Why Is the Small Intestine so Long?

The small intestine is about 20 feet long because it performs most of the body’s digestion and absorbs most of the nutrients from food, says the Children’s Hospital of Pittsburgh. It is made up of three parts: the duodenum, which takes food from the stomach and continues the digestion started there; the jejunum, which carries food to the ileum; and the ileum, which absorbs most of the nutrients.

The small intestine is so named because it has a relatively small 1-inch diameter, compared to the large intestine's 2 inches, reports Innerbody. The small intestine is part of the gastrointestinal tract, along with the stomach and the large intestine (also known as the colon), explains Healthline. The absorption surface of the small intestine is actually over 650 feet, thanks to the villi and microvilli tissues that cover the intestinal walls and increase the absorption area.

It takes food between six and eight hours on average to pass through the stomach and small intestine, according to Mayo Clinic. After the small intestine, food passes to the colon, where water is absorbed and the final stages of digestion occur, and undigested food is eliminated. The entire digestion process can take up to 53 hours, though this number is different between men and women. Food passes through the colon at a much slower rate in women.