The Stomach's Role in Digestion: Mechanical and Chemical Processes

Mechanical Digestion

• Churning: Vigorous contractions of the muscularis externa break food into smaller particles, increasing surface area for enzymatic action.
• Mixing (peristalsis): Peristaltic waves mix food with gastric juice, forming a semi-liquid mixture.

Chemical Digestion

• Acidic environment: Parietal cells secrete HCl (pH 1.5–2.5), denaturing proteins, killing microbes, activating pepsinogen.
• Enzymatic breakdown:
  • Pepsin (from chief cells' pepsinogen): Cleaves proteins into polypeptides
  • Gastric lipase: Initiates triglyceride digestion (minor role; pancreatic lipase dominates)

Mucosal Protection

• Mucus-bicarbonate barrier: Surface mucous cells secrete alkaline mucus; epithelial tight junctions prevent acid back-diffusion
• Prostaglandins: Maintain mucosal blood flow and bicarbonate secretion
• Rapid epithelial turnover: Cells replaced every 3–7 days

Chyme Formation

The resulting semi-fluid mixture (chyme) is released in controlled amounts through the pyloric sphincter into the duodenum for further digestion and absorption.