DIGESTIVE SYSTEM

The pharynx is a region that recieves air from ythe nasal cavities and food from the mouth. The palate, which forms the roof if the mouth, consist of the hard palate anteriorly and the soft palate posteriorly. The soft palate has called the uvula,which people often confuse with the tonsils. The tonsils, however, are embedded in the mucous membrane of the pharynx.

From the oral cavity the mouth, food passes through the pharynx and esophagus to the stomach, small intestine, and large intestine. The food passage and air passage cross in the pharynx because the trachea is ventral to the oesophagus along muscular tube that takes food to the stomach swallowing our process that occurs in the fairings is a reflex action performed automatically without conscious thought, usually during swallowing the soft pallet moves back to close off the nasal fairings and the trachea moves up under The epiglottis To cover the glottis, the gloss is the opening to the lyrics, and therefore the air passage. During swallowing food normally enters the oesophagus because the air passage are blocked. We do not breathe when we swallow unfortunately, we have all the had the unpleasant experience of having food go up the wrong way the wrong way may be either the nasal cavity or into the trachea, if it is a ladder coughing Will most likely force the food out of the trachea and into the fairing again the up-and-down movement of Adams apple the front part of the lyrics is easy to observe when a person swallows us. We do not breathe when we swallow.

The esophagus

esophagus is a muscular two that passes from the lyrics through the thoracic cavity and diaphragm into the abdominal cavity where it joins the stomach. The esophagus is ordinarily collapsed, but it opens and receives the list once swallowing occurs arrhythmic contraction called peristalsis Pushes the food along the digestive track. Peristalsis Begins in the esophagus and continues in all the organs of the digestive track occasionally Perstalsis Begins, even though there is no food in the esophagus, this produces the sensation of a lump in the throat. esophagus plays no role in the chemical digestion of food. Its sole purpose is to conduct the food bowls from the mouth to the stomach sphincters are muscles that In circle tubes and act as valves tubes closed when sphincter’s contract and they they open one minor relax the entrance of the oesophagus to the stomach is marked by a construction, often called a sphincter, although the muscle is not as developed as in the true sphincter relaxation of the sphincter allows. The bowl is to pass into the stomach while contraction prevents the acidic contents of the stomach from backing up into the soft heartburn, which feels like a burning pain rising up into the throat occurs when some of the stomach content escape into the oesophagus when vomiting occurs a contraction of the abdominal muscles and diaphragm propels the contents of the stomach uppers through the esophagus, the air passage and food passage cross in the fairings which takes food to the esophagus, the oesophagus conducts the bowls of the food from the fairies to the stomach peristalsis Begins in oesophagus and occurs along the entire length of the digestive track.

All the oesophagus in the Domo cavity is comparable to that of the digestive track, which has these layers

Nekoosa, a layer of the helium supported by connective tissue and smooth muscle lines of the lumen, and contains granular epithelial Cells that secrete digestive enzymes and goblet cells that secrete mucus

Submucosa abroad band of loose connective tissue that contains blood vessel lies beneath the mucosa lymph nodules called pears. Patch are in the submucosa like the tonsils they help protect us from diseases.

Muscularis Two layers of smooth muscle make up the section the inner circular layers in circles that got the outer longitudinal Layer lies in the same direction as the gut stomach also has oblique muscle.

Serosa, most of the digestive track has a very thin outermost layer of squamous epithelium supported by connective tissue the serosa Secretes a serous Fluid that keeps the outer surface of the intestines so that the organs of the abdominal cavity slide against one another the soft has an outer layer composed only of loose connective tissue called the adventitia instead of serosa

THE STOMACH

The stomach is a thick walled, j-shaped organ that lies on thee left side of the abdominal cavity deep to the liver and diaphragm. The stomach is a continuous with the esophogas above the duodenum of the small intestine below.

The length of the stomach remains at about 25cm regardless of the amount of food it holds, but the diameter varies, depending on how full it is. As the stomach expands deep fold in its wall called rugged, gradually disappear . When full, it can hold about 4 litres . The stomach receives food from the esophogas, store food, mixes food with its juices and moves foood into the small intestine.

The stomach both physically and chemically acts on food. Its wall contains three muscles layers. One layer is longitudinal, another is circular, and the third is obliquely arranged. This muscular wall not only moves the food along, bit it also churns, mixing the food with gastric juice and breaking down to small pieces.

The term gastric always refers to the stomach. The columnar epithelial lining of the stomach has millions of gastric pits, which leads into gastric glands. The gastric glands produce gastric juice which contains pepsinogen, HCL, and mucus. Chief cells secrete pepsinogen, which becomes the enzyme pepsin when exposed to hydrochloride acid released by parietal cells. The HCL cause the stomach to have a high acidity with a pH of about 2, and this is beneficial because it kills most of the bacteria present in food. Although HCL does not digest food, it does break down the connective tissue of meat and activate pepsin.

The wall of the stomach is protected by the thick layer of mucus secreted by the mucous cells. If, by chance, HCL penetrates this mucus, the wall can begin to break down, and an ulcer results. An ulcer is an open sore in the wall caused by the gradual disintegration of tissue. It now appears that most ulcers are due to a bacterial infection that impair the ability of mucous cells to produce protective mucus.

Alcohol is absorbed in the stomach, but food substance are not. Normally the stomach empties in about 2-6 hours. When food leaves the stomach, it is a thick, soupy squints by way of the pyloric sphincter, which acts like a valve, repeatedly opening and closing.

THE SMALL INTESTINE

The small intestine is named for its small diameter, but perhaps it should be called long intestine. The small intestine averages about 6 meters in length compared to the large intestine which is about 1,5 meters in length.

The first 25cm of the small intestine is called the duodenum. Ducts from the liver and pancreas join to form intestine receives bile from the liver and pancreatic juice from the pancreas via this duct. Bile emulsifies fat- emulsification slightly basic pH because pancreatic juice contains sodium bicarbonate which neutralizes chyme. The enzymes in pancreatic juice and enzymes produced by the intestinal wall complete the process of food digestion. It has been suggested that the surface area of the small intestine is approximately that of a tennis court. What factors contribute too increasing its surface area? The wall of the small intestine contains fingerlike projections called villi which gave the intestinal wall a soft, velvety epithelial cells, and each f these cells has thousands of microscopic extensions called micro villi. Collectively, in electron micrographs, microvilli give the villi a fuzzy border, known as a "brush border." Since the microvilli bear the intestinal enzymes, these enzymes are called brush-border enzymes. The microvilli greatly increase the surface area of the villus for the absorption of nutrients.

Nutrients are absorbed into the vessels of a villus. A villus contains blood capillaries and a small lymphatic cap-illary, called a lacteal. The lymphatic system is an adjunct to the cardiovascular system (its vessels carry a fluid called lymph to the cardiovascular veins). Sugars and amino acids enter the blood capillaries of a villus. Glycerol and fatty acids (digested from fats) enter the epithelial cells of the villi, and within these cells are joined and packaged as lipoprotein droplets, which enter a lacteal. After nutrients are absorbed, they are eventually carried to all the cells of the body by the bloodstream.

Regulation of Digestive Secretions

The secretion of digestive juices is promoted by the nervous system and by hormones. A hormone is a substance produced by one set of cells that affects a different set of cells, the so-called target cells. Hormones are usually transported by the bloodstream. For example, when a person has eaten a meal particularly rich in protein, the stomach produces the hormone gastrin. Gastrin enters the bloodstream, and soon the stomach is churning, and the secretory activity of gastric glands is increasing. A hormone produced by the duodenal wall, GIP (gastric inhibitory peptide), works opposite to gastrin: it inhibits gastric gland secretion.

Cells of the duodenal wall produce two other hormones that are of particular interest-secretin and CCK (cholecystokinin). Acid, especially hydrochloric acid (HCI) present in chyme, stimulates the release of secretin, while partially digested protein and fat stimulate the release of CCK. Soon after these hormones enter the bloodstream, the pancreas increases its output of pancreatic juice, which helps digest food, and the liver increases its output of bile. The gallbladder contracts to release bile. Figure 14.7 summarizes the actions of gastrin, secretin, and CCK.

The Large Intestine

absorbs water

The large intestine, which includes the cecum, the colon, the rectum, and the anal canal, is larger in diameter than the small intestine (6.5 cm compared to 2.5 cm), but it is shorter in length (see Fio 14 1). The large intestine absorbs water. salts, and some vitamins. It also stores indigestible material until it is eliminated at the anus.

The cecum, which lies below the junction with the small intestine, is the blind end of the large intestine. The cecum has a small projection called the vermiform appendix (vermiform means wormlike) (Fig. 14.8). In humans, the appendix also may play a role in fighting infections. This organ is subject to inflammation, a condition called appendicitis. I inflamed, the appendix should be removed before the fluid content rises to the point that the appendix bursts, a situation that may cause peritonitis, a generalized infection of the lining of the abdominal cavity. Peritonitis can lead The colon includes the ascending colon, which goes up the right side of the body to the level of the liver; the transverse colon, which crosses the abdominal cavity just below the liver and the stomach; the descending colon, which passes down the left side of the body; and the sigmoid colon, which enters the rectum, the last 20 cm of the large intestine. The rectum opens at the anus, where defe-cation, the expulsion of feces, occurs. When feces are forced into the rectum by peristalsis, a defecation reflex occurs. The stretching of the rectal wall initiates nerve impulses to the spinal cord, and shortly thereafter, the rectal muscles contract, and the anal sphincters relax (Fig. 14.9).

Ridding the body of indigestible remains is another way the digestive system helps maintain homeostasis. Feces are three-quarters water and one-quarter solids. Bacteria, fiber (indigestible remains), and other indigestible materials are in the solid portion. Bacterial action on indigestible materials causes the odor of feces and also accounts for the presence of gas. A breakdown product of bilirubin (see page 269) and the presence of oxidized iron causes the brown color of feces.

For many years, it was believed that facultative bacteria (bacteria that can live with or without oxygen), such as Escherichia coli, were the major inhabitants of the colon, but new culture methods show that over 99% of the colon bacteria are obligate anaerobes (bacteria that die in the presence of oxygen). Not only do the bacteria break down indigestible material, but they also produce some vitamins and other molecules that can be absorbed and used by our bodies. In this way, they perform a service for us.

Water is considered unsafe for swimming when the co-liform (nonpathogenic intestinal) bacterial count reaches a certain number. A high count indicates that a significant amount of feces has entered the water. The more feces pres-ent, the greater the possibility that disease-causing bacteria are also present.

Polyps

The colon is subject to the development of polyps, small growths arising from the epithelial lining. Polyps, whether benign or cancerous, can be removed surgically. If colon cancer is detected while still confined to a polyp, the expected outcome is a complete cure. Some investigators believe that dietary fat increases the likelihood of colon cancer because dietary fat causes an increase in bile secretion. It could be that intestinal bacteria convert bile salts to substances that promote the development of cancer. On the other hand, fiber in the diet seems to inhibit the development of colon cancer. Dietary fiber absorbs water and adds bulk, thereby diluting the concentration of bile salts and facilitating the movement of substances through the intestine.

Regular elimination reduces the time that the colon wall is exposed to any cancer-promoting agents in feces.

Diarrhea and Constipation

Two common everyday complaints associated with the large intestine are diarrhea and constipation. The major causes of diarrhea are infection of the lower intestinal tract and nervous stimulation. In the case of infection, such as food poisoning caused by eating contaminated food, the intestinal wall becomes irritated, and peristalsis increases. Water is not absorbed, and the diarrhea that results rids the body of the infectious organisms. In nervous diarrhea, the nervous system stimulates the intestinal wall, and diarrhea results. Prolongeddiarrhea can lead to dehydration because of water loss, and to disturbances in the heart's contraction, due to an imbalance of salts in the blood.

When a person is constipated, the feces are dry and hard. One reason for this condition is that socialized persons have learned to inhibit defecation to the point that the urge to defecate is ignored. Two components of the diet that can help prevent constipation are water and fiber. Water intake prevents drying out of the feces, and fiber provides the bulk needed for elimination. The frequent use of laxatives is dis-couraged. If, however, it is necessary to take a laxative, a bulk laxative is the most natural because, like fiber, it produces a soft mass of cellulose in the colon. Lubricants, such as mineral oil, make the colon slippery; saline laxatives, such as milk of magnesia, act osmotically —they prevent water from being absorbed and, depending on the dosage, may even cause water to enter the colon. Some laxatives are irri-tants, meaning that they increase peristalsis to the degree that the contents of the colon are expelled.

Chronic constipation is associated with the development of hemorrhoids, enlarged and inflamed blood vessels at the anus.

The large intestine does not produce digestive enzymes; it does absorb water, salts, and some vitamins.

THREE ACCESSORY ORGANS

The pancreas, liver, and gallbladder are accessory digestive organs. Figure 14.10a shows how the pancreatic duct from the pancreas and the common bile duct from the liver and gallbladder enter the duodenum.

The Pancreas

The pancreas lies deep in the abdominal cavity, resting on the posterior abdominal wall. It is an elongated and somewhat flattened organ that has both an endocrine and an exocrine function. As an endocrine gland, it secretes insulin and glucagon, hormones that help keep the blood glucose level within normal limits. In this chapter, however, we are interested in its exocrine function. Most pancreatic cells produce pancreatic juice, which contains sodium bicarbonate (NaHCOs) and digestive enzymes for all types of food.

Sodium bicarbonate neutralizes acid chyme from the stom-ach. Pancreatic amylase digests starch, trypsin digests pro-tein, and lipase digests fat.

The Liver

The liver, which is the largest gland in the body, lies mainly in the upper right section of the abdominal cavity, under the diaphragm (see Fig. 14.1). The liver contains approximately 100,000 lobules that serve as its structural and functional units (Fig. 14.10b). Triads consisting of these three structures are located between the lobules: a bile duct that takes bile awav from the liver: a branch of the hepatic artery that brings O-rich blood to the liver; and a branch of the hepatic portal vein that transports nutrients from the intestines. The central veins of lobules enter a hepatic vein. In Figure 14.11, trace the path of blood from the intestines to the liver via the hepatic portal vein and from the liver to the inferior vena cava via the hepatic veins.

In some ways, the liver acts as the gatekeeper to the blood (Table 142). As blood from the hepatic portal vein passes through the liver, it removes poisonous substances and detoxifies them. The liver also removes and stores iron Dand the fat-soluble vitamins A, D, E, K, and B.,. The liver makes the plasma proteins and helps regulate the quantity of cholesterol in the blood.

A The liver maintains the blood glucose level at about 100 mg /100 ml (0.1%), even though a person eats intermit: tently. When insulin is present, any excess glucose present in blood is removed and stored by the liver as glycogen.

Between meals, glycogen is broken down to glucose, which enters the hepatic veins, and in this way, the blood glucose level remains constant.

If the supply of glycogen is depleted, the liver converts glycerol (from fats) and amino acids to glucose molecules. The conversion of amino acids to glucose necessitates deamination, the removal of amino groups.

By a complex metabolic pathway, the liver then combines ammonia with carbon dioxide to form urea. Urea is the usual nitrogenous waste product from amino acid breakdown in humans.

The liver produces bile, which is stored in the gallblad-der. Bile has a yellowish green color because it contains the bile pigment bilirubin, derived from the breakdown of hemo-globin, the red pigment of red blood cells. Bile also contains bile salts. Bile salts are derived from cholesterol, and they emulsify fat in the small intestine. When fat is emulsified, it breaks up into droplets, providing a much larger surface area, which can be acted upon by a digestive enzyme from the pancreas.

Liver Disorders

Hepatitis and cirrhosis are two serious diseases that affect the entire liver and hinder its ability to repair itself. There-fore, they are life-threatening diseases. When a person has a liver ailment, jaundice may occur. Jaundice is a yellowish tint to the whites of the eyes and also to the skin of light-pigmented persons. Bilirubin is deposited in the skin, due to an abnormally large amount in the blood. Jaundice can also result from hepatitis, inflammation of the liver. Viral hepatitis occurs in several forms. Hepatitis A is usually acquired from sewage-contaminated drinking water. Hepatitis B, which is usually spread by sexual contact, can also be spread by blood transfusions or contaminated needles. The hepatitis B virus is more contagious than the AIDS virus, which is spread in the same way. Thankfully, however, a vaccine is now available for hepatitis B. Hepatitis C, which is usually acquired by contact with infected blood and for which there is no vac-cine, can lead to chronic hepatitis, liver cancer, and death.

Cirrhosis is another chronic disease of the liver. First, the organ becomes fatty, and then liver tissue is replaced by inactive fibrous scar tissue. Cirrhosis of the liver is often seen in alcoholics, due to malnutrition and to the excessive amounts of alcohol (a toxin) the liver is forced to break down.

The liver has amazing regenerative powers and can recover if the rate of regeneration exceeds the rate of dam-age. During liver failure, however, there may not be enough time to let the liver heal itself. Liver transplantation is usually the preferred treatment for liver failure, but artificial livers have been developed and tried in a few cases. One type is a cartridge that contains liver cells. The patient's blood passes through the cellulose acetate tubing of the cartridge and is serviced in the same manner as with a normal liver.

The Gallbladder

The gallbladder is a pear-shaped, muscular sac attached to the surface of the liver (see Fig. 14.1). About 1,000 ml of bile are produced by the liver each day, and any excess is stored in the gallbladder. Water is reabsorbed by the gallbladder so that bile becomes a thick, mucouslike material. When needed, bile leaves the gallbladder and proceeds to the duodenum via the common bile duct.

The cholesterol content of bile can come out of solution and form crystals. If the crystals grow in size, they form gall-stones. The passage of the stones from the gallbladder may block the common bile duct and cause obstructive jaundice.

Then, the gallbladder must be removed.

The pancreas, liver, and gallbladder are accessory digestive organs. The pancreas produces pancreatic juice, which contains enzymes for the digestion of food. Among the liver's many functions is the production of bile, which is stored in the gallbladder.

DIGESTIVE ENZYMES

The digestive enzymes are hydrolytic enzymes, which break down macromolecules by the introduction of water at specific bonds. Digestive enzymes, like other enzymes, are proteins with a particular shape that fits their substrate. They also have an optimum pH, which maintains their shape, thereby enabling them to speed up their specific reaction.

juices, mentioned previously, help break down carbohy-drates, proteins, nucleic acids, and fats, the major components of food. Starch is a polysaccharide, and its digestion begins in the mouth. Saliva from the salivary glands has a neutral pH and contains salivary amylase, the first enzyme to act on starch:

salivary amylase

starch + H,O

→

maltose

In this equation, salivary amylase is written above the arrow to indicate that it is neither a reactant nor a product in the re-action. It merely speeds the reaction in which its substrate, starch, is digested to many molecules of maltose, a disaccha-ride. Maltose molecules cannot be absorbed by the intestine; additional digestive action in the small intestine converts maltose to glucose, the monomer of a starch molecule. Glucose can be absorbed.

Protein digestion begins in the stomach. Gastric juice secreted by gastric glands has a very low pH-about 2-because it contains hydrochloric acid (HCI). Pepsinogen, a precursor that is converted to the enzyme pepsin when exposed to HCl, is also present in gastric juice. Pepsin acts on protein, a polymer of amino acids, to produce peptides:

pepsin

protein + H,O

→ peptides

Peptides vary in length, but they always consist of a number of linked amino acids. Peptides are usually too large to be absorbed by the intestinal lining, but later in the small intes-tine, they are broken down to amino acids, the monomer of a protein molecule.

Pancreatic juice, which enters the duodenum, has a basic pH because it contains sodium bicarbonate (NaHCOs).

Sodium bicarbonate neutralizes chyme, producing the slightly basic pH that is optimum for pancreatic enzymes.

One pancreatic enzyme, pancreatic amylase, digests starch:

pancreatic amylase

starch + HO

maltose

Another pancreatic enzyme, trypsin, digests protein:

trypsin

protein + H,0 →

peptides

Trypsin is secreted as trypsinogen, which is converted to trypsin in the duodenum.

juice, which contains enzymes for th production of bile, which is stored in t

The

Lipase, a third pancreatic enzyme, digests fat mole cules in the fat droplets after they have been emulsified by

bile salts:

bile salts

fat droplets

fat

lipase

fat droplets + H20

glycerol + 3 fatty acids

Fats are triglycerides, which means that each one is composed of glycerol and three fatty acids. The end products of lipase digestion, glycerol and fatty acid molecules, are small enough to cross the cells of the intestinal villi, where absorption takes place. As mentioned previousty, glycerol and fatty acids enter the cells of the villi, and within these cells, they are rejoined and packaged as lipoprotein droplets before entering the lacteals (see Fig. 14.6).

Peptidases and maltase, enzymes produced by the small intestine, complete the digestion of protein to amino acids and starch to glucose, respectively. Amino acids and glucose are small molecules that cross into the cells of the villi. Peptides, which result from the first step in protein di-gestion, are digested to amino acids by peptidases:

peptides + H,0

→

amino acids

Maltose, a disaccharide that results from the first step in starch digestion, is digested to glucose by maltase:

maltase

maltose + H,0

glucose + glucose

Other disaccharides, each of which has its own en-zyme, are digested in the small intestine. The absence of any one of these enzymes can cause illness. For example, as many as 75% of African Americans cannot digest lactose, the sugar found in milk, because they do not produce lactase, the enzyme that digests the sugar lactose. Consuming diary products often gives these individuals the symptoms oflactose intolerance (diarrhea, gas, cramps), caused by a large quantity of nondigested lactose in the intestine. How-ever, lactose-reduced products are available for most kinds of dairy products, including milk, cheese, and ice cream. It is important to have a diet that includes foods high in calcium.

Otherwise, osteoporosis, a condition characterized by weak and fragile bones, may develop.

Digestive enzymes present in digestive juices help break down food to the nutrient molecules: glucose, amino acids, fatty acids, and glycerol. The first two are absorbed into the blood capillaries of the villi, and the last two re-form within epithelial cells before entering the lacteals.