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.

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 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.

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.

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.

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.