Foods in the mouth are formed into a compact bolus by the action of the tongue and mastication. The bolus is swallowed and passes down the esophagus to enter the stomach at its cardia. The stomach has folds (rugae) that allow it to expand when filled with food. The somach has three muscle layers: longitudinal, circular, and oblique. The oblique muscle layer, which is only found in the stomach, contributes to churning contractions. The fundus and body exerts pressure on antrum of stomach which mixes and grinds food. The myenteric plexus, which is part of the enteric nervous system, innervates muscle layers in the stomach and throughout the gastrointestinal (GI) tract.
The bolus entering the stomach is mixed with gastric acid (HCl) to unfold proteins and activate the digestive enzyme pepsin. Pepsin begins digestion of proteins in the stomach and alcohol is absorbed directly into the stomach wall. The stomach mixes and breaks down foods into fine particles in suspension (chyme). Chyme is slowly released into the small intestine (duodenum) through the pyloric sphincter. Only small amounts enter the duodenum at a time to allow the small intestine enough time to fully digest what it receives from the stomach.
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