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Guide To Digestion In The Large Intestine

Digestion In The Large Intestine

After all nutrients have been absorbed from ingested food during its passage through the small intestine, the watery waste passes into the large intestine. It is the final section of the gastrointestinal tract and its main function is to remove water (plus any remaining minerals) from the food waste and compress it into a form for easy expulsion from the body. As the chyme passes through the large intestine, the water is removed and the chyme is combined with mucus and bacteria (gut flora), and is converted into feces.

How Food Waste Moves Through The Large Intestine

As in the esophagus and small intestine, undigested food is propelled through the large intestine by waves of muscular contraction and expansion, called peristalsis. However, unlike in the small intestine where these waves occur at irregular intervals, peristalsis in the large intestine is continuous. In addition, 2-3 times a day, a more vigorous type of movement (gastrocolic reflex) occurs which propels material towards the rectum and anus. As waste matter is pushed into the rectum, it triggers a desire to defecate.

Anatomy Of The Large Intestine

The large intestine (also referred to as the large bowel, or the lower gastrintestinal tract) is a thick tube of about 5 feet in length which gets progressively narrower in diameter. It consists of 4 regions: the cecum, colon, rectum, and anal canal. (The term “colon” is sometimes used to describe the entire large intestine). The cecum (or caecum) is a short pouch into which food enters from the ileum (via the ileocecal valve) and exits into the ascending colon of the large intestine. The colon is the longest segment of the large intestine. It is sub-divided into 4 sections, named after their position in the pelvis: the ascending colon, transverse colon, descending colon and sigmoid colon. The rectum is the final part of the large intestine. Feces formed in the colon collect in the rectum before being excreted via the anus. After the rectum comes the anal canal, a short passage about 1.5 inches long, terminating in two muscular rings: the internal and external sphincters. As waste products from the rectum pass into the anal canal, nerves in the rectum cause the internal sphincter to relax and open. Then the external sphincter also relaxes, permitting fecal discharge.

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