Digestion is the process whereby a biological entity processes a substance, in order to chemically convert the substance into nutrients. Digestion occurs at the multicellular, cellular, and sub-cellular levels.
Digestion usually involves mechanical manipulation and chemical action. In most vertebrates, digestion is a multi-stage process in the digestive system, following ingestion of the raw materials, most often other organisms. The process of ingestion usually involves some type of mechanical manipulation.
In humans, digestion begins in the mouth where food is chewed with the teeth. The process stimulates exocrine glands in the mouth to release digestive enzymes such as salivary amylase, which aid in the breakdown of food, particularly carbohydrates. Chewing also causes the release of saliva, which helps condense food into a bolus that can be easily passed through the esophagus to the stomach. In the stomach, food is churned and thoroughly mixed with acid and other digestive enzymes with digestive fluid to further decompose it chemically. As the acidic level changes in the stomach and later parts of the digestive tract, more enzymes are activated or deactivated to extract and process various nutrients.
After being processed in the stomach, food is passed to the small intestine where it is further mixed with secretions such as bile, which helps aid in fat digestion, and insulin, to process sugars. Most nutrient absorption takes place in the small intestine, after which food is passed to the large intestine. Blood which has absorbed nutrients passes through the liver for filtering, removal of toxins and help processing of nutrients. In the large intestine, water is reabsorbed, and leftover waste is excreted by defecation.
Organisms develop specialized organs to aid in the digestion of their food, for example different types of tongues or teeth. Insects may have a crop (or the enlargement of esophagus) while birds may develop a gizzard (or a stomach that acts as teeth and mechanically digests food). A herbivore may have a cecum that contains bacteria that helps break down the cellulose in plants. Ruminants, for example cows and sheep, have a specialised four-chambered stomach that produces cud to enable easier digestion of cellulose.
There are at least four hormones that aid and regulate the digestive system: