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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:

Not only animals digest food. Some carnivorous plants capture other organisms, usually invertebrate animals, and chemically digest them. Fungi also are very effective at digesting organic material.

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 esophogus ) while birds may develop a gizzard ( or a stomach that acts as teeth and mechanically digests food) . A Herbivore may have a caecum that contains bacteria that helps break down the cellulose in plants.

There are at least four hormones that aid and regulate the digestive system: