In order to understand the etiology of irritable bowel syndrome (IBS) one needs to learn more about the functioning of the colon.
The colon (large intestine), a tube about 2.5 in. in diameter, forms the last 4 ft of the gastrointestinal tract. The cecum forms a blind-ended pouch below the junction of the small and large intestines. The appendix, a small fingerlike projection from the end of the cecum, has no known function. The colon is not coiled but consists of three relatively straight segments, the ascending, transverse, and descending portions. The terminal portion of the descending colon is S-shaped, forming the sigmoid colon, which empties into a short section of tubing, the rectum. Although the large intestine has a greater diameter than the small intestine and is about half as long, its epithelial surface area is only about 1/30 that of the small intestine because the mucosa of the large intestine lacks villi and is not convoluted. The large intestine secretes no digestive enzymes and is responsible for the absorption of only about 4 percent of the total intestinal contents per day. Its primary function is to store and concentrate fecal material prior to defecation.
Chyme enters the colon through the ileocecal sphincter separating the ileum from the colon. This sphincter is normally closed, but after a meal when the gastroileal reflex increases the contractile activity of the ileum, the sphincter relaxes each time the terminal portion! of the ileum contracts, allowing chyme to enter the large intestine. Distension of the colon, on the other hand, produces a reflexive contraction of the sphincter, preventing further material from entering.
About 500 ml of chyme from the small intestine enters the colon each day. Most of this material is derived frame the secretions of the small intestine, since most of the ingested food has been absorbed before reaching the large intestine. The secretions of the colon are very scanty and consist mostly of mucus.
The primary absorptive process in the large intestine is the active transport of sodium from the lumen to blood with the accompanying osmotic reabsorption of water. If fecal material remains in the large intestine for a long time, almost all the water is reabsorbed, leaving behind dry fecal pellets. The cells lining the large intestine are unable to actively transport either glucose or аmino acids. There is a small net leakage of potassium into the colon, and severe depletion of total body potassium can occur as a result of repeated enemas and diarrhea.
The large intestine also absorbs some of the products synthesized by the bacteria in it. For example, small amounts of vitamins are synthesized by intestinal bacteria and absorbed into the body. Although this source of vitamins generally provides only a small part of the normal vitamin requirement per day, it may make a significant contribution when dietary intake of vitamins is low. The intestinal bacteria digest cellulose and utilize the glucose released for their own growth and reproduction.
Other bacterial products contribute to the production of intestinal gas (flatus). This gas is a mixture of nitrogen and carbon dioxide with small amounts of the inflammable gases hydrogen, methane, and hydrogen sul-fide. Bacterial fermentation produces gas in the colon at the rate of about 400 to 700 ml/day. In the cow, where bacterial fermentation makes a major contribution to the digestive process, as much as 300 to 600 liters of flatus may be produced each day.
The longitudinal smooth muscle in the human colon is incomplete, and the walls of the large intestine are folded into sacs called haustra by the contraction of the circular smooth muscle. Contractions of the circular smooth muscle produce a segmentation motion which is not propulsive. This movement is considerably slower than in the small intestine, and a contraction may occur only once every 30 min. Because of this slow movement, material entering the colon from the small intestine remains for 18 to 24 hr. Bacteria have time to grow and accumulate in the large intestine because of its slow movements; in the small intestine they do not have sufficient time to accumulate before being swept into the large intestine. During sleep and most of the day there is generally little or no movement in the large intestine, but three to four times a day, generally after meals, a marked increase in motility occurs. This usually coincides with the gastroileal reflex, described earlier, and probably has similar reflex mechanisms. This increased motility may lead to the phenomenon known as mass movement, in which large segments of the ascending and transverse colon contract simultaneously, propelling fecal material one-third to three-fourths of the length of the colon in a few seconds.