Food which is ingested moves through the digestive tract. It is both mechanically (chewing, churning) and chemically (digestive juices) broken down. Part of it is transformed into soluble nutrients which are absorbed into the blood in the small intestine and then carried to the organs. In the bowel, undigested food is eliminated as excrements.

One function of blood is to carry oxygen and nutrients to body cells. It also carries secretions and metabolic wastes away from cells.

Most of the absorption takes place in the small intestine .Because it depends on the amount of epithelial membrane exposed to the chime (partially digested food coming from the stomach), the greater the surface area of the intestinal wall is, the better is the absorption. This increased surface area is provided by folding in the intestinal wall and by the structures called villi (singular: villus)