The stomach is a temporary storage organ for large quantities of food. It can easily hold 1.5 litres, with a maximum holding capacity of about 4 litres. Its three main compartments are called the fundus, the body and the antrum. The stomach is also an important site for mechanical and chemical breakdown of the food which enters it. Digestive juices are secreted from glands covering the entire stomach wall. When food is present, approximately every 20 seconds mixing waves combine the food with gastric juices to form chyme - a creamy, semi-fluid mass. This is gradually emptied into the small intestine through a narrowed region surrounded by a thick ring of muscle at the lower end of the stomach - the pyloric sphincter. Liquids empty from the stomach faster than solids, taking about 20 minutes. A mixed meal takes about 1.5 hours to be emptied.

The stomach is also a secretory organ. The cells and glands in its wall produce several important secretions. These include: digestive enzymes ; hormones ; hydrochloric acid ; and intrinsic factor (for vitamin B12 absorption). A sticky, alkaline mucous is also produced to protect the stomach from its own acidity.