Stomach

The stomach is a sac-like organ of the upper digestive tract, located between the esophagus and the small intestine. It functions as a reservoir for ingested food and a site of mechanical mixing and chemical digestion. The interior mucosa forms folds called rugae that allow expansion. Anatomically, it extends from the gastroesophageal junction to the pylorus and is divided into the cardia, fundus, body, and antrum.

The stomach wall comprises mucosa, submucosa, muscularis externa (with inner oblique, middle circular, and outer longitudinal layers), and serosa. Gastric glands in the fundus and body include parietal cells that secrete hydrochloric acid and intrinsic factor, chief cells that secrete pepsinogen, and G cells that release gastrin. Cardia and pylorus glands largely secrete mucus.

Gastric juice contains acid, enzymes, mucus, and bicarbonate. HCl reduces pH and activates pepsinogen to pepsin, which digests proteins; lipases contribute minor fat digestion. Intrinsic factor is essential for vitamin B12 absorption in the ileum. Absorption in the stomach is limited to water, some electrolytes, alcohol, and certain drugs; most nutrients are absorbed later in the small intestine.

Regulation involves the vagus nerve and the enteric nervous system, along with hormones such as gastrin (stimulates acid secretion) and somatostatin (inhibits). The pyloric sphincter regulates chyme passage to the duodenum, and gastric emptying is influenced by meal composition, with fats typically delaying emptying. Common conditions include gastritis, peptic ulcers, gastric cancer, and gastroparesis.