Digestion in the Stomach

Stomach Anatomy

• The stomach is a widened section of the digestive tract, connecting the esophagus to the small intestine (duodenum).
• Sphincters:
  • Lower esophageal sphincter: controls passage from the esophagus to the stomach.
  • Pyloric sphincter: controls passage from the stomach to the duodenum.
• Regions of the Stomach:
  • Fundus: The superior, domed region; acts as a holding area.
  • Body: The main central region.
  • Pylorus: The narrowed, funnel-shaped terminal region.

Features of Stomach Lining

• Rugae:
  • Temporary folds in the mucosa of the stomach, visible when the stomach is empty (nondistended).
  • Allow for expansion when the stomach is full.
• Gastric Mucosa:
  • Lined by simple columnar epithelium.
  • Contains microscopic folds called gastric pits.
    • Gastric pits are infolds in the epithelium.
    • The upper portion of the pit is called the neck region.
    • Deeper portions of the pits branch into gastric glands.

Gastric Glands and Secretions

• Gastric glands are responsible for secretion.
• Cell Types in Gastric Glands:
  • Mucous neck cells: Secrete mucus and bicarbonate ions to protect the stomach lining.
    • The mucosa of the stomach is protected by a thick alkaline mucus layer.
  • Parietal cells: Secrete hydrochloric acid (HCl) and intrinsic factor.
    • Intrinsic factor is necessary for vitamin B12 absorption.
  • Enterochromaffin-like (ECL) cells: Secrete histamine, which stimulates parietal cells to secrete HCl.
  • Chief cells: Secrete pepsinogen (precursor to pepsin) and gastric lipase (in infants).
    • Pepsinogen is activated to pepsin in the acidic environment of the stomach.
  • G cells: Enteroendocrine cells that secrete gastrin.
    • Gastrin stimulates parietal cells to secrete HCl.
• Volume and pH of Gastric Juice:
  • Gastric glands produce 1-3 liters of gastric juice per day.
  • Gastric juice has a very low pH (1-2) due to HCl.

Digestion in the Stomach

• Mechanical Digestion:
  • Churning: Mixing of food boli with gastric juice.
  • The stomach has three layers of smooth muscle in the muscularis: circular, longitudinal, and oblique.
  • Contraction of these layers mixes the contents, breaking down food boli into chyme.
• Chemical Digestion:
  • Protein digestion begins in the stomach.
  • HCl denatures proteins, exposing peptide bonds.
  • Pepsin breaks down proteins into smaller peptide fragments.
  • Carbohydrate digestion does not continue in the stomach due to the acidic environment.

Absorption in the Stomach

• Limited absorption occurs in the stomach.
• Substances absorbed: small amounts of water, ions, small fatty acids, acidic drugs (e.g., aspirin), and alcohol.
• Alcohol can be absorbed quickly on an empty stomach, leading to a rapid increase in blood alcohol levels.

Metabolic Contributions of the Stomach

• The acidic environment aids in the absorption of iron by reducing it.
• Intrinsic factor, produced by parietal cells, is necessary for vitamin B12 absorption, which is important for cell division.