Key Functions of the Stomach (Review)

• Temporary storage to slow food transit to the small intestine and maximize nutrient absorption.
• Physical Breakdown (like in the mouth)
• Chemical Breakdown of proteins into their amino acids (at the same time that salivary amylase from the mouth continues to breakdown carbohydrates in the stomach).

Three Gastric Phases (Review)

1. Filling, in which food enters the stomach through the gastroesophageal sphincter.
2. Mixing, in which peristaltic contractions churn the food while the gastric lining secretes juices to produce chyme.
3. Emptying, in which peristaltic contractions propel chyme into the small intestine.

Mixing Phase – In Depth

• Peristalsis – contractions of circular smooth muscle, move from fundus to antrum
  – Pushes the stomach’s contents towards the pyloric sphincter.
  – Facilitates physical breakdown of food
• Pyloric sphincter almost closed
  – Forces the chyme to spill backwards into the antrum (stomach’s body) and continues mixing.

Exocrine Cells of Stomach

• Located in tubular gastric glands that comprise gastric pits
  – Epithelial cells at entrance of gastric pits: secrete thick mucus
  – Mucous layer
  – Submucosa layer
• Secrete products into stomach lumen
• Secretions convert food to chyme

Exocrine Cell Types

• Mucous cells (mucous neck cells): secrete alkaline, bicarbonate mucus, which protects our stomach wall from erosion in an acidic luminal environment.
• Chief cells: secrete pepsinogen, an inactive enzyme that, once activated, breaks down proteins.
• Pepsinogen is a zymogen
  – An inactive enzyme that, once activated, breaks down proteins.
  – A substance must convert to its active form, pepsin
• Pepsin
  – Breaks down peptide bonds to promote chemical breakdown.
• Parietal cells
  – Secrete HCl which denature proteins.
• HCl functions:
  – Converts pepsinogen → its active form: pepsin.
  – Aids in the breakdown of food → smaller particles.
  – Denatures proteins via its acidic environment.
  – Kills most of the microorganisms that we ingest with our food, thus, providing a protective function. (Tight junctions between the digestive tract epithelium and mucus production serve as a protective barrier).
  – Facilitates chemical breakdown; it denatures proteins but, unlike pepsin, it doesn’t break peptide bonds.

Stem cells also located in gastric pit

• Rapidly divide and mature into cells that produce gastric mucosa
  – Replenishes gastric mucosal cells every 3 days due to constant exposure to the harsh, acidic environment in the stomach.

Clinical correlation

• Peptic ulcers
  – Erosions that penetrate our gastric mucosal layer.
• Pepsin and HCl access exposed regions and erode the stomach wall.

GASTRIC EMPTYING IN DEPTH

• Induced by strong antral contraction
  – Antrum has thicker layer of smooth muscle, which allows more forceful contraction
• Antral contraction pushes chyme through the pyloric sphincter
• The volume of chyme that passes depends on the force of antral contraction.
  – Despite the force of antral contraction, only a little chyme enters the duodenum, which is where nutrient absorption occurs.
  – Pyloric sphincter limits the flow of chyme to promote slow and efficient absorption in the duodenum.