Digestion begins in the mouth, where food is chewed into small bits by the teeth. In the process of chewing, or mastication, the tongue and the palate, the roof of the mouth, help to break up the food and mix it with saliva, a secretion that moistens the food and begins the digestion of starch. The moistened food is then passed into the pharynx (throat) and through the esophagus into the stomach. Here it is further broken down by churning of the stomach as it is mixed with the enzyme pepsin and with powerful hydrochloric acid (HCl), both of which break down proteins.

The partially digested food passes through the lower portion of the stomach, the pylorus, into the first part of the small intestine, the duodenum. As the food continues through the jejunum and ileum, the remaining sections of the small intestine, digestion is completed. The substances active in digestion in the small intestine include enzymes from the intestine itself and secretions from the accessory organs of digestion. The digested nutrients, as well as water, minerals, and vitamins, are absorbed into the circulation, aided by small projections in the lining of the small intestine called villi.

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