Function of the small intestine

What leaves the stomach is no longer food; it is chyme, a thick mush of half broken-down food. That chyme enters the small intestine, and that is where almost everything is decided, because the small intestine is the body's main digestive organ: most digestion and practically all absorption happen there, absorption being the moment when nutrients cross into the body.

Absorbing that much takes surface area, and the intestine gets it with three tricks stacked one on top of the other. Its wall has circular folds, ledges that run around the inside; on the folds there are villi, projections that look like hairs; and on the cells of those villi there are microvilli, smaller still. It is a loop-pile carpet laid over a rippled wall: far more surface area fits into the same space. And there is one more thing: the chyme does not run straight through but travels in a spiral, and that detour slows its passage and gives nutrients time to be absorbed. The underlying movement is not a push, either. It is called segmentation, and it does not send the chyme forward: it mixes it with the digestive juices and presses it against the mucosa, the inner surface, so it gets absorbed, like kneading instead of sweeping.