Digestion is the process whereby a biological species processes food to produce usable nutrients or in another word is the process of dissolving food and breaking it down into smaller molecules. 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 the food is masticated with the teeth, thereby releasing enzymes from saliva into the food, and starting a reaction with the enzymes in the food itself.
• The food is then further compressed by being sent through the oesophagus to the stomach, a large pouch, where it is churned and thoroughly mixed with acid to chemically decompose it.
• As the acidic level(ph) changes in the stomach and later parts of the digestive tract, specific enzymes are activated or deactivated to extract and process various nutrients.
• Then passed into the small intestine where it is further mixed with secretions such as liver bile, to help digest fats, and pancreas insulin, to process sugars; Most nutrient absorption takes place in the small intestine.
• The final stage in human digestion is the movement of the matter into the large intestine, where water is re-absorbed before defecation.

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:

• Gastrin - is in the stomach and stimulates the grastic glands to secrete pepsinogen and HCL . Secretion of gastrin is stimulated by food arriving in stomach. The secretion is inhibited by low pH .
• Secretin - is in the duodenum and signals the secretion of sodium bicarbonate in the pancreas and in stimulates the bile secretion in the liver. This hormone responses to the acidity of the chyme.
• Cholecystokinin (CCK) - is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the gall bladder. This hormone is secreted in resonse to fat in chyme.
• Gastric Inhibitory Peptide (GIP) - is in the duodenum and decreases the stomach churning in turn slowing the emptying in the stomach