What does the small intestine do to the food after it passes through the stomach?

Answer 1

Once food is let into the small intestine, through a sphincter (valve) from the stomach, a process called peristalsis (the contracting and relaxing of muscles around the intestine) causes undigested food to be squeezed through the incredibly, long length of the small intestine (so named for its width, not length!).

During the food's journey, nutrients that have not been absorbed in the stomach are slowly absorbed through the small intestine's lining, into the tiny capillaries which surround the winding tube, thus allowing nutrient uptake into the blood and circulatory system.

Hence the huge length yet relatively thin form of the small intestine. This shape allows a maximum of surface area to allow undigested food to come in contact with the intestine's membrane.

It is mostly nutrients from food, already partially digested in the stomach, which the s. intestine absorbs, though water is also importantly transferred to the circulatory system at this stage of digestion. The large intestine, along with the rectum, however, are responsible for the major extraction of remaining water.

It is interesting to note that the appendix is a part (or adjunct) of the small intestine. Our herbivorous (plant eating) mammalian cousins have much longer appendixes, which serve a a digestion area for fibrous, plant materials. Herbivore's appendixes contain many bacteria which help to digest foods too fibrous for (near-appendixless), humans! The Koala needs an appendix of comparable length to the entire human small intestine, in order to digest Eucalyptus leaves!