The Large Intestine

The large intestine, which includes the cecum, the colon, the rectum, and the anal canal (Fig. 15.9), is larger in diameter than the small intestine (6.5 cm compared to 2.5 cm), but it is shorter in length (see Fig. 15.1). The large intestine absorbs water, salts, and some vitamins. It also stores indigestible material until it is eliminated at the anus.

Figure 15.9 The large intestine. a. The colon has four regions: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. b. The rectum and anal canal are at the distal end of the alimentary canal. c. The intestinal mucosa hasmany goblet cells.

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). In humans, the appendix also may play a role in fighting infections. This organ is subject to inflammation, a condition called appendicitis. If 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 to death.

The colon has four portions: 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 defecation, 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 (see Fig. 15B). 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 and the presence of oxidized iron cause 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 B-complex vitamins and most of the vitamin K needed by our bodies. In this way, they perform a service for us. Water is considered unsafe for swimming when the coliform (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 present, the greater is the possibility that disease-causing bacteria are also present.

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. Prolonged diarrhea 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. The Medical Focus on this page discusses the causes of constipation and how it can be prevented. Chronic constipation is associated with the development of hemorrhoids, enlarged and inflamed blood vessels at the anus.

Figure 15B Defecation reflex. The accumulation of feces in the rectum causes it to stretch, which initiates a reflex action resulting in rectal contraction.

Constipation

The colon of the large intestine has four regions: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon (see Fig. 15.9a). Water is removed from the nondigestible intestinal contents entering the ascending colon from the small intestine. At this point, bacteria begin their action; they use cellulose as an energy source as they produce fatty acids and vitamins that can also be used by their host. They also release hydrogen gas and sulfur-containing compounds that contribute to human flatulence (gas). Feces, which consist of nondigested intestinal contents, bacteria, and sloughed-off intestinal cells, begin to form in the transverse colon. From there, they are propelled down the descending colon toward the rectum by periodic, firm contractions called peristalsis. When sufficient feces are in the rectum (130-200 grams), a defecatory urge is felt. The involuntary defecation reflex contracts the rectal muscles and relaxes the internal anal sphincter, a ring of muscle that closes off the rectum (Fig. 15B). Then, feces move toward the anus. A pushing motion, along with relaxation of the external anal sphincter, propels feces from the body. Since these activities are under voluntary control, it is possible to control defecation.
Defecation normally occurs from three times a week to three times a day; therefore, some variation in occurrence is nothing to worry about. However, if the frequency of defecation declines and if defecation becomes difficult, constipation is present. If constipation is a continuing problem, a physician can help record the movement of materials through the large intestine via several tests. The patient swallows about 20 small markers that will show up on an X ray. At intervals during the following week, X rays are taken, and the number and locations of the markers are noted. If muscle contraction of the intestinal wall is insufficient, the markers move slowly along their course. Injured nerves, certain drugs, dehydration, and prolonged overuse of stimulatory laxatives can bring about this difficulty. Some or all of these problems frequently occur in the elderly. On the other hand, markers may move normally at first and then slow down considerably in the descending colon and rectum. Habitual disregard of the defecatory urge may have caused this problem, or a cancerous polyp might be obstructing normal movement. If the former is the case, it is possible to retrain the rectum to work properly. Sitting on the toilet for about 20 minutes each morning can encourage a return of the reflexes that have disappeared, but straining is not recommended.
Temporary constipation due to traveling, pregnancy, or medication can sometimes be relieved by increasing dietary fiber, drinking plenty of water, and getting moderate amounts of exercise. The use of oral laxatives (agents that aid emptying of the intestine) is a last resort. Bulk-forming laxatives, such as those that contain bran, psyllium, and methyl cellulose, are considered best because they promote the defecation reflex. Laxatives that contain osmotic agents, such as carbohydrates or salts (lactulose, milk of magnesia, or Epsom salts), cause water to move into rather than out of the colon. Stool softeners (mineral oil or those that contain docusate) should be used sparingly. Mineral oil reduces the absorption of fat-soluble vitamins, and docusate can cause liver damage. Laxatives that contain chemical stimulants (such as phenolphthalein in Ex-Lax and Feen-A-Mint) can damage the defecation reflex and lead to a dependence on their use. Aside from laxatives, rectal suppositories are sometimes helpful in providing lubrication and stimulating the defecation reflex. Enemas introduce water into the colon and, therefore, also help stimulate defecation.

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 along with a portion of the colon if necessary. If colon cancer is detected while still confined to a polyp, the expected outcome is a complete cure. If the last portion of the rectum and the anal canal must be removed, then the intestine is sometimes attached to the abdominal wall through a procedure known as a colostomy, and the digestive remains are collected in a plastic bag fastened around the opening. Recently, the use of metal staples has permitted surgeons to join the colon to a piece of rectum that formerly was considered too short.
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 cancerpromoting agents in feces.

Other Disorders of the Large Intestine

The appendix is a fingerlike projection from the cecum of the large intestine. Unfortunately, the appendix can become infected, resulting in appendicitis, a very painful condition in which the fluid content of the appendix can increase to the point that it bursts. The appendix should be removed before it bursts to avoid a generalized infection of the peritoneal membrane of the abdominal cavity. Diverticulosis is characterized by the presence of diverticula, or saclike pouches, in the colon. Ordinarily, these pouches cause no problems. But about 15% of people with diverticulosis develop an inflammation known as diverticulitis. The symptoms of diverticulitis are similar to those of appendicitis -cramps or steady pain with local tenderness. Fever, loss of appetite, nausea, and vomiting may also occur. Today, highfiber diets are recommended to prevent the development of these conditions and of cancer of the colon.