LARGE INTESTINE The large intestine is the end of the alimentary canal. Its main function is to complete the absorption of nutrients, absorption of water, vitamin synthesis, and the elimination of feces. It is a structure that starts at the appendix and ends at the anus. It is only about 3-feet long with a diameter of three inches. There are four regions to the large intestine: the cecum, colon, rectum, and anus. The start of the large intestine is the ileocecal valve, which controls the flow of chyme into the large intestine. The cecum is about six centimeters or 2.4 inches in length and contains the appendix. The appendix contains lymphoid tissue but is not considered a functional organ. The colon is divided into the ascending colon, transverse colon, descending colon, and sigmoid colon. There is right and left splenic flexure that act as “bends” in the upper right (hepatic side) and upper left (splenic side) of the colon. The rectum is about 8 inches in length and curves around the sacrum. There are three folds called the “rectal valves” that prevent the simultaneous passage of gas and feces. The anal canal has two sphincters, the smooth muscle internal anal sphincter (which is involuntary), and the skeletal muscle external anal sphincter (which is voluntary). There are no circular folds and no villi in the large intestine and there are few enzymes secreted. It consists of simple columnar epithelium (other than the anal canal), which is made from absorptive cells (called enterocytes) and interspersed goblet cells. Goblet cells secrete mucus that eases the passage of feces and protects the intestine from the effects of the acidic substances and gases made by the large intestinal bacteria. The major unique features of the large intestine are the bands of longitudinal muscle in the muscularis, divided into three parts called the teniae coli. When these muscles contract, they form the haustra, which are pouches that give the wrinkly appearance of the large intestine. Attached to the teniae coli are fat-filled sacs made by visceral peritoneum called epiploic appendages, which have no known function. These structures are seen in the entire colon except for the rectum and anal canal.
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