The small intestine is a winding, tightly folded tube about 20 ft (6.1 m) long in adults. It is connected to the stomach on the top end and to the large intestine (colon) on the bottom end.

Most of the food a person consumes is digested and absorbed in the small intestine. The lining of the small intestine has tiny fingerlike raised tissues (villi). These raised tissues greatly increase the surface area of the intestine, which absorbs calories and nutrients from food.