Small and large intestines

Folds, villi and microvilli enlarge the absorbing surface of the small intestine. A 2014 morphometric estimate put this at about 30 square metres.

Why the long one is called small

The small intestine is the longest part of the tract, about 3.05 metres in a living person. It is called small because of its width: about 2.54 cm across, compared with about 7.62 cm for the large intestine. It has three parts. The duodenum, about 25.4 cm long, curves around the head of the pancreas and receives bile and pancreatic juice. The jejunum, about 0.9 m, comes next, and the ileum, about 1.8 m, is the longest.

Almost all absorption happens here. To absorb that much, the wall needs an enormous surface, and it gets it in three steps. Circular folds of the lining make chyme spiral as it moves. On the folds stand villi, tiny finger-like projections 0.5 to 1 mm long, about 20 to 40 in every square millimetre, each with blood capillaries and a lymph vessel called a lacteal. On each absorbing cell are microvilli, an estimated 200 million per square millimetre, forming the brush border. These structures greatly enlarge the absorbing surface. A 2014 morphometric estimate put it at about 30 square metres.

The small intestine moves chyme in two ways. Segmentation squeezes small sections back and forth to mix chyme with juices and press it against the wall; in the duodenum it happens about 12 times a minute and in the ileum about 8. Slower sweeping waves then push the chyme along. It takes about 3 to 5 hours for all the chyme to leave the small intestine. At the end, the ileocecal valve controls the flow into the large intestine. The small intestine absorbs about 90 percent of the water you take in.

The large intestine

The large intestine has four regions: the cecum, the colon, the rectum and the anal canal. The cecum is a pouch where the small intestine joins, and the appendix, a winding tube, hangs from it. OpenStax mentions one suggestion that the appendix may hold a reserve of gut bacteria. The colon rises on the right, crosses the abdomen and descends on the left. Bands of muscle bunch its wall into a chain of pouches called haustra. The rectum, about 20.3 cm long, stores stool until it leaves the body.

Trillions of bacteria live in the large intestine. OpenStax counts more than 700 species, most of them harmless as long as they stay inside the gut. They ferment some of the carbohydrates we cannot digest and make certain vitamins, mainly biotin, pantothenic acid and vitamin K. The large intestine absorbs more water and salts. The leftovers stay in it typically for 12 to 24 hours. Movement there is slow, but strong waves called mass movements happen three or four times a day, often while you eat or right after. Of every 500 mL of food residue that enters the cecum each day, about 150 mL becomes feces.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.