Human Biology: Cardiovascular System (Chapter 5)

Overview of the Cardiovascular System

The cardiovascular system is a fundamental organ system comprised of two primary components: the heart and the blood vessels. The heart serves as a muscular pump that propels blood throughout the vast network of blood vessels. This system perform several critical functions essential for survival. Primarily, it acts as a transport system, delivering vital nutrients to cells and facilitating the removal of metabolic waste products. The actual exchange of substances between the blood and the body's tissues does not occur directly but rather through an intermediary known as interstitial fluid.

Circulation within the cardiovascular system involves systemic interactions with all other organ systems through thousands of miles of blood vessels. For example, in conjunction with the respiratory system, the blood drops off carbon dioxide ($CO_2$) and picks up oxygen ($O_2$) at the lungs via gas exchange. In coordination with the digestive system, nutrients enter the bloodstream at the intestines to be transported to the body’s cells. The liver also plays a crucial role, working with the cardiovascular system to support metabolism, detoxification, and the maintenance of homeostasis. Furthermore, at the kidneys, the blood is purified, and specific levels of water and salts are retained or excreted as necessary.

The functions of the cardiovascular system can be categorized into transport, protection, and regulation. It transports oxygen ($O_2$), carbon dioxide ($CO_2$), waste products, nutrients, hormones, and cells of the immune system. Protection is provided by immune system cells, their associated antibodies, and chemical signals that defend the body against infection. Regulation occurs as the system plays a central role in maintaining homeostasis for internal conditions such as body temperature, pH balance, and levels of water and electrolytes.

Additionally, the lymphatic system works in tandem with the immune and cardiovascular systems. Its primary role in circulation is to collect excess interstitial fluid and return it to the blood. Once this fluid enters the lymphatic vessels, it is referred to as lymph.

The Three Types of Blood Vessels

There are three distinct types of blood vessels responsible for the transport of blood to and from the tissues of the body: arteries, capillaries, and veins.

Arteries are designed to carry blood away from the heart. Their walls are composed of three layers: the thin inner endothelium (epithelium), a middle layer consisting of smooth muscle and elastic tissue that allows the vessel to expand and recoil, and an outer layer made of connective tissue. Arterioles are smaller versions of arteries. Their middle layer is dominated by smooth muscle; when this muscle contracts, the vessel constricts, reducing blood flow and raising blood pressure. Conversely, when the muscle relaxes, the vessel dilates, increasing blood flow and reducing blood pressure.

Capillaries are microscopic vessels situated between arterioles and venules. Their walls are uniquely composed of only a single layer of endothelium, which facilitates the exchange of gases, nutrients, and wastes. Capillaries form networks known as capillary beds. These beds contain precapillary sphincters that control the flow of blood; if these sphincters are closed, blood bypasses the bed and flows through an arteriovenous shunt.

Veins and venules are responsible for carrying blood toward the heart. Venules are small veins that receive blood directly from capillaries. While vein and venule walls consist of the same three layers as arteries, they possess significantly less smooth muscle in the middle layer. Veins that must transport blood against the pull of gravity are equipped with valves to ensure blood flows only toward the heart. Because the walls of veins are thinner than those of arteries, they can expand to hold a larger volume of blood; at any given time, veins store approximately $70\%$ of the total blood volume. In the event of blood loss (hemorrhage), the nervous system stimulates the veins to constrict, thereby increasing the effective blood volume.

The Heart: Anatomy and Physiology

The heart is a muscular organ located between the lungs, pointing toward the left hip. It is primarily composed of the myocardium, which consists of cardiac muscle tissue. The muscle fibers within the myocardium are branched and interconnected by intercalated disks. These disks contain gap junctions, which enable the cells to contract in unison, and desmosomes, which are cell junctions that hold adjacent cells together to prevent overstretching during contraction.

The heart is encased in a protective sac called the pericardium, which secretes pericardial fluid to provide lubrication. Internally, the heart is divided into right and left sides by a wall called the septum. The heart contains four chambers: two upper atria and two lower ventricles. Blood flow within these chambers is regulated by two types of valves: atrioventricular (AV) valves and semilunar valves. The AV valves are reinforced by structures called chordae tendineae. The left AV valve is known as the bicuspid or mitral valve, while the right AV valve is the tricuspid valve. The semilunar valves include the pulmonary valve and the aortic valve.

For its own operation, the myocardium requires a dedicated blood supply provided by the coronary circulation. Coronary arteries, which are the first branches off the aorta, supply the heart muscle with blood, while coronary veins drain the blood into the right atrium. Blockages in these coronary arteries can lead to coronary artery disease and potentially a myocardial infarction (heart attack).

The Passage of Blood and the Cardiac Cycle

Blood follows a specific pathway through the heart. Deoxygenated (O2-poor, CO2-rich) blood enters the right atrium via the superior and inferior vena cavae. The right atrium contracts, pushing blood through the tricuspid valve into the right ventricle. The right ventricle then pumps blood through the pulmonary valve into the pulmonary trunk, which divides into the right and left pulmonary arteries leading to the lungs. After gas exchange in the lungs, oxygenated (O2-rich, CO2-poor) blood returns to the left atrium through the pulmonary veins. This blood flows through the bicuspid (mitral) valve into the left ventricle, which then pumps it through the aortic valve into the aorta for distribution to the rest of the body. Notably, the walls of the left ventricle are thicker than those of the right ventricle because it must generate enough pressure to pump blood throughout the entire body.

The cardiac cycle describes the sequence of events during a single heartbeat, occurring on average $70$ times per minute. It involves systole (contraction) and diastole (relaxation). First, the atria contract together, followed by the ventricles, and finally, the heart relaxes. The "lub-dub" sounds are the result of valve closures: "lub" occurs when the AV valves close, and "dub" occurs when the semilunar valves close. A swishing sound known as a murmur may occur if there is a backflow of blood due to leaky valves.

Heartbeat control is managed both internally and externally. The internal conduction system is led by the SA node (sinoatrial node) in the right atrium, which acts as the pacemaker by initiating the electrical signal for atrial contraction. This impulse travels to the AV node (atrioventricular node), then through the AV bundle and Purkinje fibers to trigger ventricular contraction. Externally, the cardiac control center in the brain and various hormones can increase or decrease the heart rate based on metabolic needs.

An electrocardiogram (ECG) records these electrical changes. It features a P wave (atrial stimulation by the SA node), a QRS complex (electrical current traveling through ventricles prior to contraction), and a T wave (ventricular recovery). Abnormalities like ventricular fibrillation—uncoordinated electrical signals—can prevent the heart from pumping, requiring defibrillation to reset the SA node.

Blood Pressure and Cardiovascular Pathways

Blood pressure is the force exerted by blood against vessel walls. It is highest in the aorta and decreases as it moves through arteries, arterioles, capillaries, venules, and veins, reaching its lowest point in the venae cavae. A pulse, usually measured at the radial or carotid artery, reflects the heart rate and averages $60–80\,\text{beats per minute}$. Blood pressure is measured using a sphygmomanometer, typically in the brachial artery.

Systolic pressure is the highest pressure reached during heart ejection, while diastolic pressure is the lowest during ventricular relaxation. Average blood pressure is $120/80\,\text{mmHg}$. Higher values are categorized as follows:

• Normal: below $120\,\text{systolic}$ and below $80\,\text{diastolic}$.

• Elevated: $120–129\,\text{systolic}$ and below $80\,\text{diastolic}$.

• Stage 1 Hypertension: $130–139\,\text{systolic}$ or $80–89\,\text{diastolic}$.

• Stage 2 Hypertension: $140\,\text{or higher systolic}$ or $90\,\text{or higher diastolic}$.

• Hypertension Crisis: $180\,\text{or higher systolic}$ and/or $120\,\text{or higher diastolic}$.

Blood flow is slowest in the capillaries to maximize the exchange of materials. In veins, where pressure is very low, return to the heart is aided by the skeletal muscle pump, the respiratory pump, and one-way valves.

The body utilizes two circuits: the pulmonary circuit (circulating blood through the lungs for gas exchange) and the systemic circuit (circulating blood to all other body tissues). A specialized systemic route is the hepatic portal system, where the hepatic portal vein carries nutrient-rich blood from the digestive tract to the liver for filtration, protein synthesis, and toxin removal before the blood returns to the inferior vena cava via the hepatic veins.

Capillary Exchange and Cardiovascular Disorders

Material exchange at the capillaries is driven by two competing forces: blood pressure, which pushes fluid out (typically at the arterial end), and osmotic pressure, which draws water in (typically at the venule end). Fluid that does not return to the capillaries is collected by the lymphatic system as lymph.

Cardiovascular Disease (CVD) is a major cause of death and includes several disorders:

• Hypertension: High blood pressure, often called the "silent killer," treated with diuretics.

• Atherosclerosis: Plaque buildup in vessels, which can lead to a thrombus (stationary clot), embolus (moving clot), or thromboembolism (lodged clot).

• Stroke (Cerebrovascular Accident): Death of brain tissue due to a blocked or burst cranial artery.

• Myocardial Infarction: Death of heart muscle due to blocked coronary arteries; often preceded by chest pain (angina pectoris).

• Aneurysm: Ballooning of a weakened blood vessel wall.

Treatments for these conditions include biotechnology drugs like tissue plasminogen activator (t-PA) to dissolve clots, aspirin for prevention, coronary bypass surgery, gene therapy using vascular endothelial growth factor (VEGF) to grow new vessels, and angioplasty with stents. Heart failure treatments range from ICDs and LVADs (Left Ventricular Assist Devices) to heart transplants or Total Artificial Hearts (TAH).