(See also Overview of the Digestive System.)
The stomach is a large, bean-shaped, hollow muscular organ consisting of three regions:
Food and fluids enter the stomach from the esophagus by passing through the lower esophageal sphincter.
The upper stomach serves as a storage area for food. Here, the cardia and body of the stomach relax to accommodate food that enters the stomach. Then the antrum (lower stomach) contracts rhythmically, mixing the food with acid and enzymes (stomach juices) and grinding the food down into small pieces so that it is more easily digested. The cells lining the stomach secrete three important substances: mucus, hydrochloric acid, and the precursor of pepsin (an enzyme that breaks down proteins). Mucus coats the cells of the stomach lining to protect them from being damaged by acid and enzymes. Any disruption of this layer of mucus, such as that resulting from infection by the bacterium Helicobacter pylori or from aspirin, can result in damage that leads to a stomach ulcer.
Hydrochloric acid provides the highly acidic environment needed for pepsin to break down proteins. The stomach's high acidity also serves as a barrier against infection by killing most bacteria. Acid secretion is stimulated by nerve impulses to the stomach, gastrin (a hormone released by the stomach), and histamine (a substance released by the stomach). Pepsin is the only enzyme that digests collagen, which is a protein and a major part of meat.
Only a few substances, such as alcohol and aspirin, can be absorbed directly into the bloodstream from the stomach and only in small amounts.