Stomach

The stomach serves as a muscular reservoir that stores ingested food and progressively releases it into the duodenum. Its thick muscular walls churn food through peristaltic contractions, mechanically breaking it into smaller particles. Gastric glands secrete hydrochloric acid (creating a pH of 1.5-3.5) and pepsinogen (activated to pepsin), which begin protein digestion. Parietal cells also produce intrinsic factor, essential for vitamin B12 absorption in the terminal ileum.

Helicobacter pylori is a gram-negative bacterium that colonizes the gastric mucosa and disrupts its protective mechanisms by producing urease (which neutralizes acid locally), protease, and cytotoxins. It weakens the mucus layer, damages epithelial cells, and triggers chronic inflammation, predisposing the mucosa to erosion by gastric acid. H. pylori infection is present in over 90% of duodenal ulcers and about 70% of gastric ulcers, and eradication with antibiotics substantially reduces ulcer recurrence.

GERD occurs when gastric contents reflux into the esophagus due to dysfunction of the lower esophageal sphincter (LES)—the muscular barrier between the stomach and esophagus. Risk factors include obesity (increased intra-abdominal pressure), hiatal hernia (which displaces the LES above the diaphragm), pregnancy, certain foods (fatty foods, caffeine, alcohol), and lying flat after meals. Chronic acid exposure causes heartburn, regurgitation, and can lead to Barrett's esophagus—a precancerous condition.

Intrinsic factor is a glycoprotein secreted by gastric parietal cells that is essential for the absorption of vitamin B12 (cobalamin). It binds to B12 in the stomach and protects it from digestion, then delivers it to specific receptors in the terminal ileum for absorption. Conditions that destroy gastric parietal cells—such as autoimmune gastritis (causing pernicious anemia) or total gastrectomy—eliminate intrinsic factor production and lead to vitamin B12 deficiency, causing megaloblastic anemia and neurological damage.

The stomach is divided into four anatomical regions: the cardia (immediately below the gastroesophageal junction) transitions food from the esophagus; the fundus (upper dome) stores swallowed air and food; the body (largest region) contains most of the acid-secreting parietal cells and pepsinogen-secreting chief cells; and the pyloric antrum and canal control the rate of gastric emptying into the duodenum through the pyloric sphincter, which prevents duodenogastric reflux.