The stomach is a hollow, muscular organ of the gastrointestinal tract (digestive system), between the esophagus and the small intestine. It is where digestion of protein begins. The stomach has three tasks. It stores swallowed food. It mixes the food with stomach acids. Then it sends the mixture on to the small intestine.
In humans, the stomach has a relaxed, empty volume of about 45 ml. It generally expands to hold about 1 litre of food, but can hold as much as 1.5 litres.
The stomach is supplied by both the parasympathetic and sympatethic parts of the autonomic nervous system.
Many stomach diseases are associated with infection. Historically, it was widely believed that the highly acidic environment of the stomach would keep the stomach immune from infection. However, a large number of studies have indicated that most cases of stomach ulcers, gastritis, and stomach cancer are caused by Helicobacter pylori infection.