End section of the intestine. It is about 5 ft (1.5 m) long, is wider than the small intestine, and has a smooth inner wall. In the first half, enzymes from the small intestine complete digestion, and bacteria produce many B vitamins and vitamin K. Over 24–30 hours, churning movements break down tough cellulose fibres and expose chyme to the colon's walls, which absorb water and electrolytes; absorption is its main function, along with storing fecal matter for expulsion. The more vigorous “mass movement” (gastrocolic reflex) occurs only two or three times a day to propel waste material toward the anal canal. Common afflictions include ulcerative colitis, diverticulosis (see diverticulum), polyps, and tumours.

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