intestine, muscular hoselike portion of the gastrointestinal tract extending from the lower end of the stomach (pylorus) to the anal opening. In humans this fairly narrow (about 1 in./2.5 cm) tubelike structure winds compactly back and forth within the abdominal cavity for about 23 ft (7 m), and is known as the small intestine. It is not only an organ of digestion (for that part of the process not completed by the stomach) but is the chief organ of absorption. By contraction of its muscular walls (peristalsis) the food mass is propelled onward and, as it is carried along, it is subject to the digestive action of the secretions of the intestinal lining as well as to that of bile and pancreatic juice which enter the upper intestine (duodenum) from ducts leading from the liver and pancreas. Innumerable minute projections (villi) in the intestinal mucous lining absorb the altered food for distribution by the blood and lymphatic systems to the rest of the body. Food continues to pass into the middle (jejunum) and end (ileum) of the small intestines. The small intestine joins the large intestine (colon) at the cecum in the right lower abdominal cavity. Here, also, is the appendix, a blind pouch projecting from the cecum. The large intestine is wider in diameter. Its direction as it leaves the cecum is upward (ascending colon), across the abdominal cavity (transverse colon) beneath the stomach, and then downward (descending colon) on the left side of the abdominal cavity, making a sharp turn in the left lower portion (sigmoid) to merge with the rectum. In all, the large intestine is about 5 ft (1.5 m) long. Bacteria, the indigestible residue of food, and mucus form the bulk of matter in the large intestine. The water content of the bulk is absorbed through the walls of the large intestine, and the solid matter is excreted through the rectum. See digestive system.

Long, narrow, convoluted tube in which most digestion takes place. It extends 22–25 ft (6.7–7.6 m), from the stomach to the large intestine. The mesentery, a membrane structure, supports it and contains its blood supply, lymphatics, and insulating fat. The autonomic nervous system supplies it with parasympathetic nerves that initiate peristalsis and sympathetic nerves that suppress it. It is lined with minute fingerlike projections (villi) that greatly increase its surface area for enzyme secretion and food absorption. Its three sections, the duodenum, jejunum, and ileum, have distinct characteristics. Food takes three to six hours to pass through the small intestine unless a disorder such as gastroenteritis, diverticulosis, or obstruction impedes it.

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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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