The Digestive System and the Liver - The large intestine

The large intestine, also called the large bowel, is shaped like a great, lumpy, drooping question mark—arching, from its beginning at the ileocecal valve, over the folds of the small intestine, then curving down and descending past more coiled small intestine to the anus, which marks the end of the GI tract. From ileocecal valve to anus, the large intestine is five to six feet in length.

The junction between the ileum and the cecum , the first section of the large intestine, occurs very low in the abdomen, normally on the right-hand side. The cecum is a bowl-like receptacle at the bottom of the colon, the longest section of the large intestine.

Just below the entrance of the ileum, a dead-end tube dangles down from the cecum. This is the appendix vermiformis (Latin, “worm-shaped appendage”) commonly known as the appendix. Three to six inches long and one-third inch in diameter, the appendix may get jammed with stray pieces of solid food, become infected, swell, and rupture, spewing infection into the abdominal cavity. This is why early diagnosis of appendicitis and removal ( appendectomy ) are critically important.

Sections of the Colon

The colon is divided into three sections by pronounced flexures , or bends, where the colon makes almost right-angle changes of direction. Above the bowl of the cecum, the ascending colon rises almost vertically for about a foot and a half.

Then there is a flexure in the colon, after which the transverse colon travels horizontally for a couple of feet along a line at navel height. At another flexure, the colon turns vertically down again, giving the name of descending colon to this approximately two feet of large intestine. At the end of the descending colon, the large intestine executes an S-shaped curve, the sigmoid flexure , after which the remaining several inches of large intestine are known as the rectum .

Some people confuse the terms rectum and anus: the rectum refers specifically to the last section of the large intestinal tube, between sigmoid flexure and the anal sphincters, while the anus refers only to the opening controlled by the outlet valves of the large intestine. These valves consist of two ringlike voluntary muscles called anal sphincters.

Any solid materials that pass into the large intestine through the ileocecal valve (which prevents backflow into the small intestine) are usually indigestible, such as cellulose, or substances that have been broken down in the body and blood in the normal process of cell death and renewal, such as some bile components. But what the cecum mainly receives is water.

Functions of the Large Intestine

The principal activity of the large intestine—other than as a channel for elimination of body wastes—is as a temporary storage area for water, which is then reabsorbed into the circulation through the walls of the colon. Villi are absent in the large intestine, and peristalsis is much less vigorous than in the small intestine.

As water is absorbed, the contents of the large intestine turn from a watery soup into the semisolid feces. Meanwhile, bacteria—which colonize the normal colon in countless millions—have begun to work on and decompose the remaining solid materials. These bacteria do no harm as long as they remain inside the large intestine, and the eliminated feces is heavily populated with them. Nerve endings in the large intestine signal the brain that it is time for a bowel movement.

The Peritoneum

Lining the entire abdominal cavity, as well as the digestive and other abdominal organs, is a thin, tough, lubricated membrane called the peritoneum . In addition to protecting and supporting the abdominal organs, the peritoneum permits these organs to slip and slide against each other without any harm from friction. The peritoneum also contains blood and lymph vessels that serve the digestive organs. See Ch. 11, Diseases of the Digestive System .