Colon (anatomy)
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In anatomy of the digestive system, the colon, also called the large intestine or large bowel, 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 cecum, 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.
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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 cells 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
- Angiodysplasia of the colon
- Chronic functional abdominal pain
- Colitis
- Colon cancer
- Constipation
- Crohn's disease
- Diarrhea
- Diverticulosis & Diverticulitis
- Hirschsprung's disease (aganglionosis)
- Irritable bowel syndrome
- Polyposis (see also Polyp)
- Pseudomembranous colitis
- Ulcerative colitis and toxic megacolon
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.
Cecum
The cecum or caecum is a pouch connected to the large intestine and the ileum. It is separated from the ileum by the ileocecal valve (ICV) or Bauhin's valve, and is considered to be the beginning of the large intestine and part of the colon.
The cecum is present in mammals, birds, and some reptiles. Most herbivores have a relatively large cecum, and exclusive carnivores have a smaller cecum or none at all. This size difference results from the large number of bacteria in the cecum of herbivores, which aid in the enzymatic breakdown of plant materials such as cellulose. Carnivores, whose diets contain little or no plant material, have a reduced cecum, often partially or wholly replaced by the vermiform appendix.
The appendix is a branch of the cecum. Like the human appendix, the cecum (from the Latin caecus meaning blind) was once believed to have no function. Unlike the appendix, however, there is no current debate about the use of the cecum.
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. That perceptual ability is important in knowing what can be passed appropriately in what circumstance.
| Digestive system - edit |
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| Mouth | Pharynx | Esophagus | Stomach | Pancreas | Gallbladder | Liver | Small intestine (duodenum, jejunum, ileum) | Colon | Cecum | Rectum | Anus |
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