The stomach stores food and performs preliminary steps of digestion. The stomach is a large, muscular sac situated in the upper left abdomen and separated from the left lung and the heart by the diaphragm.

It is divided into two parts: the corpus and the antrum. The gullet (esophagus) opens into the corpus, the larger part, but is separated from it by a circular band of muscle called the cardiac sphincter. The antrum is the area of stomach closest to the duodenum, separated from it by the pyloric sphincter. Both sphincters act as one–way valves, ensuring that food passes only in one direction. The stomach with elastic walls can stretch to accommodate about 2 liters of food and fluid. The stomach stores food, secretes a digestive fluid called gastric juice and mixes this secretion with the food by the churning action of the smooth muscles in the stomach wall.

Gastric juice with a high concentration of hydrochloric acid has a pH of about 2, acidic enough to dissolve iron nails. The acid disrupts the extracellular matrix that binds cells together in meat and plant material and also kills most bacteria that are swallowed with food. Pepsin, an enzyme present in gastric juice begins the hydrolysis of proteins. Pepsin breaks peptide bonds adjacent to specific amino acids, cleaving proteins into smaller polypeptides, which are later digested completely to amino acids in the small intestine. Pepsin is one of the few enzymes that work best in a strongly acidic environment. The low pH of gastric juice unfolds the proteins in food, increasing exposure of their peptide bonds to pepsin. The release of pepsin stimulates the muscles of the stomach wall to contract weakly in a wave pattern (peristalsis). This mixes the food thoroughly with the gastric juices, the final mixture being known as chyme, which is passed to small intestine for further processing.

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