Colon (anatomy)

For the article about the punctuation symbol, see Colon (punctuation). In anatomy of the digestive system, the colon is the part of the intestine from the cecum ('caecum' in British English) to the rectum. Its primary purpose is to extract water from feces. In mammals, it consists of the ascending colon, transverse colon, the descending colon, the sigmoid colon, and the rectum. The colon from cecum to the mid transverse colon is also known as the right colon. The remainder is known as the left colon.

Role in digestion

The large intestine comes after the small intestine in the digestive tract and measures approximately 1.5 meters in length. Although there are differences in the large intestine between different organisms, the large intestine is mainly responsible for storing waste, reclaiming water, maintaining the water balance, and absorbing some vitamins, such as vitamin K.

By the time the chyme has reached this tube, almost all nutrients and 90% of the water have been absorbed by the body. At this point some electrolytes like sodium, magnesium, and chloride are left as well as indigestible carbohydrates known as dietary fiber. As the chyme moves through the large intestine, most of the remaining water is removed, while the chyme is mixed with mucus and bacteria known as gut flora, and becomes feces. The bacteria break down some of the fiber for their own nourishment and create acetate, propionate, and butyrate as waste products, which in turn are used by the cell lining of the colon for nourishment. This is an example of a symbiotic relationship and provides about 100 Calories a day to the body. The large intestine produces no digestive enzymes — chemical digestion is completed in the small intestine before the chyme reaches the large intestine. The pH in the colon varies between 5.5 and 7 (slightly acidic to neutral).

Diseases and disorders of the colon

Colon subsections

The location of the parts of the colon are either in the abdominal cavity or behind it in the retroperitoneum. The colon in those areas is fixed in location.

Ascending colon

The ascending colon is on the right side of the abdomen. It is the part of the colon from the cecum to the hepatic flexure (the turn of the colon by the liver). It is retroperitoneal in most humans. In grazing animals the cecum empties into the spiral colon.

Transverse colon

The transverse colon is the part of the colon from the hepatic flexure (the turn of the colon by the liver) to the splenic flexure (the turn of the colon by the spleen). The transverse colon hangs off the stomach, attached to it by a wide band of tissue called the mesocolon. The transverse colon is mobile (unlike the parts of the colon immediately before and after it), and is very mobile in the abdomen of some individuals.

Descending colon

The descending colon is the part of the colon from the splenic flexure to the beginning of the sigmoid colon. It is retroperitoneal in two-thirds of humans. In the other third, it has a (usually short) mesentery.

Sigmoid colon

The sigmoid colon is the part of the large intestine after the descending colon and before the rectum. The name sigmoid means S-shaped (see sigmoid). The walls of the sigmoid colon are muscular, and contract to increase the pressure inside the colon, causing the stool to move into the rectum.

Due to the intermittent high pressure within it, the colon can develop pockets called diverticuli in its walls. The presence of diverticuli, whether harmful or not, is called diverticulosis. An infection of the diverticuli is called diverticulitis.

Sigmoidoscopy is a common diagnostic technique used to examine the sigmoid colon.

If long objects are inserted forcibly into the anus, the rectum or sigmoid colon may be damaged.

Rectum

The rectum is the last part of the colon. It holds stool prior to defecation. The last few centimeters of the rectum are lined by tissue which is similar to skin. This area is known as the "social part" of the rectum, since it can distinguish between solid, liquid and gas. This perceptual ability is important in knowing what can be passed appropriately and under what circumstances.

Abdomen | Digestive system

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