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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.
This entry comes from Encyclopædia Britannica Concise. For the full entry on small intestine, visit Britannica.com.