Stomach

The stomach lies in the upper left abdomen, just below the diaphragm, between the end of the esophagus and the duodenum, where the small intestine begins. It has four regions: the cardia, where food enters; the dome-shaped fundus; the large central body; and the funnel-shaped pylorus, which ends in the pyloric sphincter that controls emptying. When empty it is roughly fist-sized, with its lining gathered into folds called rugae, but it can stretch to hold several liters.

An extra, oblique layer of smooth muscle in its wall lets it churn food forcefully. Gastric glands in the lining produce gastric juice. Parietal cells secrete hydrochloric acid, which makes the contents strongly acidic, kills many microbes and unfolds proteins, along with intrinsic factor, needed later to absorb vitamin B12. Chief cells release pepsinogen, which the acid converts into the protein-digesting enzyme pepsin, and G cells release the hormone gastrin. The result is a thick liquid called chyme, passed a little at a time into the duodenum. Very little is absorbed in the stomach apart from alcohol and some drugs.

A coat of alkaline mucus and tight junctions between lining cells stop the stomach digesting itself. When this defense fails, often because of Helicobacter pylori infection or regular use of NSAIDs, peptic ulcers can form.