Overview
This lecture covers the physiology of the stomach, its structure, secretions, mechanisms of acid production, functions, and the regulation of gastric secretions.
Structure of the Stomach
- The stomach consists of the cardiac region, fundus, body, pylorus, and pyloric sphincter.
- The stomach wall contains folds called rugae.
Functions of the Stomach
- Acts as a reservoir and controls the release of digested food into the small intestine.
- Mixes and mechanically breaks down food.
- Performs hydrolytic digestion, mainly of proteins.
- Kills bacteria and secretes intrinsic factor for vitamin B12 absorption.
- Forms food products and regulates emptying into the small intestine.
Gastric Mucosa and Secretory Cells
- Surface mucous and mucous neck cells secrete mucus to protect the lining.
- Parietal cells secrete hydrochloric acid (HCl) and intrinsic factor.
- Chief (peptic) cells secrete pepsinogen and gastric lipase.
- G cells secrete the hormone gastrin.
Gastric Secretions
- Stomach secretes 2.5–3 liters/day at pH 1–2.
- HCl lowers pH, kills bacteria, activates pepsinogen, and aids iron absorption.
- Mucus protects the lining and acts as a lubricant.
- Pepsinogen is activated by HCl to pepsin, which digests proteins.
- Other secretions: lipase, intrinsic factor, electrolytes.
Mechanism of HCl Secretion
- CO₂ and H₂O form H₂CO₃, which splits into H⁺ and HCO₃⁻ in parietal cells.
- H⁺ is pumped into the stomach lumen via an H⁺/K⁺ ATPase pump.
- HCO₃⁻ is exchanged for Cl⁻, which then moves into the lumen.
- H⁺ and Cl⁻ combine in the lumen to form HCl.
- HCO₃⁻ entering the blood causes postprandial alkaline tide.
Functions of HCl
- Kills bacteria and aids protein digestion.
- Activates pepsinogen to pepsin.
- Provides optimum pH (2–3) for pepsin activity.
- Stimulates bile and pancreatic juice flow.
Regulation of Gastric Secretion
- Cephalic Phase: Initiated by sight, smell, or thought of food; mediated by vagus nerve.
- Gastric Phase: Triggered by food in the stomach; involves vagal and hormonal (gastrin) mechanisms.
- Intestinal Phase: Triggered by food in the duodenum; inhibits gastric secretion through nerves and hormones (secretin, cholecystokinin).
Gastrin Hormone
- Secreted by G cells in response to protein digestion products and stomach distension.
- Stimulates gastric mucosa growth, motility, and, slightly, gallbladder contraction.
- Zollinger-Ellison syndrome involves excessive gastrin production.
- Acid in the antrum inhibits further gastrin release.
Key Terms & Definitions
- Rugae — folds in the stomach wall aiding expansion.
- Parietal cells — cells that secrete HCl and intrinsic factor.
- Chief cells — cells secreting pepsinogen and gastric lipase.
- G cells — cells releasing the hormone gastrin.
- Postprandial alkaline tide — rise in blood bicarbonate after a meal due to HCl secretion.
- Gastrin — hormone stimulating gastric juice secretion and motility.
Action Items / Next Steps
- Review the three phases of gastric secretion and their regulation.
- Memorize the function and mechanism of HCl secretion.
- Prepare to answer short questions on stomach secretions for exams.