymphocytes are a key component of the immune system. They circulate in the blood and lymph and also take up residence under the linings of the respiratory and digestive systems. Many also reside in the spleen, thymus, lymph nodes, and tonsils, all known as lymphoid organs.

The immune response is triggered by large foreign molecules, such as proteins and long-chained carbohydrates. These molecules are called antigens (AN-tah-gins), an abbreviation for antibody-generating substances. Antigens are found in the outer membranes of viruses, bacteria, and parasites. As a result, the immune system will react to viruses and bacteria and other microbes. In addition, it responds to parasites such as the organism that causes malaria.

FIGURE 6-3 The Inflammatory Response

Some small molecules like penicillin, poison ivy toxin, and formaldehyde can also stimulate an immune reaction. They do so by binding to naturally occurring proteins in the body. The resulting combination is viewed by the immune system as a foreign substance.

Cells transplanted from one person to another also stimulate an immune response. That’s because the cell membranes of each individual’s cells contain a unique set of proteins—not found on the cells of recipients. When an organ is transplanted from one individual to another, then, the immune system of the recipient reacts to the cell membrane proteins of the donor and mounts an attack on them. This response must be counteracted by drugs that suppress the immune system of the tissue or organ recipient.

Cancer cells also have a slightly different set of membrane proteins than normal body cells, even though they arise from normal body cells. Cancer cells are therefore viewed as foreign cells and are attacked by the immune system. Although cancer cells evoke an immune response, it is often not sufficient to stop the disease.

The human body contains two types of lymphocytes: T lymphocytes, commonly called T cells, and B lymphocytes, also called B cells. B cells and T cells respond to different types of antigens. B cells, for instance, react to bacteria, bacterial toxins, and a few viruses. When activated, B cells produce antibodies.

T cells recognize and respond to our own body cells that have gone awry. This includes cancer cells as well as body cells that have been invaded by viruses. T cells also respond to transplanted tissue cells and larger disease-causing agents, such as single-celled fungi and parasites. Unlike B cells, T cells attack their targets directly.

By various estimates, several million distinct B and T cells are produced in the body early in life. Each one is programmed to respond to a specific antigen. Over a lifetime, only a relatively small fraction of these cells will be called into duty.

How B Cells Work

When a bacterium first enters the body, B cells preprogrammed to respond to the unique proteins in the bacterium’s cell membrane bind to the bacterium. The B cells begin to divide. Some of the cells produced in this process form a new kind of cell, called the plasma cell. Plasma cells produce antibodies. Antibodies are proteins that help eliminate antigens, such as bacteria and bacterial toxins. Antibodies circulate in the blood and lymph, where they bind to the antigens that triggered the response.

The first time an antigen enters the body, it elicits an immune response, but the initial reaction—or primary response—is relatively slow (FIGURE 6-4a). In fact, antibody levels in the blood do not begin to rise until approximately the beginning of the second week after the intruder was detected, which explains why it takes most people about 7–10 days to combat a cold or the flu. This delay occurs because it takes time for B cells to form a sufficient number of plasma cells.

If the same antigen enters the body at a later date, however, the immune system acts much more quickly (FIGURE 6-4b). This stronger reaction constitutes the secondary response. During a secondary response, antibody levels increase within a few days after the antigen has entered the body. The amount of antibody produced during this infection also greatly exceeds quantities produced during the primary response. Consequently, the antigen is quickly destroyed, and a recurrence of the illness is prevented.

FIGURE 6-4 Primary and Secondary Responses (a) The primary (initial) immune response is slow. It takes about 10 days for antibody levels to peak. Almost no antibody is produced during the first week as plasma cells are being formed. (b) The secondary response is much more rapid. Antibody levels rise almost immediately after the antigen invades. T cells show a similar response pattern.

The reason the secondary response occurs so quickly is that during the primary response, some lymphocytes divide to produce a large population of memory cells. Memory cells remain in the body awaiting the antigen’s re-entry. During the secondary response, the memory cells proliferate rapidly, producing numerous plasma cells that produce lots of antibody to combat the foreign invaders.

Antibodies destroy foreign organisms and antigens via one of several ways. Some antibodies bind directly to antigens, forming a complete coating around them, which prevents them from doing any harm. Other antibodies bind to numerous antigens, causing them to clump together, again rendering them ineffective. Still others bind to multiple antigens, forming large water-insoluble complexes that precipitate out of solution.

How T Cells Work

Like B cells, T cells respond to the presence of antigens by undergoing rapid cell division. However, T cells produce four different cells.

One of the products is the cytotoxic T cell. Cytotoxic T cells attack and kill viruses, body cells infected by viruses, parasites, cancer cells, and foreign cells introduced during blood transfusions or tissue or organ transplants.

Memory T cells are also produced when antigens are present. As in B cells, the memory T cells form a cellular reserve. They’re there to protect the body in the event of a second or third invasion.

Helper T cells are also produced when the body detects an antigen. Helper T cells greatly enhance the immune response. They do so by stimulating both B cells and cytotoxic T cells. Helper T cells are the most abundant of all the T cells (comprising 60–70% of the circulating T cells). Without them, antibody production and T-cell activity is greatly reduced. Incidentally, it is the helper T cells that are targeted by HIV, human immunodeficiency virus, which is responsible for AIDS. Infection of helper T cells by HIV therefore disables a person’s immune system.

The final type of lymphocyte is suppressor T cells. Research suggests that they “turn off” the immune reaction as the antigen begins to disappear.

Active and Passive Immunity

One of the major medical advances of the 1800s was the discovery of vaccines (vac-SEENS). Used to prevent bacterial and viral infections, vaccines contain inactivated or greatly weakened viruses, bacteria, or bacterial toxins. When injected into the body, the “disabled” antigens in vaccines elicit an immune response just as if the real thing had entered the body. The immune system responds by producing antibodies or T cells.

Vaccines stimulate the immune reaction because the weakened or deactivated organisms (or toxins) they contain still possess the antigenic proteins or carbohydrates that trigger B- and T-cell activation. However, because the infectious agents have been seriously weakened or deactivated, viruses, bacteria, and bacterial toxins in vaccines do not cause disease. Some individuals may develop minor symptoms, but they’re not life-threatening.

Vaccination provides a form of protection that immunologists call active immunity—so named because the body actively produces memory T and B cells to protect a person against future infections. The immune response is the same as the one that occurs when a disease-causing organism enters the body. Most vaccines provide immunity or protection from microorganisms for long periods, sometimes for life. Others need to be administered several times during one’s lifetime.

Vaccinations are vital in controlling deadly diseases such as polio, typhus, and smallpox—diseases that can cripple or kill people before their immune system mounts an effective response.

The second type of immunity, called passive immunity, is a temporary form of protection. It results from the injection of antibodies to disease-causing microorganisms. These antibodies are produced by first injecting antigens in animals such as sheep. Antibodies produced in the sheep are then extracted from the blood and bottled. They can then be injected into humans to prevent disease. Antibodies so produced do not activate the immune system, but rather destroy antigens directly. This type of protection is called passive because T cells and B cells are not called into duty; the antibodies combat potential invaders directly.

Antibodies remain in the blood for a few weeks, protecting an individual from infection. Because the liver slowly removes these molecules from the blood, a person gradually loses protection. This is the kind of protection one needs when traveling for a short period in a foreign country where infectious diseases are more prevalent. For example, antibodies are administered to individuals who are about to travel in less-developed nations to protect against viral hepatitis (liver infection) and other common prevalent organisms. Antibodies are also used to treat individuals who have been bitten by poisonous snakes to counteract the venom (FIGURE 6-5). Venom is a mixture of proteins, enzymes, and polypeptides (long-chain molecules made up of amino acids, but not long enough to be classified as a protein). These molecules damage body cells, especially nerve cells and heart muscle cells. Antibodies in antivenoms destroy or deactivate the harmful molecules, preventing or reversing their adverse effects, if injected early enough.

Vaccines have lowered the incidence of many infectious diseases in the United States and other industrialized nations by 99% or more. Despite the success of vaccines, publicity concerning their rare side effects has caused many parents to choose not to have their children vaccinated. In addition, because immunization programs have greatly reduced the incidence of most infectious diseases, some parents falsely believe that their children are safe without vaccines.

Public health officials are quick to point out that disease-causing microorganisms that once took a huge toll on humans have not been eradicated. Without widespread vaccinations, outbreaks could occur again.

FIGURE 6-5 Poison and Antidote Poisonous snakes like this rattler inject venom into their victims. Venom can be milked from the snake and used to produce antivenom, a serum containing antibodies that neutralize the venom.

Blood Transfusions and Tissue Transplantation

Although the immune system is important in protecting us from microorganisms, it can work against doctors when they perform life-saving blood transfusions and tissue transplants.

Blood transfusions require careful cross-matching of donors and recipients to be certain their blood types match. Humans have four blood types: A, B, AB, and O. The letters refer to the type of antigens found on the surface of the red blood cells. In type A blood, the A antigen is present. In Type B, B antigen is present. In Type AB both antigens are present. In Type O there are no antigens. Cross-matching blood is essential to prevent life-threatening immune reactions. Blood from a Type B person cannot be safely transfused into a person with Type A blood.

Organ and tissue transplantation is a much more complex matter. In fact, only three conditions exist in which a person can receive a transplant and not reject it. One is if the tissue comes from an individual’s own body. For burn victims, surgeons often transplant healthy skin from one part of the body to cover a badly damaged region elsewhere.

The second instance is when a tissue is transplanted between identical twins—individuals derived from a single fertilized egg that splits early in embryonic development to form two embryos. These individuals are genetically identical and have identical cell membrane antigens.

A third instance occurs when tissue rejection is inhibited by drugs. For example, in heart and kidney transplants treatment with these drugs must be continued throughout the life of the patient. Unfortunately, most of these drugs leave patients vulnerable to bacterial and viral infections.

Diseases of the Immune System

The immune system, like all other body systems, can malfunction. One of the most common malfunctions results in allergies.

An allergy is an overreaction to some antigens such as pollen or certain foods like milk (FIGURE 6-6). Antigens that stimulate allergic reactions are called allergens (A-ler-gens). Allergens cause the production of a special group of antibodies from plasma cells. These antibodies are known as IgE antibodies. As Figure 6-6 shows, IgE produced when an allergen is present bind to a type of cell known as a mast cell. Mast cells are found in many tissues, and contain large quantities of histamine.

FIGURE 6-6 Allergic Reaction Antigen stimulates the production of massive amounts of IgE, a type of antibody produced by plasma cells. IgE attaches to mast cells. This is the sensitization stage. When the antigen enters again, it binds to the IgE antibodies on the mast cells, triggering a massive release of histamine and other chemicals. Histamine, in turn, causes blood vessels to dilate and become leaky. This triggers the production of mucus in the respiratory tract. In some people, the chemicals released by the mast cells also cause the small air-carrying ducts in the lungs to constrict, making breathing difficult.

As shown in Figure 6-6, allergens that stimulated the production of the IgE antibodies bind to the antibodies attached to the mast cells. This causes the mast cells to release massive amounts of histamine (Figure 6-6). Histamine released in the lungs causes tiny tubules carrying air in the lung to constrict, which reduces airflow and makes breathing difficult. This condition is called asthma (AS-mah).

Allergic reactions usually occur in specific body tissues, where they create local symptoms that, while irritating, are generally not life-threatening. For example, an allergic response may occur in the eyes, causing redness and itching. Or, it may occur in the nasal passageway, causing stuffiness.

An allergic response can occur in the bloodstream, where it can be fatal if not treated quickly. For example, penicillin or bee venom in the bloodstream of certain people can cause a massive release of histamine and other chemicals. Histamine released by mast cells also causes severe constriction of the ducts in the lungs that deliver oxygen to the air sacs, making breathing difficult. Histamine also causes extensive dilation of blood vessels in the skin and other tissues. The blood pressure then falls, shutting down the circulatory system. The decline in blood pressure and constriction of the bronchioles in the lungs result in a condition known as anaphylactic shock (AN-ah-fah-LACK-tic). Death may follow if measures are not taken quickly. One such measure is an injection of the hormone epinephrine (commonly known as adrenalin), which rapidly reverses the constriction of the bronchioles.

Allergies can be treated by avoiding allergens— for instance, avoiding milk or staying clear of dogs and cats. Patients can also take antihistamines (an-tee-HISS-tah-meens), drugs that counteract the effects of histamine. Patients may also be given allergy shots, injections of increasing quantities of the allergen to which they’re allergic. In many cases, this treatment makes an individual less and less sensitive to the allergen.

Occasionally, the immune system really runs amok, mounting an attack on the body’s own cells. This unfortunate state of affairs is known as an autoimmune disease. Autoimmune diseases result from many causes. For example, in some instances, normal body proteins can be modified by environmental pollutants, viruses, or genetic mutations so that they are no longer recognizable by the body.

AIDS: The Deadly Virus

Many millions of people the world over have died from a disease known as AIDS (acquired immune deficiency syndrome). AIDS is caused by a virus that attacks and weakens the immune system. This virus, known as human immunodeficiency virus or HIV for short, attacks the helper T cells, severely impairing the immune system. Patients die from infections.

HIV is transmitted by sexual contact, usually from infected men to uninfected men and women. It is also transmitted in blood transfusions and by sharing hypodermic needles among drug addicts. It can also be transmitted from infected mothers to their babies at birth.

The number of cases of AIDS and the number of deaths from AIDS in the United States increased dramatically since the early 1980s. HIV is a global epidemic with more than 34 million people infected worldwide in 2010 and approximately 2.7 million new infections each year. Most, if not all, of these people will die from the disease, which is still spreading rapidly through many places such as Africa and China and through certain populations, such as African-Americans in the United States.

AIDS progresses through three stages. During the first phase, no symptoms appear, although an individual is highly infectious—that is, able to transmit the disease to others. During the second phase, patients grow progressively weaker as their immune systems falter. Lymph nodes swell and patients report persistent or recurrent fevers and persistent coughs. Mental deterioration may also occur. During the last phase, patients suffer from severe weight loss and weakness. Many develop cancer and bacterial infections because of their diminished immune response (FIGURE 6-7).

Stopping the virus has proved difficult, in large part because symptoms of AIDS do not appear until several months to several years after the initial HIV infection. Abstinence and the use of condoms are both effective in helping to stop the spread of the disease. Several drugs have also been developed that slow down the progression of the disease. Used in combination, they are greatly prolonging the lives of many people infected with HIV, although treatment is extremely expensive. Numerous researchers are developing vaccines which they hope will protect people and eventually eradicate the virus.

The immune system is one of our greatest allies in maintaining our health. It is called into duty every day of your life dozens of times to protect you from hidden dangers. But like other body systems, it can run amok. And, it is not invincible. You can, however, increase your chances of maintaining a strong immune system by eating right, getting plenty of sleep, minimizing stress, and living a healthy lifestyle.

FIGURE 6-7 HIV and Kaposi’s Sarcoma (a) AIDS viruses. (b) Kaposi’s sarcoma on the foot found only in AIDS patients.

he human respiratory system functions automatically, drawing air into the lungs, and then releasing it. This cycle repeats itself about 16 times per minute at rest—or about 23,000 times per day. Like so many body systems, the respiratory system is vital to our health and well-being. Not only does it supply the blood with oxygen required by cells to make energy, it helps get rid of carbon dioxide, a waste product of cellular energy production.

In this module, we will explore the structure and function of the respiratory system—and some of the diseases that affect it.

Structure and Function of the Respiratory System

The respiratory system consists of two parts: one part that transports air into and out of the lungs and a second part, the lungs, in which gas exchange occurs between the air we breathe and our blood stream.

The air-conducting portion of the respiratory system is an elaborate set of passageways that transports air to and from the lungs, two large, saclike organs in the chest (FIGURE 7-1a). Like the arteries of the body, these passageways start out large then become progressively smaller and more numerous, branching extensively in the lungs.

The lungs are the gas exchange portion of the respiratory system. Each lung contains millions of tiny, thin-walled air sacs called alveoli (al-vee-OH-lie) (FIGURE 7-1b). The walls of the alveoli are surrounded by numerous capillaries that absorb oxygen from the inhaled air and release carbon dioxide carried in the blood that circulates through the lungs (Figure 7-1b).

Air enters the respiratory system through the nose and mouth, then is drawn backward through the pharynx, and then into the larynx (LAIR-inks). The larynx is a rigid, hollow structure that houses the vocal cords, two folds of tissue that vibrate when we talk, sing, or hum (FIGURE 7-2). From here, air flows into the windpipe or trachea (TRAY-kee-ah).

Food is prevented from entering the larynx by the epiglottis (eh-peh-GLOT-tiss), a flap of tissue that closes off the opening to the larynx during swallowing. Occasionally, however, food accidentally enters the larynx. This leads to violent coughing, a reflex that usually successfully ejects the food and keeps us from choking to death—suffocating because we can’t get oxygen into our lungs.

FIGURE 7-1 The Human Respiratory System (a) This drawing shows the air-conducting portion and the gas exchange portion of the human respiratory system. The insert shows a higher magnification of the alveoli, where oxygen and carbon dioxide exchange occurs. (b) A scanning electron micrograph of the alveoli, showing the rich capillary network surrounding them.

The trachea enters the chest cavity, where it divides into two large branches. They enter the lungs alongside the arteries and veins. Inside the lungs, these tubes branch extensively, forming progressively smaller tubules that carry air to the alveoli.

The smallest of these tubules branch to form bronchioles (BRON-kee-oles). Bronchioles are small ducts that lead directly to the alveoli. Smooth muscle in the walls of the bronchioles permits the bronchioles to open and close, and thus provides a means of controlling airflow in the lungs. During exercise or times of stress, the bronchioles open, allowing air to flow more readily into the alveoli. This homeostatic mechanism helps meet the body’s need for additional oxygen.

Besides moving air to and from the lungs, the conducting portion of the respiratory system filters out particles, such as dust, found in the air we breathe. The large and medium-sized particles drop out as inhaled air passes through the nasal cavity and the passageways of the upper respiratory system. Smaller particles, also known as fine particulates, however, are so tiny that they remain suspended in the air breathed into our lungs. As a result, they can penetrate deeply into the lungs. Some of these particles may contain toxic metals such as mercury, which can cause lung cancer.

Particles that precipitate out of the inhaled air in the upper portion of the respiratory system are trapped in a layer of mucus (MEW-kuss). Mucus is a thick, slimy secretion produced by certain cells in the epithelial lining of the upper respiratory tract (FIGURE 7-3).

The epithelium of the respiratory tract also contains numerous ciliated cells. Cilia are tiny organelles that project from the surface of the cells. They beat upward toward the mouth, and thus help transport mucus containing bacteria and dust particles to the oral cavity. When the mucus reaches the mouth, it may be swallowed or spit out. This mechanism protects much of the respiratory tract and the lungs from bacteria and potentially harmful particulates.

FIGURE 7-2 Uppermost Portion of the Respiratory System Bony protrusions into the nasal cavity (not shown here) create turbulence that causes dust particles to settle out on the mucous coating. Notice that air passing from the pharynx enters the larynx. Food is kept from entering the respiratory system by the epiglottis, which covers the laryngeal opening during swallowing.

Like all homeostatic mechanisms, the respiratory mucous trap is not invincible. Bacteria and viruses do occasionally penetrate the lining, causing respiratory infections. In addition, sulfur dioxide in cigarette smoke temporarily paralyzes, and may even destroy, cilia in the respiratory track. Sulfur dioxide gas in the smoke of a single cigarette paralyzes the cilia for an hour or more, permitting bacteria and toxic particulates to be deposited in the respiratory tract, even enter the lungs. Unfortunately, the cilia of a smoker are paralyzed when they are needed the most!

Because of this, smokers suffer more frequent respiratory infections than nonsmokers. Research shows that alcohol also paralyzes the respiratory system cilia, explaining why alcoholics are more prone to respiratory infections than others.

Beneath the epithelium of the respiratory tract is a rich network of capillaries. These tiny blood vessels release heat and moisture into the incoming air. As air passes through the conducting portion it is warmed and moistened. Moisture protects the lungs from drying out, and heat protects them from cold temperatures. By the time inhaled air reaches the lungs, it is nearly saturated with water and is warmed to body temperature, except in extremely cold areas.

The Alveoli. The air we breathe consists principally of two gases: nitrogen and oxygen. It also contains small amounts of carbon dioxide and a few other gases.

Oxygen is transported to the lungs in the air we breathe. It enters the blood stream via an estimated 150 million alveoli in the lungs. The alveoli are lined by a single layer of flattened cells surrounded by an extensive network of capillaries, shown in Figure 7-1b. Each capillary is made of an equally thin layer of flattened cells. Together, the cells lining the alveoli and the capillary walls present a fairly easy route for the movement of oxygen into the alveoli.

FIGURE 7-3 Mucous Trap (a) Drawing of the lining of the trachea. Mucus produced by the mucous cells of the lining of much of the respiratory system traps bacteria, viruses, and other particulates in the air. The cilia transport the mucus toward the mouth. (b) Higher magnification of the lining showing a mucous cell and ciliated epithelial cells.

FIGURE 7-4 The Alveolar Macrophage Drawing of the alveolus showing Type I and Type II alveolar cells and macrophages or dust cells.

Oxygen travels from the alveoli to the blood stream by diffusion. Once in the blood, it diffuses into red blood cells where it attaches to hemoglobin molecules. It is then transported via the arteries to capillaries to the cells of the tissues and organs of the body.

Carbon dioxide travels in the opposite direction, also by diffusion, leaving the blood and entering the alveoli. It is then released into the air in the lungs, and then exhaled.

Inside the lungs in the alveoli are cells called alveolar macrophages, also known as the dust cells. Alveolar macrophages wander freely through the alveoli, engulfing dust, bacteria, viruses, and other particulates that escape filtration in the upper portions of the respiratory system (FIGURE 7-4). Once filled, they retire to the sidelines, taking up residence in the connective tissue surrounding the alveoli. Because there are so many particulates in tobacco smoke, a smoker’s lungs are often blackened by dust cells packed with fine dust and soot particles. The lungs of urban residents and marijuana smokers may also be blackened by the accumulation of smoke and dust particles.

The walls of the alveoli also contain large, round cells that produce a chemical known as surfactant (sir-FACK-tant). It dissolves in the thin layer of water lining the alveoli. Surfactant keeps the tiny alveoli from collapsing.

Some premature babies lack sufficient surfactant. As a result, the larger alveoli collapse, making it difficult to breath. The condition, known as respiratory distress syndrome, is usually treated with an artificial surfactant that keeps the alveoli open until the lungs produce enough of their own.

Making Sounds

The chief functions of the respiratory system are to replenish the blood’s oxygen supply and rid the blood of excess carbon dioxide. However, the respiratory system serves other functions as well. The vocal cords, located in the larynx, for example, produce sounds that allow us to communicate our thoughts, ideas, and feelings.

All the sounds we make from the most primitive grunts and groans to the most elegant speech and songs are produced by vibrations in the vocal cords. The vocal cords are two elastic ligaments located inside the larynx (FIGURE 7-5). The vocal cords vibrate as air is expelled from the lungs. The sounds generated by the vocal cords are then modified by changing the position of the tongue and by changes in the shape of the oral cavity.

FIGURE 7-5 Vocal Cords (a) This drawing of the larynx shows the location of the vocal cords. Note the presence of the false vocal cord, so named because it does not function in sound production. (b) View into the larynx of a patient showing the true vocal cords from above.

The vocal cords vary in length and thickness from one person to the next, which accounts for differences in our voices. Differences in thickness also account for the differences in the voices of men and women. Most men, for example, have longer, thicker vocal cords than women, resulting in deeper voices. The difference in thickness is attributable to the male sex hormone, testosterone, which is produced by the testes.

Bacterial and viral infections of the larynx can cause the vocal cords to swell. This thickens the cords, causing a person’s voice to lower. This condition is known as laryngitis (lair-in-JITE-iss). Laryngitis may also be caused by tobacco smoke, alcohol, excessive talking, shouting, coughing, or singing—all activities that can irritate the vocal cords.

The respiratory system also houses the receptors for smell. They are located in the epithelium in the roof of the nasal cavity. You’ll learn more about this in the module on the senses.

Breathing and the Control of Respiration

Air moves in and out of the lungs in much the same way that it moves in and out of the bellows that a blacksmith uses to fan a fire. Breathing, however, is largely an involuntary action, controlled by the nervous system.

To begin, air must first be drawn into the lungs. This process, known as inhalation, is followed by exhalation, the expulsion of the air.

Inhalation is controlled by the brain. Nerve impulses traveling from the brain stimulate the diaphragm, a dome-shaped muscle that separates the abdominal and chest cavities (FIGURE 7-6a). Nerve impulses cause the diaphragm to contract, which causes it to flatten and lower. This, in turn, draws air into the lungs in much the same way that pulling the plunger of a syringe out draws air into the device.

Inhalation also involves the muscles between the ribs. They are stimulated by nerve impulses from the brain. These muscles therefore contract as the diaphragm is lowered. Contraction of these muscles lifts the rib cage up and out, helping to draw air into the lungs.

Together, the contractions of the diaphragm and the muscles between the ribs increase the volume of the chest cavity (FIGURE 7-6b). Air naturally flows in through the mouth and nose. The lungs expand like balloons.

Inhalation can also be increased by a conscious, forceful contraction of the diaphragm and the muscles of the ribs. This increases the amount of air entering the lungs. Athletes often actively inhale and exhale just before an event to increase oxygen levels in their blood.

Once the lungs are full, the diaphragm and rib muscles relax, returning to their previous state. This forces air out of the lungs. Exhalation is aided by elastic fibers in the walls of the lung. When inhalation ceases, the elastic fibers recoil, helping to force air out.

FIGURE 7-6 The Bellows Effect (a) The rising and falling of the chest wall through the contraction of the intercostal muscles (muscles between the ribs) is shown in the diagram, illustrating the bellows effect. Inspiration is assisted by the contraction of the diaphragm. The rising of the chest wall and the lowering of the diaphragm draws air into the lungs. (b) X-rays showing the size of the lungs in full exhalation (top) and full inspiration (bottom).

Although exhalation does not involve the contraction of muscles in an individual at rest, forceful muscle contraction can be used to push air out more quickly and more completely. (Try it and you’ll see.)

Breathing is controlled by a region of the brain called the breathing center. It contains nerve cells that produce periodic electric impulses. These impulses stimulate the rib muscles and the diaphragm to contract, initiating inhalation. When the lungs fill, the nerve impulses cease and the muscles relax. Air is then forced out of the lungs, as just explained.

Breathing is controlled by other processes, too. Chemical receptors inside the brain, for instance, monitor carbon dioxide concentration in the blood. When levels of carbon dioxide are high, for instance, during vigorous exercise, the receptors send impulses to the breathing center. This increases the depth and rate of breathing to ensure adequate oxygen supply. Next time you run up a flight of stairs and start breathing hard, you’ll know why.

Diseases of the Respiratory System

The respiratory system like other body systems can become diseased. It is particularly vulnerable to certain airborne bacteria and viruses that can cause debilitating infections like the flu and colds. As readers know, both can result in considerable discomfort. In infants and older adults, the flu can be fatal.

Infections may occur in different locations in the respiratory system and are named by the affected site. An infection in the large tubes that branch from the trachea, that is, the bronchi, is known as bronchitis (bron-KITE-iss). An infection of the sinuses is known as sinusitis (sigh-nu-SITE-iss).

The lungs are also susceptible to air pollution, including airborne asbestos fibers. Asbestos is a naturally occurring fiber that has been used in thousands of products from pipe and sound insulation to car brake pads. Inhalation of asbestos fibers can cause serious health problems. Some workers exposed to asbestos, for example, develop a fatal lung cancer known as mesothelioma. Others develop a debilitating disease known as asbestosis (as-bes-TOE-sis). Asbestosis is a buildup of scar tissue that reduces lung capacity.

Because asbestos is so dangerous, virtually all of its uses have been banned in the United States. Asbestos used for insulation and decoration in years past is now either being removed from or is being stabilized (coated with a sealant) in existing buildings, especially schools, to prevent potential health problems. Those most prone to health effects are smokers.

Another common disease of the respiratory system is asthma. Asthma is an allergic reaction to substances like dust, pollen, mold, or skin cells (dander) from pets or other animals. It is characterized by periodic episodes of wheezing and difficult breathing. In some individuals, asthma is brought on by foods, such as eggs, milk, chocolate, beer, and food preservatives. Still other cases are triggered by drugs, such as antibiotics.

In asthmatics, irritants cause a rapid increase in the production of mucus by the linings of the bronchi and bronchioles. In addition, they stimulate constriction of the bronchioles. Together, mucus production and constriction of the bronchioles can make breathing quite difficult. Asthmatics also suffer from a chronic inflammation of the lining of the respiratory tract. As a result, asthma attacks can be quite disabling. Severe reactions lead to death. Victims are generally elderly individuals suffering from other diseases.

Although asthma is fairly common in school children, it often disappears as they grow older. Unfortunately, the incidence of asthma is increasing in children, and asthma is also starting to appear in older adults.

The severity of asthma attacks can be greatly lessened by two medications. One of the most common is an oral spray (inhalant) containing the hormone adrenalin (epinephrine). It stimulates the bronchioles to open. Anti-inflammatory drugs (steroids) are also commonly administered to treat chronic inflammation. Screening tests can help a patient find out what substances trigger an asthmatic attack so they can be avoided.

Another common disease is lung cancer. In 2010, lung cancer claimed the lives of over 157,000 men and women in the United States (FIGURE 7-7). Lung cancer is primarily caused by smoking; in fact, 90% of all cases of lung cancer in men and 80% of all cases of lung cancer in women are believed to be caused by smoking. Smokers are 11–25 times more likely to develop lung cancer than nonsmokers. Moreover, the more cigarettes one smokes, the higher the risk of developing cancer.

FIGURE 7-7 The Normal and Cancerous Lung (a) The normal lung appears spongy. (b) The cancerous lung from a smoker is filled with particulates and a large tumor.

Clearly, the respiratory system is vital to human life, but like other body systems it must be well taken care of to ensure a good, healthy life. Living and working in an unpolluted environment and avoiding tobacco smoke are keys to maintaining the lungs and the rest of the respiratory system in peak condition. These measures and a healthy diet help you live a long, healthy, and enjoyable life.

he human respiratory system functions automatically, drawing air into the lungs, and then releasing it. This cycle repeats itself about 16 times per minute at rest—or about 23,000 times per day. Like so many body systems, the respiratory system is vital to our health and well-being. Not only does it supply the blood with oxygen required by cells to make energy, it helps get rid of carbon dioxide, a waste product of cellular energy production.

In this module, we will explore the structure and function of the respiratory system—and some of the diseases that affect it.

Structure and Function of the Respiratory System

The respiratory system consists of two parts: one part that transports air into and out of the lungs and a second part, the lungs, in which gas exchange occurs between the air we breathe and our blood stream.

The air-conducting portion of the respiratory system is an elaborate set of passageways that transports air to and from the lungs, two large, saclike organs in the chest (FIGURE 7-1a). Like the arteries of the body, these passageways start out large then become progressively smaller and more numerous, branching extensively in the lungs.

The lungs are the gas exchange portion of the respiratory system. Each lung contains millions of tiny, thin-walled air sacs called alveoli (al-vee-OH-lie) (FIGURE 7-1b). The walls of the alveoli are surrounded by numerous capillaries that absorb oxygen from the inhaled air and release carbon dioxide carried in the blood that circulates through the lungs (Figure 7-1b).

Air enters the respiratory system through the nose and mouth, then is drawn backward through the pharynx, and then into the larynx (LAIR-inks). The larynx is a rigid, hollow structure that houses the vocal cords, two folds of tissue that vibrate when we talk, sing, or hum (FIGURE 7-2). From here, air flows into the windpipe or trachea (TRAY-kee-ah).

Food is prevented from entering the larynx by the epiglottis (eh-peh-GLOT-tiss), a flap of tissue that closes off the opening to the larynx during swallowing. Occasionally, however, food accidentally enters the larynx. This leads to violent coughing, a reflex that usually successfully ejects the food and keeps us from choking to death—suffocating because we can’t get oxygen into our lungs.

FIGURE 7-1 The Human Respiratory System (a) This drawing shows the air-conducting portion and the gas exchange portion of the human respiratory system. The insert shows a higher magnification of the alveoli, where oxygen and carbon dioxide exchange occurs. (b) A scanning electron micrograph of the alveoli, showing the rich capillary network surrounding them.

The trachea enters the chest cavity, where it divides into two large branches. They enter the lungs alongside the arteries and veins. Inside the lungs, these tubes branch extensively, forming progressively smaller tubules that carry air to the alveoli.

The smallest of these tubules branch to form bronchioles (BRON-kee-oles). Bronchioles are small ducts that lead directly to the alveoli. Smooth muscle in the walls of the bronchioles permits the bronchioles to open and close, and thus provides a means of controlling airflow in the lungs. During exercise or times of stress, the bronchioles open, allowing air to flow more readily into the alveoli. This homeostatic mechanism helps meet the body’s need for additional oxygen.

Besides moving air to and from the lungs, the conducting portion of the respiratory system filters out particles, such as dust, found in the air we breathe. The large and medium-sized particles drop out as inhaled air passes through the nasal cavity and the passageways of the upper respiratory system. Smaller particles, also known as fine particulates, however, are so tiny that they remain suspended in the air breathed into our lungs. As a result, they can penetrate deeply into the lungs. Some of these particles may contain toxic metals such as mercury, which can cause lung cancer.

Particles that precipitate out of the inhaled air in the upper portion of the respiratory system are trapped in a layer of mucus (MEW-kuss). Mucus is a thick, slimy secretion produced by certain cells in the epithelial lining of the upper respiratory tract (FIGURE 7-3).

The epithelium of the respiratory tract also contains numerous ciliated cells. Cilia are tiny organelles that project from the surface of the cells. They beat upward toward the mouth, and thus help transport mucus containing bacteria and dust particles to the oral cavity. When the mucus reaches the mouth, it may be swallowed or spit out. This mechanism protects much of the respiratory tract and the lungs from bacteria and potentially harmful particulates.

FIGURE 7-2 Uppermost Portion of the Respiratory System Bony protrusions into the nasal cavity (not shown here) create turbulence that causes dust particles to settle out on the mucous coating. Notice that air passing from the pharynx enters the larynx. Food is kept from entering the respiratory system by the epiglottis, which covers the laryngeal opening during swallowing.

Like all homeostatic mechanisms, the respiratory mucous trap is not invincible. Bacteria and viruses do occasionally penetrate the lining, causing respiratory infections. In addition, sulfur dioxide in cigarette smoke temporarily paralyzes, and may even destroy, cilia in the respiratory track. Sulfur dioxide gas in the smoke of a single cigarette paralyzes the cilia for an hour or more, permitting bacteria and toxic particulates to be deposited in the respiratory tract, even enter the lungs. Unfortunately, the cilia of a smoker are paralyzed when they are needed the most!

Because of this, smokers suffer more frequent respiratory infections than nonsmokers. Research shows that alcohol also paralyzes the respiratory system cilia, explaining why alcoholics are more prone to respiratory infections than others.

Beneath the epithelium of the respiratory tract is a rich network of capillaries. These tiny blood vessels release heat and moisture into the incoming air. As air passes through the conducting portion it is warmed and moistened. Moisture protects the lungs from drying out, and heat protects them from cold temperatures. By the time inhaled air reaches the lungs, it is nearly saturated with water and is warmed to body temperature, except in extremely cold areas.

The Alveoli. The air we breathe consists principally of two gases: nitrogen and oxygen. It also contains small amounts of carbon dioxide and a few other gases.

Oxygen is transported to the lungs in the air we breathe. It enters the blood stream via an estimated 150 million alveoli in the lungs. The alveoli are lined by a single layer of flattened cells surrounded by an extensive network of capillaries, shown in Figure 7-1b. Each capillary is made of an equally thin layer of flattened cells. Together, the cells lining the alveoli and the capillary walls present a fairly easy route for the movement of oxygen into the alveoli.

FIGURE 7-3 Mucous Trap (a) Drawing of the lining of the trachea. Mucus produced by the mucous cells of the lining of much of the respiratory system traps bacteria, viruses, and other particulates in the air. The cilia transport the mucus toward the mouth. (b) Higher magnification of the lining showing a mucous cell and ciliated epithelial cells.

FIGURE 7-4 The Alveolar Macrophage Drawing of the alveolus showing Type I and Type II alveolar cells and macrophages or dust cells.

Oxygen travels from the alveoli to the blood stream by diffusion. Once in the blood, it diffuses into red blood cells where it attaches to hemoglobin molecules. It is then transported via the arteries to capillaries to the cells of the tissues and organs of the body.

Carbon dioxide travels in the opposite direction, also by diffusion, leaving the blood and entering the alveoli. It is then released into the air in the lungs, and then exhaled.

Inside the lungs in the alveoli are cells called alveolar macrophages, also known as the dust cells. Alveolar macrophages wander freely through the alveoli, engulfing dust, bacteria, viruses, and other particulates that escape filtration in the upper portions of the respiratory system (FIGURE 7-4). Once filled, they retire to the sidelines, taking up residence in the connective tissue surrounding the alveoli. Because there are so many particulates in tobacco smoke, a smoker’s lungs are often blackened by dust cells packed with fine dust and soot particles. The lungs of urban residents and marijuana smokers may also be blackened by the accumulation of smoke and dust particles.

The walls of the alveoli also contain large, round cells that produce a chemical known as surfactant (sir-FACK-tant). It dissolves in the thin layer of water lining the alveoli. Surfactant keeps the tiny alveoli from collapsing.

Some premature babies lack sufficient surfactant. As a result, the larger alveoli collapse, making it difficult to breath. The condition, known as respiratory distress syndrome, is usually treated with an artificial surfactant that keeps the alveoli open until the lungs produce enough of their own.

Making Sounds

The chief functions of the respiratory system are to replenish the blood’s oxygen supply and rid the blood of excess carbon dioxide. However, the respiratory system serves other functions as well. The vocal cords, located in the larynx, for example, produce sounds that allow us to communicate our thoughts, ideas, and feelings.

All the sounds we make from the most primitive grunts and groans to the most elegant speech and songs are produced by vibrations in the vocal cords. The vocal cords are two elastic ligaments located inside the larynx (FIGURE 7-5). The vocal cords vibrate as air is expelled from the lungs. The sounds generated by the vocal cords are then modified by changing the position of the tongue and by changes in the shape of the oral cavity.

FIGURE 7-5 Vocal Cords (a) This drawing of the larynx shows the location of the vocal cords. Note the presence of the false vocal cord, so named because it does not function in sound production. (b) View into the larynx of a patient showing the true vocal cords from above.

The vocal cords vary in length and thickness from one person to the next, which accounts for differences in our voices. Differences in thickness also account for the differences in the voices of men and women. Most men, for example, have longer, thicker vocal cords than women, resulting in deeper voices. The difference in thickness is attributable to the male sex hormone, testosterone, which is produced by the testes.

Bacterial and viral infections of the larynx can cause the vocal cords to swell. This thickens the cords, causing a person’s voice to lower. This condition is known as laryngitis (lair-in-JITE-iss). Laryngitis may also be caused by tobacco smoke, alcohol, excessive talking, shouting, coughing, or singing—all activities that can irritate the vocal cords.

The respiratory system also houses the receptors for smell. They are located in the epithelium in the roof of the nasal cavity. You’ll learn more about this in the module on the senses.

Breathing and the Control of Respiration

Air moves in and out of the lungs in much the same way that it moves in and out of the bellows that a blacksmith uses to fan a fire. Breathing, however, is largely an involuntary action, controlled by the nervous system.

To begin, air must first be drawn into the lungs. This process, known as inhalation, is followed by exhalation, the expulsion of the air.

Inhalation is controlled by the brain. Nerve impulses traveling from the brain stimulate the diaphragm, a dome-shaped muscle that separates the abdominal and chest cavities (FIGURE 7-6a). Nerve impulses cause the diaphragm to contract, which causes it to flatten and lower. This, in turn, draws air into the lungs in much the same way that pulling the plunger of a syringe out draws air into the device.

Inhalation also involves the muscles between the ribs. They are stimulated by nerve impulses from the brain. These muscles therefore contract as the diaphragm is lowered. Contraction of these muscles lifts the rib cage up and out, helping to draw air into the lungs.

Together, the contractions of the diaphragm and the muscles between the ribs increase the volume of the chest cavity (FIGURE 7-6b). Air naturally flows in through the mouth and nose. The lungs expand like balloons.

Inhalation can also be increased by a conscious, forceful contraction of the diaphragm and the muscles of the ribs. This increases the amount of air entering the lungs. Athletes often actively inhale and exhale just before an event to increase oxygen levels in their blood.

Once the lungs are full, the diaphragm and rib muscles relax, returning to their previous state. This forces air out of the lungs. Exhalation is aided by elastic fibers in the walls of the lung. When inhalation ceases, the elastic fibers recoil, helping to force air out.

FIGURE 7-6 The Bellows Effect (a) The rising and falling of the chest wall through the contraction of the intercostal muscles (muscles between the ribs) is shown in the diagram, illustrating the bellows effect. Inspiration is assisted by the contraction of the diaphragm. The rising of the chest wall and the lowering of the diaphragm draws air into the lungs. (b) X-rays showing the size of the lungs in full exhalation (top) and full inspiration (bottom).

Although exhalation does not involve the contraction of muscles in an individual at rest, forceful muscle contraction can be used to push air out more quickly and more completely. (Try it and you’ll see.)

Breathing is controlled by a region of the brain called the breathing center. It contains nerve cells that produce periodic electric impulses. These impulses stimulate the rib muscles and the diaphragm to contract, initiating inhalation. When the lungs fill, the nerve impulses cease and the muscles relax. Air is then forced out of the lungs, as just explained.

Breathing is controlled by other processes, too. Chemical receptors inside the brain, for instance, monitor carbon dioxide concentration in the blood. When levels of carbon dioxide are high, for instance, during vigorous exercise, the receptors send impulses to the breathing center. This increases the depth and rate of breathing to ensure adequate oxygen supply. Next time you run up a flight of stairs and start breathing hard, you’ll know why.

Diseases of the Respiratory System

The respiratory system like other body systems can become diseased. It is particularly vulnerable to certain airborne bacteria and viruses that can cause debilitating infections like the flu and colds. As readers know, both can result in considerable discomfort. In infants and older adults, the flu can be fatal.

Infections may occur in different locations in the respiratory system and are named by the affected site. An infection in the large tubes that branch from the trachea, that is, the bronchi, is known as bronchitis (bron-KITE-iss). An infection of the sinuses is known as sinusitis (sigh-nu-SITE-iss).

The lungs are also susceptible to air pollution, including airborne asbestos fibers. Asbestos is a naturally occurring fiber that has been used in thousands of products from pipe and sound insulation to car brake pads. Inhalation of asbestos fibers can cause serious health problems. Some workers exposed to asbestos, for example, develop a fatal lung cancer known as mesothelioma. Others develop a debilitating disease known as asbestosis (as-bes-TOE-sis). Asbestosis is a buildup of scar tissue that reduces lung capacity.

Because asbestos is so dangerous, virtually all of its uses have been banned in the United States. Asbestos used for insulation and decoration in years past is now either being removed from or is being stabilized (coated with a sealant) in existing buildings, especially schools, to prevent potential health problems. Those most prone to health effects are smokers.

Another common disease of the respiratory system is asthma. Asthma is an allergic reaction to substances like dust, pollen, mold, or skin cells (dander) from pets or other animals. It is characterized by periodic episodes of wheezing and difficult breathing. In some individuals, asthma is brought on by foods, such as eggs, milk, chocolate, beer, and food preservatives. Still other cases are triggered by drugs, such as antibiotics.

In asthmatics, irritants cause a rapid increase in the production of mucus by the linings of the bronchi and bronchioles. In addition, they stimulate constriction of the bronchioles. Together, mucus production and constriction of the bronchioles can make breathing quite difficult. Asthmatics also suffer from a chronic inflammation of the lining of the respiratory tract. As a result, asthma attacks can be quite disabling. Severe reactions lead to death. Victims are generally elderly individuals suffering from other diseases.

Although asthma is fairly common in school children, it often disappears as they grow older. Unfortunately, the incidence of asthma is increasing in children, and asthma is also starting to appear in older adults.

The severity of asthma attacks can be greatly lessened by two medications. One of the most common is an oral spray (inhalant) containing the hormone adrenalin (epinephrine). It stimulates the bronchioles to open. Anti-inflammatory drugs (steroids) are also commonly administered to treat chronic inflammation. Screening tests can help a patient find out what substances trigger an asthmatic attack so they can be avoided.

Another common disease is lung cancer. In 2010, lung cancer claimed the lives of over 157,000 men and women in the United States (FIGURE 7-7). Lung cancer is primarily caused by smoking; in fact, 90% of all cases of lung cancer in men and 80% of all cases of lung cancer in women are believed to be caused by smoking. Smokers are 11–25 times more likely to develop lung cancer than nonsmokers. Moreover, the more cigarettes one smokes, the higher the risk of developing cancer.

FIGURE 7-7 The Normal and Cancerous Lung (a) The normal lung appears spongy. (b) The cancerous lung from a smoker is filled with particulates and a large tumor.

Clearly, the respiratory system is vital to human life, but like other body systems it must be well taken care of to ensure a good, healthy life. Living and working in an unpolluted environment and avoiding tobacco smoke are keys to maintaining the lungs and the rest of the respiratory system in peak condition. These measures and a healthy diet help you live a long, healthy, and enjoyable life.

The Endocrine System

The nervous system controls many body functions, especially those that require immediate action. Control is also vested in the endocrine system, the subject of this chapter.

What Is the Endocrine System?

The endocrine system is a diffuse body system that consists of numerous small glands scattered throughout the body (FIGURE 13-1). These glands produce and secrete chemical substances called hormones (HOR-monz). A hormone is a chemical produced by an endocrine gland that is transported in the blood stream to distant sites where it exerts some effect. The cells affected by a hormone are called its target cells. The blood carries dozens of hormones at any one time.

Hormones affect five distinct areas of body function: (1) homeostasis; (2) growth and development; (3) reproduction; (4) energy production, storage, and use; and (5) behavior.

Target Cells, Receptors, and Cellular Responses

Despite the fact that they’re exposed to many different hormonal signals, target cells respond only to specific hormones. The reason for this is that target cells contain very specific protein receptors that bind to specific hormones. Each cell contains receptors for the hormones to which it is genetically programmed to respond. In some target cells, the hormone receptors are in the cell membrane; in others, they’re located in the cytoplasm.

Hormones fall into two broad categories. The first is the tropic hormones (TRAW-pik; “to nourish”). Tropic hormones stimulate the production and secretion of hormones by other endocrine glands. An example is thyroid-stimulating hormone (TSH). Produced by the pituitary gland, TSH travels in the blood to the thyroid gland in the neck on either side of the voice box. Here it stimulates the release of yet another hormone, thyroxine (thigh-RAW-xin). Thyroxine released by the thyroid circulates in the blood and stimulates metabolism in many types of body cells.

The second group of hormones is the nontropic hormones. They stimulate cellular growth, metabolism, or other functions. Thyroxine is one of many nontropic hormones produced in the body.

FIGURE 13-1 The Human Endocrine System The endocrine system consists of a scattered group of glands that produce hormones, chemicals that regulate growth and development, homeostasis, reproduction, energy metabolism, and behavior.

The production and release of hormones are controlled by negative feedback, described in the module on structure and function. As you learned in that module, negative feedback helps to maintain the concentration of many ions, nutrients, and other chemicals in the body, which is necessary for proper functioning of the human body. You’ll see many examples of this phenomenon in this module. Negative feedback is particularly important when it comes to maintaining proper hormone levels.

The fact that hormones are controlled by negative feedback loops does not mean that hormone concentrations in the blood are constant 24/7. In fact, virtually all hormones undergo daily changes in their release. These natural fluctuations in hormone levels are mirrored by natural changes in body function and are known as biological cycles, or biorhythms, described in the module on structure and function.

With these basics in mind, let’s take a look at the main endocrine glands and the hormones they produce.

The Pituitary and Hypothalamus

Attached to the underside of the brain by a thin stalk is the pituitary gland (peh-TOO-eh-TARE-ee) (FIGURE 13-2). It’s about the size of a pea.

FIGURE 13-2 The Pituitary Gland (a) A cross section of the brain showing the location of the pituitary and hypothalamus. (b) The structure of the pituitary gland. (c) Releasing and inhibiting hormones travel from the hypothalamus to the anterior pituitary, where they affect hormone secretion.

The pituitary gland is divided into two parts: the anterior pituitary and the posterior pituitary. Together, they secrete a large number of hormones that affect many of the body’s functions. Let’s begin with the anterior pituitary.

The anterior pituitary produces seven hormones, six of which are discussed in this module. The production and release of these hormones are controlled by a region of the brain just above the pituitary, known as the hypothalamus.

The hypothalamus contains receptors that monitor blood levels of hormones, nutrients, and ions. When activated, the receptors stimulate highly specialized nerve cells in the hypothalamus. These cells are a type of neuron known as a neurosecretory neuron. Their name derives from the fact that they synthesize and secrete (release) hormones. These hormones are released into the blood stream in the hypothalamus and then flow to the anterior pituitary where they control the production and release of pituitary hormones (Figure 13-2).

The hypothalamus produces two types of hormones: releasing hormones (RH) that stimulate the release of pituitary hormones and inhibiting hormones (IH) that inhibit the release of hormones from the anterior pituitary.

The releasing and inhibiting hormones travel from the hypothalamus to the pituitary via a special network of blood vessels known as the portal system (Figure 13-2). When they reach the pituitary, these hormones bind to their target cells.

Growth Hormone. The anterior pituitary controls growth of other cells through the release of a protein hormone known as growth hormone (GH). It stimulates an increase in the size of cells like muscle cells and the number of cells. Although growth hormone affects virtually all body cells, it acts primarily on bone and muscle.

As a rule, the more growth hormone that is produced when one is growing, the taller and heftier he or she will be. As FIGURE 13-3 shows, the highest blood levels are present during sleep and during strenuous exercise. It is no wonder that sleep is so important to a growing child. Growth hormone secretion decreases gradually as we age.

The secretion of growth hormone is controlled by a releasing hormone (GH-RH) produced in the hypothalamus. Growth hormone participates in a classic negative feedback loop. When levels of growth hormone are low, GH-RH is secreted by the hypothalamus. It then flows to the anterior pituitary where it stimulates the secretion of GH. Growth hormone release is also stimulated directly through the nervous system. Studies show that stress and exercise both stimulate the hypothalamus to release GH-RH.

FIGURE 13-3 Growth Hormone Secretion in an Adult Growth hormone stimulates muscle and bone growth and is released during exercise and at night.

Deficiencies in growth hormone can result in dramatic changes in body shape and size. If the deficiency occurs during the growth phase, a child may be stunted. This condition is known as dwarfism (FIGURE 13-4a). If the excess occurs during the growth phase, oversecretion results in giantism (JIE-an-tizm) (FIGURE 13-4b).

Thyroid-Stimulating Hormone. Thyroid-Stimulating Hormone (TSH) is a protein hormone produced by the anterior pituitary. Like growth hormone, its production and release are controlled by the hypothalamus. Ultimately, though, TSH release by the anterior pituitary is regulated by the level of thyroid hormone, thyroxine, in the blood in a negative feedback loop. Receptors in the hypothalamus detect the level of thyroxine. When levels are low, these receptors signal the hypothalamus to release TSH-RH. It flows to the anterior pituitary, where it stimulates the production and release of TSH. TSH, in turn, flows to the thyroid, where it stimulates the production and release of thyroxine. As the level of thyroxine increases in the blood, TSH-RH secretion begins to decline (FIGURE 13-5). As shown in Figure 13-5, TSH-RH secretion is also stimulated by stress and cold (in infants).

Thyroid hormones circulate in the bloodstream and influence many body cells. One of their chief functions is to stimulate the breakdown of glucose by body cells to produce energy and heat.

ACTH. Another important hormone is ACTH or adrenocorticotropic hormone (ad-REE-noe-cor-tick-oe-TRO-pik). ACTH is produced by the anterior pituitary. Its target organ is a layer of hormone-producing cells in the outer region of the two adrenal glands, known as the adrenal cortex. The adrenal cortex produces a group of steroid hormones known as the glucocorticoids (GLEW-coe-COR-teh-KOIDS). Their main function is to increase blood glucose levels, thus helping maintain homeostasis.

As in other anterior pituitary hormones, ACTH secretion is controlled by the hypothalamus via a releasing hormone, ACTH-RH, as shown in FIGURE 13-6. ACTH-RH secretion is also controlled by stress. When you are under stress, your hypothal-amus increases ACTH-RH secretion. This increases ACTH release by the anterior pituitary that stimulates an increase in the release of gluco-corticoids by the adrenal cortex. Glucocorticoids increase blood glucose levels, which ensures that we have the additional energy we need to operate body systems, especially muscles, when we are under stress.

FIGURE 13-4 Disorders of Growth Hormone Secretion (a) Pituitary dwarf. (b) Pituitary giant.

FIGURE 13-5 Negative Feedback Control of TSH Secretion Thyroxine levels are detected by receptors in the hypothalamus. When levels are low, the hypothalamus releases TSH-RH, which stimulates the pituitary to release TSH. Other factors such as stress and cold influence the release of TSH via the hypothalamus. (1 denotes stimulation; 2 denotes inhibition.)

The Gonadotropins. Reproduction in both males and females is primarily under the control of the anterior pituitary. It produces two hormones that affect the gonads, appropriately known as gonadotropins (goe-NAD-oe-TROE-pinz).

Prolactin. Milk production in women is stimulated by the hormone prolactin (proe-LACK-tin), also from the anterior pituitary. Prolactin secretion, shown in FIGURE 13-7, is stimulated by suckling. During suckling, nerve impulses travel from the breast to the hypothalamus. Here, they stimulate the release of prolactin releasing hormone. It travels to the anterior pituitary in the blood stream, where it stimulates the secretion of prolactin.

Prolactin production continues as long as suckling continues. As babies begin to eat solid food, however, reduced suckling shuts down prolactin secretion and the breasts cut back on milk production.

FIGURE 13-6 Feedback Control of ACTH Cortisol regulates hypothalamic and pituitary activity, but stress and the biological clock also influence the release of ACTH-RH.

FIGURE 13-7 Neuroendocrine Reflex and Prolactin Secretion Suckling stimulates prolactin release by the anterior pituitary. Prolactin stimulates milk production by the breast. Milk release requires another hormone, oxytocin, from the posterior pituitary.

The Posterior Pituitary

The posterior pituitary produces two hormones: antidiuretic hormone (an-tie-DIE-yur-ET-ick) (ADH) and oxytocin (OX-ee-TOE-sin). As shown in FIGURE 13-8, ADH and oxytocin are not produced in the posterior pituitary. They are produced by the cell bodies of the neurosecretory cells in the hypothalamus. These hormones then travel down the axons of the neurosecretory nerves into the posterior pituitary. ADH and oxytocin are stored in the ends of the nerves until released into capillaries inside the posterior pituitary. They then flow to their target organs.

Antidiuretic Hormone. ADH regulates water balance in humans by increasing water absorption in the nephrons of the kidneys (FIGURE 13-9). That is, it stimulates the movement of water from the nephrons into the bloodstream, thus helping us conserve water.

Oxytocin. Prolactin from the anterior pituitary stimulates milk production in the breasts. But it’s oxytocin from the posterior pituitary that actually “pumps” the milk from the glands. It does this by stimulating the contraction of the smooth-musclelike cells that surround the milk-producing glands in the breast.

Like prolactin, oxytocin release is stimulated by suckling. Sensory fibers in the breast conduct nerve impulses to the hypothalamus. There they trigger the release of oxytocin. It then travels in the blood to the breast, where it stimulates the ejection of milk soon after suckling begins.

Oxytocin is also released during birth. It travels in the blood to the uterus, where it stimulates smooth muscle contraction, aiding in the expulsion of the baby.

FIGURE 13-8 The Posterior Pituitary Neurosecretory neurons that produce oxytocin and ADH originate in the hypothalamus and terminate in the posterior pituitary. Hormones are produced in the cell bodies of the neurons and are stored and released into the blood stream in the posterior pituitary.

FIGURE 13-9 Role of ADH in Regulating Fluid Levels ADH secretion is stimulated by an increase in the blood concentration of sodium caused by dehydration. ADH increases water reabsorption in the kidney, thus eliminating the stimulus for ADH secretion.

The Thyroid Gland

The thyroid gland is a U- or H-shaped gland in the neck (FIGURE 13-10). The thyroid gland produces three hormones: (1) thyroxine or T4, (2) a chemically similar compound, called T3 for short, and (3) calcitonin. The first two are involved in controlling metabolism and heat production, as explained earlier. Calcitonin helps to regulate blood levels of calcium.

Thyroxine and T3 accelerate the rate of glucose breakdown in most body cells and therefore help to maintain glucose levels especially during times of stress. Thyroid hormones also stimulate cellular growth and development and are therefore especially important early in life. Bones and muscles are especially dependent on them during growth.

In some individuals, the thyroid glands produce insufficient amounts of T3 and T4. This condition, known as hypothyroidism, results in a dramatic decrease in the metabolic rate. People suffering from this disease feel cold much of the time and may also feel tired and worn out. Even simple mental tasks become difficult. Their heart rate may slow to 50 beats per minute. Hypothyroidism is treated by pills containing artificially produced thyroid hormone.

In adults, excess thyroid activity, known as hyperthyroidism, results in elevated metabolism. Patients suffer from excessive sweating (due to overheating). They may become thin, even if they eat a lot. The increase in thyroid hormone levels results in increased mental activity, resulting in nervousness and anxiety. People with hyper - thyroidism often find it difficult to sleep. Their heart rate may accelerate, and they may lose their sensitivity to cold.

FIGURE 13-10 The Thyroid Gland The thyroid gland is located in the neck on either side of the larynx.

Hyperthyroidism is treated with antithyroid medications, that is, drugs that block the effects of thyroid hormones. Surgery may also remove part or all of the gland if a tumor has developed and it is producing excess thyroid hormone. The most common treatment for hyperthyroidism, however, is radioactive iodine. Iodine is a key component of the hormones thyroxine and T4. Cells in the thyroid actively pump iodine from the blood into the gland to provide sufficient amounts needed to make these two hormones. Radioactive iodine administered by physicians accumulates in the hormone-producing cells in the thyroid. The radioactive iodine then begins to irradiate thyroid cells, killing them and reducing the gland’s production of thyroid hormones.

Calcitonin. Calcitonin is produced by certain cells in the thyroid gland. It lowers blood calcium levels in part by stimulating the formation of new bone (Figure 13-10). It also inhibits bone-destroying cells, the osteoclasts.

Calcitonin secretion is regulated by a negative feedback loop with calcium ions in the blood. When the calcium-ion concentration increases, calcitonin secretion increases. As calcium concentrations fall, calcitonin secretion falls.

The Parathyroid Glands

The parathyroid glands are four small nodules of tissue embedded in the back side of the thyroid gland. These glands produce a hormone known as parathyroid hormone or PTH.

Parathyroid hormone has the opposite effect of calcitonin. That is, it increases blood calcium levels by stimulating bone destruction via osteoclasts and also by increasing the amount absorbed by the small intestine.

Calcium levels in the blood are influenced by a number of other factors, including vitamin D intake from certain foods (cheese and tofu) and milk. Vitamin D is also produced in the skin when it is exposed to sunlight. Vitamin D increases calcium in the blood by increasing absorption in the small intestine. It also increases the responsiveness of bone to parathyroid hormone.

As in other glands, the parathyroid glands may malfunction. Excess secretion of parathyroid hormone is the most common condition. It results from a tumor in the parathyroid gland that causes the secretion of excess PTH. Excess PTH, in turn, results in a loss of calcium from the bones and teeth. Because bones contain enormous amounts of calcium, most symptoms do not appear until 2–3 years after the onset of the disease. Therefore, by the time the disease is discovered, kidney stones may already have formed from calcium, cholesterol, and other substances. Bones may have become more fragile and susceptible to breakage. To prevent further complications, parathyroid tumors must be removed.

The Pancreas

Insulin is a hormone produced by the pancreas, an organ that also produces digestive enzymes. Insulin has many functions but one of the most important is that it stimulates the uptake of glucose by body cells. It also stimulates the synthesis of glycogen in liver and muscle cells. Glycogen is a molecule made up of many glucose molecules. It therefore serves as a way to store glucose for use between meals. (Glucose is released from glycogen stores in the muscle and liver between meals to provide energy we need!)

Insulin also stimulates fat storage and stimulates lipid synthesis, creating storage depots we can use between meals.

The pancreas also produces a hormone known as glucagon. Glucagon has the opposite effect of insulin. That is, it increases blood levels of glucose by stimulating the breakdown of glycogen in liver cells (FIGURE 13-11). In other words, insulin stores hormones and is released when we eat; glucagon releases glucose from storage depots and is released between meals so we can tap into stored glucose.

Glucagon secretion, like that of insulin, is regulated by a negative feedback mechanism. It is controlled by glucose concentrations in the blood. When glucose levels fall, the pancreas releases glucagon into the blood stream. It stimulates the breakdown of glycogen, which releases glucose into the blood stream. When glucose levels rise, glucagon secretion declines.

Diabetes Mellitus. Most readers have heard about a disease called diabetes or diabetes mellitus. In actuality, there are two types of diabetes. Type I diabetes begins early in life and is also called early-onset diabetes. This disease is believed to be caused by damage to the insulin-producing cells of the pancreas. In some individuals diabetes is the result of an autoimmune reaction. Others may develop it as a result of a viral infection or an environmental pollutant that damages the cells that produce insulin. Insulin production varies in patients with Type I diabetes. In some, it is only slightly reduced; in others, it is completely suppressed.

FIGURE 13-11 The Role of the Liver and Pancreas in Controlling Blood Glucose Levels Glucagon and insulin are antagonistic hormones that regulate blood glucose levels through different mechanisms.

Type II diabetes usually occurs in people over 40 and is also called late-onset diabetes. In this disease, the cells of the pancreas produce normal or above-normal levels of insulin. However, the target cells of the body are unresponsive to the hormone.

Type II diabetes is caused by obesity, a problem growing rapidly in the United States where people consume high-fat, high carbohydrate, and otherwise high-calorie diets. Because obesity in adults and children is on the rise, Type II diabetes is also increasing—and increasing very rapidly.

Although Type I and Type II diabetes have different causes, both forms of this disease exhibit similar symptoms. Excess urination and thirst are generally the first signs of trouble. Patients often feel tired, weak, and apathetic. Weight loss and blurred vision are also common. Excess glucose in the urine may result in frequent bacterial infections in the bladder.

Early-onset diabetes (Type I) is treated with insulin injections. Patients give themselves regular injections of insulin—usually two to three times per day. Patients are also required to eat meals and snacks at regular intervals to maintain constant glucose levels in the blood and to ensure that regular insulin injections always act on approximately the same amount of blood glucose.

To mimic the body’s natural release, biomedical researchers have developed a device called an insulin pump. This device is worn by the patient 24 hours a day. It delivers predetermined amounts of insulin after each meal. Patients can also manually adjust the amount of insulin to accommodate snacks or extra heavy meals. Researchers are also experimenting with ways to transplant healthy insulin-producing cells in the pancreases of diabetics in hopes of curing the disease.

Type 2 diabetes is the most common form of diabetes. It affects 90-95% of the 21 million people in the United States with diabetes. Although insulin injections help in treating Type I diabetes, they’re useless in treating Type II diabetes—also known as non-insulin dependent diabetes. Rather, Type II diabetes is treated by weight loss, proper diet, and exercise. Physicians recommend reductions in carbohydrate intake and instruct their patients to eat small meals at regular intervals during the day. Candy, sugar, cakes, and pies are off-limits.

Although treatments for both kinds of diabetes have improved people’s lives, the health effects, even with treatment, can be serious. Type I diabetics, for example, may suffer from diabetic comas, or unconsciousness. This occurs when an individual injects an insufficient amount of insulin or if he skips an insulin injection or two. Without insulin, the body cells become starved for glucose (even though blood levels are high) and begin breaking down fat. Excessive fat catabolism releases toxic chemicals (called ketones) that cause the patient to lose consciousness.

Diabetics may also suffer from insulin shock caused by an overdose of insulin. This reduces blood glucose levels, creating hypoglycemia. In mild cases, symptoms include tremor, fatigue, sleepiness, and the inability to concentrate. These symptoms result from a lack of glucose in the brain. In severe cases, unconsciousness and death may occur.

Over the long term, patients with both forms of diabetes often experience additional serious health effects. For example, some individuals suffer from loss of vision, nerve damage, and kidney failure. These symptoms appear 20–30 years after the onset of the disease, even if they are being treated. Loss of vision occurs because the blood vessels in the retina of the eye rupture or hemorrhage. Blood vessels become damaged when blood glucose levels rise in diabetics. Elevated levels of glucose also damage nerve cells.

Damage to the blood vessels can cause gangrene (GAN-green), a condition in which tissues die because their blood supply is interrupted. In diabetics, gangrene is caused by a restriction of blood circulation caused by damage to blood vessels. Gangrene most commonly occurs in the extremities, that is, the toes, fingers, arms, and legs; however, internal organs and muscles may also become gangrenous. Diabetics may require amputation of limbs, especially the lower extremities.

The Adrenal Glands

Atop the kidneys are two endocrine organs: the adrenal glands. The adrenal glands (ah-DREE-nal), shown in FIGURE 13-12, consists of two zones. The central region, or adrenal medulla, produces the hormones that increase the heart rate and accelerate breathing when a person is excited or frightened. The outer zone, the adrenal cortex, produces a number of steroid hormones, discussed shortly.

Hormones of the Adrenal Medulla. The adrenal medulla produces two hormones: adrenalin (epi-nephrine) and noradrenalin (norepinephrine). In humans, about 80% of the adrenal medulla’s output is adrenalin. Helping us meet the stresses of life, adrenalin and noradrenalin are instrumental in the fight-or-flight response—the physiological reactions that take place when an animal is threatened. These hormones enhance our ability to either fight or flee.

Adrenalin and noradrenalin are secreted under stress—for example, when a careless driver cuts in front of you in traffic or, perhaps, as you wait outside a lecture hall to take an exam. Nerve impulses traveling from the brain to the adrenal medulla trigger the release of adrenalin and noradrenalin.

FIGURE 13-12 Adrenal Glands The adrenal glands sit atop the kidney and consist of an outer zone of cells, the adrenal cortex, which produces a variety of steroid hormones, and an inner zone, the adrenal medulla. The adrenal medulla produces adrenalin and noradrenalin.

Adrenalin and noradrenalin elevate blood glucose levels, making more energy available to cells, particularly skeletal muscle cells. They also increase breathing rate and heart rate. In addition, these hormones cause tiny air-carrying tubules in the lungs to expand (dilate). This permits greater movement of air in and out of the lungs, increasing oxygen levels in the blood required during times of stress or when flight or fight is required. Furthermore, these hormones cause blood vessels in the intestinal tract to constrict, putting digestion on temporary hold. This diverts blood to the dilated blood vessels in skeletal muscles increasing flow through them. Mental alertness increases as a result of increased blood flow and hormonal stimulation. You are ready to fight or flee.

Hormones of the Adrenal Cortex. The adrenal cortex produces two main types of hormones, each of which has a different function. The first group, the glucocorticoids, helps to maintain blood glucose levels. Several chemically distinct glucocorticoids are secreted, the most important being cortisol.

The second group is the mineralocorticoids (MI-ner-al-oe-COR-teh-KOIDS). They regulate the concentration of ions in the blood and tissue fluids. The mineralocorticoids are involved in maintaining the proper concentration of certain ions, notably potassium and sodium. The most important mineralocorticoid is aldosterone.

Diseases of the Adrenal Glands. Like other endocrine glands, the adrenal can malfunction. One of the most common disorders is Addison’s disease. Most cases of Addison’s disease are thought to be autoimmune reactions in which cells of the adrenal cortex are destroyed by the immune system.

Addison’s disease results in a decrease in the production and release of hormones from the adrenal cortex. The absence of cortisol upsets the body’s homeostatic mechanism for controlling glucose. The lack of aldosterone results in low levels of sodium and low blood pressure. Symptoms include loss of appetite, weight loss, fatigue, and weakness.

Addison’s disease can be treated with steroid tablets that replace the missing hormones. Treatment allows patients to lead fairly normal, healthy lives.

Like the nervous system, the endocrine system plays an extremely important role in maintaining the many and diverse functions of the body—functions required for us to live well, reproduce, and survive trying times. Working in concert with the nervous system, the endocrine system helps ensure our day-to-day survival as well as our long-term health.

Most body systems you’ve studied so far function to keep us alive and well. The reproductive system is unlike them. That is, it is not necessary for our health and survival. Rather, it functions to keep our species alive. In this module, we’ll begin our exploration of human reproduction, starting with the male reproductive system.

Overview of the Male Reproductive System

The male reproductive system consists of a number of organs. The penis is the organ of copulation. Below it are the testes. They are suspended in a sac of skin called the scrotum. The testes (TESS-teaz) or testicles produce sperm and male hormones. The male reproductive system also consists of several additional organs that produce part of the semen and a duct system that transports the sperm into the vagina during sexual intercourse.

The Testes

Although the testes reside in the scrotum, they are formed inside the body cavity during fetal development. After formation, the testes descend into the scrotum. The scrotum provides just the right temperature to permit normal sperm production. In a very small percentage of boys, the testes fail to descend. Drugs may be given to stimulate the testes to descend. If drug treatment is unsuccessful, surgery is required. If the condition is not corrected, sterility may result because the temperature inside the body cavity is too high for sperm development.

As FIGURE 14-1 shows, each testis is surrounded by a dense layer of connective tissue. It contains numerous pain fibers, a fact to which most men will attest. Each testis contains numerous tubules, in which sperm are formed. They are called the seminiferous tubules (SE-meh-NI-fe-russ).

FIGURE 14-1 Interior View of the Testis Sperm are produced in the seminiferous tubules and stored in the epididymal duct.

Sperm produced in the seminiferous tubules empty into a network of connecting tubules in the “back” of the testes. These tubules, in turn, empty into a duct where they are stored until released during ejaculation.

During ejaculation, the sperm from each testis enter much larger, muscular ducts known as the vas deferentia (DE-fe-ren-she-ah). These ducts pass from the scrotum into the body cavity through two canals in the body wall, known as the inguinal canals (FIGURE 14-2a). As shown in FIGURE 14-3, each vas deferens empties into the urethra. It carries the sperm through the penis to the outside during ejaculation.

FIGURE 14-2 The Inguinal Canal and Hernia (a) During development, the testis descends through the inguinal canal, an opening through the musculature in the lower abdominal wall. (b) Loops of intestine may push through the inguinal canal if the muscles surrounding it are weak.

The inguinal canals are potential weak spots in the lower abdominal wall. In some men, loops of the small intestine may enter the weak-walled inguinal canals (FIGURE 14-2b). This condition is known as an inguinal hernia or simply, a hernia, and can be corrected surgically. Weaknesses in the canal can be detected by a doctor who during a physical exam places a finger over the inguinal canal, then asks a man to cough. If the canal is weak, coughing will push the intestines into the opening.

During ejaculation, sperm are joined by fluids produced by several additional glands, known as the sex accessory glands. They are located around the neck of the urinary bladder (Figure 14-3). The sex accessory glands produce fluid that makes up 99% of the volume of the ejaculate, or semen (SEE-men). The remaining 1% consists of sperm produced in the testes.

One of the most prominent sex accessory glands is the prostate gland (PROS-tate). The prostate surrounds the neck of the urinary bladder and empties its contents directly into the urethra during ejaculation. Routine medical examinations of men over the age of 45 show that nearly all have enlarged prostates. This condition results from the formation of small nodules inside the gland. Although they usually cause no trouble, in some cases the nodules grow quite large and can block the flow of urine, making urination painful.

The prostate is also a common site for cancer in men over 40 and should be checked every year by a physician once men reach that age. Prostate cancer typically remains undetected until the tumor begins to press on the urethra, making urination difficult and painful. Prostate cancers are detected by digital rectal exams during which doctors feel the prostate. A blood test is also used to determine cancer.

If cancer is confined to the gland, it can be removed surgically. Radiation can also be used to treat prostate cancer. If the cancer has spread, however, it may be too late. Drugs that inhibit testosterone’s secretion or its actions can be given to slow the growth of the tumors. The testes may also have to be removed to reduce testosterone secretion.

On average, men produce 200–300 million sperm every day. The average ejaculate contains 240 million or more. In humans, sperm are formed during a special type of cell division known as meiosis. When formed, each sperm contains 23 single-stranded chromosomes—half the number in a normal body cell. Thus, when the sperm unites with an egg (also containing 23 single-stranded chromosomes), they produce a cell that contains 46 single-stranded chromosomes. One-half of its chromosomes come from each parent.

FIGURE 14-3 Anatomy of the Male Reproductive System

The testes also produce male sex hormones. These hormones are produced by cells that lie between the seminiferous tubules. These cells produce a group of sex steroid hormones known as androgens, so named because they have a masculinizing effect. The most important androgen is testosterone.

Testosterone has many functions. It stimulates the formation of sperm. In the absence of testosterone, sperm cell production declines, then stops, and the walls of the seminiferous tubules shrink.

Testosterone also stimulates the growth of bone and muscle and explains in large part why men are generally taller and more muscular than women. In addition, testosterone promotes facial hair growth and thickening of the vocal cords, giving men deeper voices than women.

In addition, testosterone affects the hair follicles on the heads of many men, causing baldness. It is not the absence of testosterone, as some believe, but the presence of testosterone and certain genes that lead to this condition, which known as pattern baldness (FIGURE 14-4).

Testosterone also stimulates the oil glands of the skin in both sexes (FIGURE 14-5a). During puberty (sexual maturation) in boys, testosterone levels increase dramatically. This causes a sharp increase in oil gland activity. If the skin is not kept clean and well scrubbed, dead skin cells may block the openings of the oil glands on the skin’s surface (FIGURE 14-5b). As a result, oil builds up inside the glands. Bacteria on the skin often invade and proliferate in the small pools of oil. This causes inflammation, pus formation, and swelling. The result is an acne pimple.

Mild acne can be treated by washing the skin twice a day with warm water and a mild, unscented soap. Be sure to wash gently. You should also apply a cream containing benzoyl peroxide daily to affected areas. Benzoyl peroxide is an antibacterial agent and a peeling agent, that is, it increases skin turnover. This helps clear pores, which reduces the bacterial count. Women with acne should avoid makeup that has an oil base or use a non-oily type of foundation and wash their faces thoroughly each night. Sunlight also helps clear up acne, because it dries the oil on the skin and the ultraviolet light kills bacteria. In addition, doctors may prescribe antibiotic creams. Ideally, pimples should not be picked at either, as this may worsen acne and cause scarring.

FIGURE 14-4 Male Pattern Baldness

FIGURE 14-5 Formation of an Acne Pimple (a) Testosterone stimulates oil production in the sebaceous (oil) glands. (b) If the outlet is blocked, sebum builds up in the gland and (c) the gland may become infected.

Moderate acne can be successfully treated with antibiotics. Severe acne can be treated by special ointments and antibiotics given by a skin doctor or dermatologist. One relatively new and fairly successful drug is Retin-A, a derivative of vitamin A.

The Penis

Sperm are deposited in the female reproductive tract via the penis. The penis consists of a shaft of varying length and an enlarged tip (FIGURE 14-6). The tip is covered by a sheath of skin at birth, the foreskin. The foreskin gradually becomes separated from the glands in the first two years of life. At puberty, the inner lining of the foreskin begins to produce an oily secretion. Bacteria can grow in the protected, nutrient-rich environment created by the foreskin, so special precautions must be taken to keep the area clean.

Because of potential health problems or religious reasons, many parents opt to have the foreskin removed in the first few days of their son’s life. The operation, called circumcision (sir-come-SI-zhen; literally, “to cut around”), may help reduce penile cancer in men and may also reduce cervical cancer in the wives or sexual partners of circumcised men.

FIGURE 14-6 Anatomy of the Penis The penis consists principally of spongy tissue that fills with blood during sexual arousal. The urethra passes through the penis, carrying urine or semen.

For successful copulation, the penis must become rigid, or erect. During sexual arousal, nerve impulses cause arterioles in the penis to dilate. Blood flows into a spongy erectile tissue (eh-REK-tile) in the shaft of the penis, making it harden. Swelling compresses a large vein on the dorsal surface of the penis, blocking the outflow of blood and further stiffening the organ.

In the center of the penis is the urethra, a duct that carries urine from the bladder to the outside of the body during urination. The urethra also transports semen—sperm and secretions of the sex accessory glands—during ejaculation.

Some men lose their ability to achieve or sustain an erection. This condition is known as impotence (IM-poe-tents). Most men experience impotence at some time in their life, but it is usually temporary. Persistent impotence, however, is a more serious condition, and is more common in middle-age and elderly men.

Persistent impotence may be caused by a number of factors. Marital conflict, stress, fatigue, and anxiety, for example, all contribute to impotence. Nerve damage may also cause impotence. Alcohol ingestion and some medications may contribute to the condition as well.

Smoking is also a major contributor to impotence. In fact, smokers are twice as likely to be impotent as nonsmokers. That’s because smoking causes arterioles in the penis to constrict, reducing blood flow. Smoking also causes the buildup of plaque, which permanently blocks blood flow to this organ.

If impotence is caused by psychological factors, such as marital conflict and resulting stress, therapy is often advised. Rest and relaxation help. If the problem is a result of plaque buildup in the arteries supplying the penis, resulting from high cholesterol levels in the blood, surgery on the arteries can be helpful. If the problem results from a medication one is taking, a change of prescription can help. Patients with nerve damage, however, are likely to suffer permanent impotence.

Nerve damage may result from diabetes or from an accident. For patients with irreversible impotence, urologists can surgically insert an inflatable plastic implant in the penis. The implant is attached to a small, fluid-filled reservoir in the scrotum. The fluid is manually pumped into the implant, making the penis erect upon demand, thus permitting sexual intercourse. Other types of implants are also available. Vacuum pumps can be used, too. These devices use a plastic cylinder that is fitted over the penis. A handheld pump is used to create a vacuum. Blood rushes into the penis, causing an erection. After the vacuum aspirator is removed, special rubber bands are placed around the base of the penis to maintain an erection.

A number of drugs are also available. Viagra and Cialis, for example, promote erection when men are sexually aroused. Viagra works by causing the muscle in the walls of the arteries supplying the penis to relax. This permits blood to flow into the organ more readily when stimulated.

Ejaculation

Ejaculation is a nervous system reflex. Sexual stimulation results in nerve impulses that travel in sensory nerves supplying the penis to the spinal cord. When stimulation becomes intense, these impulses activate neurons in the spinal cord. They send impulses to the smooth muscle in the walls of the tubes that store sperm just outside each teste and to the sex accessory glands and the vas deferens. Sperm and secretions of the sex accessory glands are released and propelled to the urethra. Semen is then propelled onward by smooth muscle contractions in the walls of the urethra and is released in spurts.

Control of the Male Reproductive System

As noted earlier, the testes produce sex steroid hormones—mainly, testosterone. Testosterone production and release, however, are controlled by a hormone from the pituitary gland. This hormone is known as luteinizing hormone (LH). In males, LH is also known as interstitial cell-stimulating hormone (ICSH). It gets this name because it stimulates the testosterone-producing cells (known as interstitial cells).

ICSH secretion is controlled by a releasing hormone produced by the hypothalamus known as gonadotropin releasing hormone (GnRH). As FIGURE 14-7 shows, the secretion of GnRH is controlled by testosterone levels in the blood in a negative feedback loop. When testosterone levels in the blood decline, receptors in the hypothalamus detect the change and signal an increase in GnRH secretion.

FIGURE 14-7 Hormonal Control of Testicular Function Testosterone, FSH, and ICSH participate in a negative feedback loop. The testes also produce a substance called inhibin, which controls GnRH secretion.

The pituitary also produces the gonadotropin follicle-stimulating hormone or FSH. Like testosterone, FSH stimulates sperm formation (Figure 14-7).

Human reproduction involves many processes finely controlled by the nervous system and endocrine system. Although the reproductive system is not essential to our own lives, it does provide us with enjoyment and the means to propagate our species.

he female reproductive system like the male reproductive system is not vital to day-to-day survival, but plays an obvious role in propagating our species. The female reproductive system consists of external parts, known as the external genitalia, and several internal organs. We’ll start with the internal structures: (1) the uterus, (2) the uterine tubes, (3) the ovaries, and (4) the vagina.

The Uterus and Uterine Tubes

The uterus or womb is a pear-shaped organ in the pelvic cavity. It houses and nourishes the developing fetus. The uterus is about 7 centimeters (3 inches) long and about 2 centimeters wide (less than 1 inch) at its widest point in nonpregnant women. The wall of the uterus contains a thick layer of smooth muscle.

Attached to the uterus are two hollow, muscular tubes, known as the uterine tubes (YOU-ter-in). In humans, the uterine tubes are often referred to as the Fallopian tubes (fal-OH-pee-an). As FIGURE 15-1a shows, the ends of the uterine tubes are widened like a catcher’s mitt and fit loosely over the ovaries (OH-var-ees), two almond-shaped organs that also lie in the pelvic cavity.

Fertilization—the union of the sperm and egg—occurs in the upper third of the uterine tubes. The fertilized egg is then transported down the uterine tubes to the uterus. Inside the uterus, it attaches to the lining, known as the endometrium (EN-doe-MEE-tree-um). The developing embryo then embeds itself in the lining and remains here for the duration of pregnancy.

At birth, the baby is expelled from the uterus through the cervix (SIR-vix), the lowermost portion of the uterus. As FIGURE 15-1b shows, the cervix protrudes into the vagina (vah-GINE-ah). The canal running through the cervix is quite narrow, so the cervix must stretch considerably at birth to allow passage of the baby.

The vagina or birth canal is a 7-centimeter (3-inch), tubular organ that leads to the outside of the body. The vagina serves as the receptacle for sperm during sexual intercourse. To reach the ovum, sperm must travel through a tiny opening in the cervix and then move through a narrow canal in the center of the organ that leads to the uterus. From here, sperm move upwards into the uterine tubes.

FIGURE 15-1 Female Reproductive System (a) Frontal view. (b) Midsagittal view.

The external genitalia are the externally visible parts of the female reproductive system. They consist of two flaps of skin on either side of the vaginal opening (Figure 15-1). The outer folds are the labia majora (LAY-bee-ah ma-JOR-ah). These large folds of skin are covered with hair. The inner flaps are the labia minora (meh-NOR-ah). They meet to form a hood over a small knot of tissue called the clitoris (CLIT-er-iss). The clitoris is a highly sensitive organ involved in female sexual arousal. It is formed from the same embryonic tissue as the penis. The various components of the female reproductive system, and their functions, are listed in TABLE 15-1.

TABLE 15-1

The Female Reproductive System

Component Function

Ovaries Produce ova and female sex steroids

Uterine tubes Transport sperm to ova; transport fertilized ova to uterus

Uterus Nourishes and protects embryo and fetus

Vagina Site of sperm deposition, birth canal

The Ovaries

During each menstrual cycle, one ovary releases an egg or ovum, the female gamete. This process is called ovulation (O-vue-LAY-shun). The ovum or egg (as it is commonly called) is drawn into the uterine tube.

The release of an egg occurs approximately once a month in women during their reproductive years—from puberty (age 11–15) to menopause (age 45-55)—although ovulation does not occur when a woman is pregnant.

The structure of an ovary is shown in FIGURE 15-2. Eggs are surrounded by one or more layers of cells derived from the loose connective tissue of the ovary. Together, the eggs and surrounding cells form follicles.

FIGURE 15-2 Structure of the Ovary (a) This drawing illustrates follicle development and also shows the formation and destruction of the corpus luteum (CL). (b) A recently ovulated egg.

During each cycle about a dozen follicles begin to enlarge, as shown in Figure 15-2. As they get bigger, a clear liquid begins to accumulate between the follicle cells. The fluid creates small spaces among the follicle cells, which enlarge as additional fluid is generated. Eventually, the cavities join, forming one large cavity.

Although a dozen or so follicles begin developing during each cycle, as a rule only one ovulates. The rest stop growing and degenerate. The follicle that survives continues to enlarge by accumulating more fluid. As the fluid builds up, the follicle begins to bulge from the surface of the ovary, not unlike a pimple. This weakens the wall of the ovary. Eventually, the wall of the follicle and ovary break down, and the egg is released. The follicle collapses.

The collapsed follicle does not degenerate. Rather, it forms a structure known as the corpus luteum (CORE-puss LEU-tee-um; “yellow body”) or CL for short—so named because of the yellow pigment it contains in cows and pigs (Figure 15-2).

The CL is a temporary endocrine gland. It produces two sex hormones: estrogen and progesterone. If the egg is fertilized, the CL remains active for several months, producing hormones needed to maintain early pregnancy. If fertilization does not occur, the CL soon disappears.

The Menstrual Cycle

Women of reproductive age undergo a series of changes called the menstrual cycle (MEN-strel). This cycle lasts from 25–35 days, although the length of the cycle may also vary from month to month in the same woman. On average, however, the cycle repeats itself every 28 days. Ovulation occurs approximately at the midpoint of the cycle.

The menstrual cycle involves changes in the ovary and the uterus. These are brought about by changes in hormones. As shown in FIGURE 15-3, estrogen concentrations in the blood increase during the first half of the menstrual cycle. Estrogen production is stimulated by luteinizing hormone or LH, a hormone from the pituitary.

Estrogen causes the lining of the uterus to increase in size, in preparation for a possible pregnancy. Follicle growth is also stimulated by a pituitary hormone, FSH, short for follicle-stimulating hormone. My research as a graduate student showed that estrogen also stimulates follicles to grow by stimulating cell division of the follicle cells.

In the middle of each cycle, LH and FSH secretion peak. This surge in hormones causes ovulation.

During the second half of the menstrual cycle, the collapsed follicle forms the CL, which produces estrogen and progesterone, as just noted. These hormones stimulate the uterine lining, causing it to thicken in preparation for possible pregnancy. During this time, glands in the uterine lining swell with a glycogen-rich secretion used to nourish the early embryo.

If fertilization does not occur, the uterine lining starts to shrink approximately 4 days before the end of the menstrual cycle. It then begins to be shed, starting an event called menstruation. The shedding of the uterine lining (menstruation) is triggered by a decline in estrogen and progesterone concentrations in the blood.

When progesterone levels fall, the uterus begins to undergo contractions. These contractions push the detached lining and blood from broken blood vessels out of the uterus into the vagina, resulting in vaginal bleeding. These contractions cause the cramps that many women experience during menstruation.

If fertilization occurs, the newly formed embryo produces an LH-like hormone known as HCG (human chorionic gonadotropin). HCG stimulates the corpus luteum, maintaining its structure and function. When HCG is present, estrogen and progesterone continue to be secreted from the CL. The uterine lining remains intact. When the newly formed embryo arrives in the uterus, it attaches to the thickened lining. It then embeds in the lining from which it derives its nutrients. If the newly forming embryo successfully embeds in the uterine lining, HCG will maintain the CL for approximately 6 months.

HCG shows up in detectable levels in a woman’s blood and urine about 10 days after fertilization. Pregnancy tests available through a doctor’s office or drugstore detect HCG in a woman’s urine, and thus allow a woman to determine if she is pregnant. The tests use a commercially prepared antibody to HCG, which binds to the hormone. The home pregnancy tests are relatively inexpensive, fairly reliable, and fast.

FIGURE 15-3 The Menstrual Cycle (a) Hormonal cycles. (b) The ovarian cycle. (c) The uterine cycle.

Estrogen and Progesterone

Like testosterone in boys, estrogen secretion in girls increases dramatically at puberty. As the levels of estrogen in the blood increase, the hormone begins to stimulate follicle development in the ovaries. Estrogen also stimulates the growth of the external genitalia and the breasts as well as internal structures: the uterus, uterine tubes, and vagina.

Estrogen’s influence extends far beyond the reproductive system. For example, estrogen promotes rapid bone growth in the early teens. Because estrogen secretion in girls usually occurs earlier than testosterone secretion in boys, girls experience a growth spurt before similarly aged boys. However, estrogen also stimulates the closure of the growth zones in the long bones of the body. This ends the female growth spurt well before male bone growth period is over. Thus, most girls reach their full adult height by the age of 15–17. Boys experience their most rapid growth later in adolescence and continue growing until the age of 19–21. Besides stimulating bone growth, estrogen stimulates the deposition of fat in women in their hips, buttocks, and breasts, giving the female body its characteristic shape.

Premenstrual Syndrome

Many women (4 out of 10) become irritable, depressed, and suffer from headaches and fatigue just before menstruation—the shedding of the uterine lining that occurs if pregnancy does not occur. Many women also complain of bloating, tension, joint pain, and swelling and tenderness of the breasts. These complaints are symptoms of a condition known as PMS, or premenstrual syndrome.

Physicians recommend that women suffering from PMS see their family doctor to be certain that the symptoms are not caused by some other medical condition. To treat PMS, doctors often recommend relaxation and avoidance of stress. Light exercise may help relieve symptoms, as may warm baths. More frequent light meals with plenty of carbohydrates and fiber may help, too. Reductions in salt intake and avoiding excess chocolate consumption are also recommended. Cutting out caffeine drinks and taking vitamin B6 supplements are also generally advised.

Menopause

The menstrual cycle continues throughout the reproductive years. However, when a woman reaches 20, her ovaries begin to become less responsive to FSH and LH. As a result, estrogen levels gradually decline. Ovulation and menstruation eventually stop, resulting in menopause— literally a cessation of the menses or menstruation.

Menopause generally occurs between the ages of 45 and 55, but can occur earlier. The decline in estrogen secretion causes the breasts and reproductive organs such as the uterus to begin to shrink. Vaginal secretions often decline, so sexual intercourse may become painful in some women without artificial lubrication.

The decline in estrogen levels also often results in behavioral changes. Many women, for instance, become more irritable and suffer periods of depression as menopause begins. Many women experience “hot flashes” and “night sweats,” symptoms caused by extensive dilation of vessels in the skin. Fortunately, these symptoms usually pass.

Declining estrogen levels also reduce calcium levels in bones, resulting in a condition known as osteoporosis. To counter osteoporosis, physicians often prescribe small amounts of estrogen or, more commonly a combination of estrogen and progesterone. This treatment is called hormone replacement therapy. Doctors also recommend exercise and a diet rich in calcium and vitamin D. Most women are treated with estrogen for 5–10 years after the onset of menopause, but no longer, for long-term treatment may increase the risk of breast cancer.

Birth Control

Birth control is any method or device that prevents births. Birth control measures fit into two broad categories: (1) contraception, ways of preventing pregnancy, and (2) induced abortion, the deliberate expulsion of a fetus.

FIGURE 15-4 summarizes the effectiveness of the most common means of contraception. A 95% effectiveness rating means that 95 women out of 100 using a method in a year will not become pregnant.

FIGURE 15-4 Effectiveness of Contraceptive Measures Percent effectiveness is a measure of the number of women in a group of 100 who will not become pregnant in a year.

Not listed in the figure is a form of birth control known as abstinence, that is, refraining from sexual intercourse. This method is appropriate for many people as a means of reducing unwanted pregnancy and has the added benefit of preventing the spread of AIDS and other sexually transmitted diseases (discussed later).

Sterilization is one of the most effective birth control measures. In women, sterilization is performed by cutting and tying off the uterine tubes. This procedure is known as tubal ligation (TOO-bal lie-GAY-shun) (FIGURE 15-5a). It prevents sperm from reaching the eggs.

Male sterilization, known as a vasectomy, can be carried out in a physician’s office under local anesthesia (FIGURE 15-5b). During this operation, the physician makes a small incision in the scrotum. Each vas deferens is then cut. The free ends are tied off or cauderized—that is, burned shut.

Vasectomies prevent the sperm from passing into the urethra during ejaculation. They do not decrease sex drive, and because they do not block fluids from the sex accessory glands, they have virtually no effect on ejaculation.

The birth control pill is also a highly effective means of birth control. The most commonly used birth control pill contains a mixture of estrogen and progesterone. These hormones inhibit the release of LH and FSH. This, in turn, inhibits follicle development and ovulation. Birth control pills must be taken throughout the menstrual cycle. Skipping a few days may result in ovulation and possible pregnancy.

Although effective, birth control pills have some adverse health effects. Although rare, some like heart attacks, strokes, and blood clots can be serious, even fatal. The risk of a nonsmoker dying from taking birth control pills is 1 in 63,000 in any given year. For comparison, the risk of dying in an auto accident is 1 in 6,000.

Women who take birth control pills are also more likely to develop cervical cancer than women who do not. Physicians therefore recommend annual Pap smears for women on the pill. During a Pap smear, the cervical lining is swabbed. The swab picks up cells that are examined under a microscope for signs of cancer.

Smoking increases the likelihood of side effects from birth control pills. If a woman is a smoker and takes the pill, she is four times more likely to die from a heart attack or stroke than a nonsmoker. The risk of side effects also increases with age.

Birth control pills also have beneficial effects, not the least of which is that they prevent pregnancy. National statistics show that one out of every 10,000 women who become pregnant and deliver will die from complications, usually during delivery. Thus, even with the risks associated with the pill, using this mode of contraception is six times safer than pregnancy.

Birth control pills also reduce the incidence of a number of medical disorders including ovarian cysts, breast lumps, anemia, rheumatoid arthritis, osteoporosis, and pelvic infection. They may also protect a woman from cancer of the ovary and uterus.

The next most effective means of birth control is the intrauterine device (IUD) (FIGURE 15-6). The IUD consists of a small plastic or metal object with a short string attached to it. These devices are inserted into the uterus by a doctor.

FIGURE 15-5 Sterilization Methods (a) In a tubal ligation, the uterine tubes are cut, then tied off or cauterized. (b) In a vasectomy, the vasa deferentia are cut, and then tied off.

FIGURE 15-6 The IUD IUDs come in a variety of shapes and sizes and are inserted into the uterus, where they prevent implantation. Only one type is currently legal in the United States.

Like other forms of birth control, IUDs may have adverse effects. In some cases, the uterus expels the device, leaving a woman unprotected. The IUD may also cause slight pain and increase menstrual bleeding. Although rare, IUDs can cause uterine infections and have, in rare instances, perforated the uterus, that is, penetrated the uterine wall, causing bleeding.

The next most effective means of birth control are the barrier methods—the diaphragm, condom, and vaginal sponge—all of which prevent the sperm from entering the uterus. The diaphragm (DIE-ah-FRAM) is a rubber cup that fits over the entire end of the cervix (FIGURE 15-7). To increase its effectiveness, a spermicidal (sperm-killing) jelly, foam, or cream is applied to the rim and inside surface of the cup.

Smaller versions of the diaphragm, called cervical caps, are also available. Cervical caps fit only over the end of the cervix, and are held in place by suction. When used with spermicidal jelly or cream, the caps are as effective as full-sized diaphragms.

Condoms are thin, latex rubber sheaths that fit onto the erect penis (FIGURE 15-8). Sperm released during ejaculation are trapped inside. Besides preventing fertilization, condoms also protect against sexually transmitted diseases, including AIDS, a benefit not offered by any other birth control measure except abstinence.

Yet another barrier method is the vaginal sponge (FIGURE 15-9). This small absorbent piece of foam is impregnated with spermicidal jelly. Inserted into the vagina, the sponge is positioned over the end of the cervix. The sponge is effective immediately after placement and remains effective for 24 hours.

One of the oldest, but least successful means of birth control is withdrawal, removal of the penis just prior to ejaculation. This method requires tremendous willpower and frequently fails. Males may not pull out or may pull out too late. In addition, a few drops of semen may be released before ejaculation. The sperm they contain may fertilize the egg, if present.

FIGURE 15-7 The Diaphragm Worn over the cervix, the diaphragm is coated with spermicidal jelly or cream and is an effective barrier to sperm.

FIGURE 15-8 The Condom Worn over the penis during sexual intercourse, it prevents sperm from entering the vagina.

FIGURE 15-9 The Vaginal Sponge Impregnated with a spermicidal chemical, the vaginal sponge is inserted into the vagina and is effective for up to 24 hours.

FIGURE 15-10 The Natural Method The shaded areas indicate an unsafe period for sexual intercourse, assuming ovulation occurs at the midpoint of the cycle.

As mentioned earlier, spermicidal jellies, creams, foams, and films contain chemicals that kill sperm and can be used with other methods, like diaphragms. They can also be used alone, but are only about as effective as withdrawal in such cases.

If a couple knows the exact time of ovulation, they can time sexual intercourse to prevent pregnancy more precisely. Abstaining from sexual intercourse around the time of ovulation is known as the rhythm method. Because eggs remain viable 12–24 hours after ovulation and sperm may remain alive in the female reproductive tract for up to 3 days, abstinence 4 days before and 4 days after the probable ovulation date should provide a margin of safety (FIGURE 15-10).

The time of ovulation can be determined by measuring body temperature because in most women body temperature rises slightly after ovulation. The time of ovulation can also be determined by taking samples of cervical mucus. Cervical mucus varies in consistency during the menstrual cycle. By testing its thickness on a daily basis, a woman can tell fairly accurately when she has ovulated.

Abortion

Some couples may elect to terminate pregnancy through abortion, surgical removal of the fetus early in pregnancy. Abortion is not suitable or morally acceptable to all people. Those opposed to abortion argue that it should be outlawed or severely restricted—for example, allowed only in cases of rape, incest, and threat to the life of the mother. They often advise unmarried women to abstain from sexual intercourse or, if they become pregnant, to give birth and either keep the baby or put it up for adoption.

Pro-choice advocates, on the other hand, argue that women should have the freedom to choose whether to terminate a pregnancy or have a child. Abortion, they say, reduces unwanted pregnancies and untold suffering among unwanted infants, especially in poor families, and gives women more options than motherhood. Although they often view abortion as a legitimate means of family planning, pro-choice advocates point out that it should not be practiced as a primary means of birth control. Contraception is less costly, less traumatic, and more morally acceptable.

In the first 12 weeks of pregnancy, abortions can be performed surgically in a doctor’s office via vacuum aspiration. In this procedure, the cervix is first dilated by a special instrument. Next, the contents of the uterus are drawn out via an aspirator tube—a suction device.

Vacuum aspiration is a fairly simple and relatively painless procedure. Usually no anesthesia is required. Although women bleed for a week or so after the procedure, they generally experience few complications.

Most abortions are performed by the end of the twelfth week of pregnancy. After 16 weeks, abortions are more difficult and more risky. Solutions of salt or prostaglandins, hormones that stimulate uterine contractions, are injected into the sac of fluid surrounding the fetus to induce labor. The hormone oxytocin may be administered to the woman with the same effect.

Women can also take a pill that prevents fertilized eggs from implanting in the lining of the uterus. It is called RU486, and incorrectly referred to as the abortion pill.

In the United States, there is another chemical treatment that has the same effect. Two pills are administered. This first pill restricts blood flow to the uterus. Two days after the initial pill is given, a second pill is taken. It prevents the fertilized ovum from implanting.

Sexually Transmitted Diseases

Certain bacteria and viruses can be transmitted by sexual contact. These organisms can penetrate the lining of the reproductive tracts of men and women and proliferate in the moist, warm environment of the body causing sexually transmitted diseases (STDs). Most of the infectious agents that cause STDs are spread by vaginal intercourse, but other forms of sexual contact such as anal and oral sex can also play a role in the transmission of these diseases. AIDS, for example, can be transmitted by anal sex as well as vaginal and oral sex. So can syphilis.

Although STDs pass from one person to another during sexual contact, the symptoms are not confined to the reproductive tract. AIDS, for instance, is a sexually transmitted disease that affects the immune system.

One complicating factor in controlling STDs is that some of them, like gonorrhea, initially may produce no obvious symptoms in an infected individual. In AIDS, symptoms may not appear for weeks after the initial infection. Thus, sexually active individuals can transmit the disease to many people before they know they are infected.

Gonorrhea (GON-or-REE-ah) is caused by a bacterium that commonly infects the urethra of men and the cervical canal of women. Although gonorrhea sometimes produces no symptoms, in many individuals it results in painful urination and a puslike discharge from the urethra. Because several other sexually transmitted diseases have similar symptoms, doctors must perform a blood test to determine the exact cause of these systems. Symptoms of gonorrhea usually appear about 1–14 days after sexual contact.

Gonorrhea is treated with antibiotics and clears up quickly, usually within 3–4 days, if treatment begins early. If left untreated, gonorrhea in men can spread to the prostate gland, which makes it more difficult to treat. Infections can spread to the urethra and may lead to the formation of scar tissue. This may narrow the urethra and make urination difficult. In some women, bacterial infection spreads to the uterus and uterine tubes, causing the buildup of scar tissue in these organs. When located in the uterine tubes, scar tissue may block the passage of sperm and ova, resulting in infertility. Gonorrheal infections can also spread into the abdominal cavity through the opening of the uterine tubes. If the infection enters the bloodstream in men or women, it can travel throughout the body, making it even more difficult to treat.

Syphilis is an STD caused by a bacterium that penetrates the linings of the oral cavity, vagina, and penile urethra. It may also enter through breaks in the skin. If untreated, syphilis proceeds through three stages. In stage 1, between 1 and 8 weeks after exposure, a small, painless red sore develops, usually in the genital area. Easily visible when on the penis, these sores often go unnoticed when they occur in the vagina or cervix. The sore heals in 1–5 weeks, leaving a tiny scar.

Approximately 6 weeks after the sore heals, stage 2 begins. Individuals complain of fever, headache, and loss of appetite. Lymph nodes in the neck, groin, and armpit swell as the bacteria spread throughout the body. Stage 2 lasts about 4–12 weeks.

The symptoms of syphilis disappear for several years. Then, without warning, the disease flares up again. This is stage 3. During stage 3, patients experience a loss of their sense of balance and a loss of sensation in their legs. As the disease progresses, patients experience paralysis, senility, and even insanity.

Syphilis can be treated with antibiotics, but only if the treatment begins early. In stage 3, antibiotics are useless. Tissue and organ damage is usually permanent.

One of the most common sexually transmitted diseases, affecting 3–10 million people each year, is chlamydia (clah-MI-dee-ah). Caused by a bacterium, this disease is characterized by a burning sensation during urination and a discharge from the penis and vagina. If the bacterium spreads, it can cause more severe infection and infertility. Like other STDs, however, many people experience no symptoms at all and therefore risk spreading the disease to others. Antibiotics are effective in treating this STD.

Genital herpes (HER-peez) is another very common STD. It is caused by a virus. The first sign of infection is pain, tenderness, or an itchy sensation on the penis or female external genitalia. These symptoms usually occur 6 days or so after contact with someone infected by the virus. Soon afterward, painful blisters appear on the external genitalia, thighs, buttocks, and cervix, or in the vagina (FIGURE 15-11).

FIGURE 15-11 Genital Herpes Blisters on the External Genitalia and Inner Thigh

The blisters break open and become painful ulcers that last for 1–3 weeks, then disappear. Unfortunately, the herpes virus is not rid from the body. It becomes a lifelong resident. New outbreaks can occur from time to time, especially when an individual is under stress. Recurrent outbreaks are generally not as severe as the initial one, and, in time, the outbreaks generally cease.

Unlike other STDs, herpes can be transmitted to other individuals during sexual contact only when the blisters are present or just beginning to emerge. When the virus is inactive, sexual intercourse can occur without infecting a partner. Although herpes cannot be cured, physicians can suppress outbreaks with antiviral drugs.

Herpes is not a particularly dangerous STD, except in pregnant women. These women run the risk of transferring the virus to their infants at birth. Because the virus can be fatal to newborns, women with herpes are often advised to deliver by cesarean section (an incision made just above the pubic bone) if the virus is active at the time of birth.

The most common sexually transmitted disease is nongonococcal urethritis (YUR-ee-THRIGHtiss), or NGU for short. Moreover, NGU is one of several STDs whose incidence is steadily rising in the United States. Caused by any of several different bacteria, this infection is generally less threatening than gonorrhea, syphilis, and chlamydia, although some infections can result in sterility.

Many men and women often exhibit no symptoms whatsoever and can therefore spread the disease without knowing it. Symptoms, when they exist, resemble those of gonorrhea. NGU can be treated by antibiotics.

The vast majority of Americans carry a virus known as human papillomavirus (pap-ILL-oh-mah) or HPV. Transmitted by sexual contact, this virus can cause genital warts. Genital warts are benign growths that appear on the external genitalia and around the anuses of men and women. Warts also grow inside the vaginas of women. Warts generally occur in individuals whose immune systems are suppressed, for example, after long periods of stress.

Genital warts may remain small and limited in number, or they may grow to cover large areas, creating cosmetically unsightly growths. They may cause mild irritation, and certain strains of HPV are associated with cervical cancer in women.

Genital warts can be treated with chemicals applied directly to the wart or removed surgically—although rates of recurrence are quite high. Getting rid of the virus, however, is impossible, for it resides in the body forever.

Infertility

A surprisingly large percentage (about one in six) of American couples cannot conceive. The inability to conceive (to become pregnant) is called infertility. According to statistics, in about 50% of the couples, infertility results from problems occurring in the woman. Approximately 30% of the cases are due to problems in the man alone, and about 20% are the result of problems in both partners.

If after a year of actively trying to conceive a couple remains unsuccessful, they can consult a fertility specialist who will first check obvious problems such as infrequent or poorly timed sex. If the frequency and timing of sexual intercourse is not the problem, and it rarely is, the physician tests the man’s sperm count. A low sperm count is one of the most common causes of male infertility and is relatively easy to test.

A low sperm count may result from overwork, emotional stress, and fatigue. Tobacco and alcohol consumption can also contribute. The testes are also sensitive to a wide range of environmental chemicals and drugs that reduce sperm production.

If infertility appears to be caused by a low sperm count, a couple may choose to undergo artificial insemination, using sperm from a sperm bank. These sperm are generally acquired from anonymous donors.

If sperm production and ejaculation appear normal, a physician checks the woman, starting with ovulation. If ovulation is not occurring, fertility drugs may be administered. Unfortunately, fertility drugs often result in the ovulation of many viable eggs, which, if fertilized, result in four to six babies. Most of the multiple births you hear about on the news are the result of fertility drugs.

If ovulation is occurring normally, the physician then examines the uterine tubes to see if they are obstructed. In some instances, a previous gon-orrheal or chlamydial infection that spread into the tubes resulted in scarring that obstructs the passageway. In such instances, couples may be advised to adopt a child or to try in vitro (in VEE-troe) fertilization.

During in vitro fertilization, eggs are removed from the woman and fertilized by the partner’s sperm outside her body. The fertilized ovum is then injected into the interior of the uterus of the woman. Besides being expensive and time-consuming, this procedure has a low success rate.

Female reproduction, like male reproduction, is vital to the continuation of our species. We humans have been quite successful in continuing our kind—so successful that the size of the human population now threatens our long-term future.