Lecture Notes: Digestive Processes in the Stomach

Overview

• The stomach serves as a holding area for consumed material and plays a role in digestion.
• Minimal digestion occurs in the stomach, primarily mechanical and chemical breakdown.

Mechanical and Chemical Breakdown

• Mechanical Breakdown:
  • Churning of food into smaller morsels by oblique muscle groups.
• Chemical Breakdown:
  • Highly acidic environment denatures proteins, increasing surface area for enzymes.
  • Enzymes in Stomach:
    • Pepsin: Activated from pepsinogen by low pH, begins protein breakdown.
    • Renin: In infants, breaks down milk protein casein, causing curdling.

Absorption in the Stomach

• Limited absorption occurs:
  • Alcohol and aspirin are absorbed directly due to lipid solubility.

Role of Intrinsic Factor

• Essential for vitamin B12 absorption in the small intestine.
• Lack of intrinsic factor (due to surgeries or pernicious anemia) leads to anemia.
• Treatments include vitamin B12 injections or nasal gels.

Regulation of Stomach Activity

• Neural Regulation:
  • Parasympathetic (vagus nerve) stimulates gastric activity.
  • Sympathetic activity decreases gastric secretion.
• Hormonal Regulation:
  • Gastrin: Increases hydrochloric acid production.
  • Countered by hormones from the small intestine (e.g., enterogastrones).

Phases of Gastric Secretion

• Cephalic Phase:
  • Triggered by thought, smell, or taste of food, initiating stomach acidity.
• Gastric Phase:
  • Stimulation by stomach distension and peptides in food.
  • Peptides act as buffers, affecting pH and stimulating gastrin.
• Intestinal Phase:
  • Initially excitatory, but primarily inhibitory to prevent overload of duodenum.

Enterogastric Reflex and Hormones

• Hormones: Secretin, Cholecystokinin (CCK), Vasoactive Intestinal Peptide (VIP).
• These hormones inhibit gastrin to manage stomach acid production and prevent duodenal overload.

Hydrochloric Acid Secretion

• Reaction: Water + CO₂ → Carbonic Acid → H+ + Bicarbonate.
• Chloride shift allows Cl- and H+ into stomach, forming HCl.

Issues and Syndromes

• Dumping Syndrome: Excessive chyme entering the small intestine can lead to nausea/vomiting.
  • Common in gastric bypass patients if overeating occurs.

Summary of Phases

• Cephalic and gastric phases are mainly stimulatory for acid production.
• Intestinal phase is primarily inhibitory to prevent excessive acid secretion.

These notes capture the essential elements of stomach digestion, hormonal and neural regulation, and related conditions discussed in the lecture.