Gastric Physiology: Anatomy, Secretory Function, and Digestive Role

The stomach is a J-shaped muscular organ in the left upper quadrant (LUQ) serving as a reservoir, mixer, and chemical processor of ingested food.

Anatomical Regions

• Cardia: Esophageal junction; lower esophageal sphincter (LES)
• Fundus: Superior dome; gas accumulation, accommodation
• Body (corpus): Main secretory region; parietal cells (HCl, IF), chief cells (pepsinogen)
• Antrum: Distal 30%; G-cells (gastrin), antral mixing waves
• Pylorus: Pyloric sphincter (tonic contraction + phasic waves) regulates gastric emptying

Secretory Function

Cephalic Phase (30% of acid)

Vagal (sight, smell, taste) → ACh → parietal cells (M3) + ECL cells (histamine) → H⁺/K⁺-ATPase activation

Gastric Phase (60%)

Distension + peptides → local reflexes + gastrin (G-cells) → CCK2 on ECL → histamine → parietal cells

Intestinal Phase (10%)

Duodenal distension, acid, fats → enterogastric reflex (inhibitory) + CCK, secretin, GIP → slow emptying

Key Secretory Products

Motor Function

• Receptive relaxation: Fundus accommodates volume without pressure rise (vagal, NO/VIP)
• Peristalsis: Pacemaker (interstitial cells of Cajal) → 3/min antral waves → grinding/sieving (1-2mm particles pass)
• Gastric emptying: Liquids: exponential (t½ 10-20min); Solids: linear (after lag phase) — regulated by duodenal feedback

Clinical Correlates

• Gastroparesis: Delayed emptying (diabetes, post-viral, post-surgical)
• PUD: Acid/pepsin > mucosal defense (H. pylori, NSAIDs)
• Gastric cancer: Intestinal vs. diffuse type; H. pylori major risk
• Dumping syndrome: Rapid emptying post-gastrectomy

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Cell/Product	Function	Regulation
Parietal: HCl	Protein denaturation, pepsin activation, microbial kill, intrinsic factor conformation	ACh, gastrin, histamine (H2); inhibited by somatostatin, PGE2, secretin
Parietal: Intrinsic Factor	B12 absorption (terminal ileum)	Parallel with acid
Chief: Pepsinogen	→ Pepsin (pH<4); protein hydrolysis	ACh, acid
G-cell: Gastrin	Stimulates acid, mucosal growth, motility	Peptides, distension; inhibited by acid (pH<3), somatostatin
Surface mucous: Mucus/HCO₃⁻	Epithelial barrier (pH gradient 1→7)	PGE2, trefoil peptides