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.
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).
*
Angiodysplasia of the colon
*
Chronic functional abdominal pain*
Colitis*
Colon cancer*
Constipation*
Crohn's disease*
Diarrhea*
Diverticulitis*
Diverticulosis*
Hirschsprung's disease (aganglionosis)
*
Irritable bowel syndrome*
Polyposis (see also
Polyp)
*
Pseudomembranous colitis*
Ulcerative colitis and
toxic megacolonThe 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
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Diagram of the Human Intestine. |
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.
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Overview and diagrams at seer.cancer.gov*
Photo at mgccc.cc.ms.us
