Lecture 5: The Stomach

Overview of the Stomach

• Resembles a long, wrinkled sausage when empty.
• Connects esophagus to pyloric sphincter.
• Main functions:
  • Mixing and storing food
  • Starting protein digestion
  • Continuing triglyceride digestion
  • Converting bolus into chyme

Functions of the Stomach

1. Storage of Food
2. Mechanical Breakdown
3. Chemical Digestion

Regions of the Stomach

• Cardia
• Fundus
• Body
• Pylorus
  • Connected to pyloric sphincter

Structure

• Muscle Layers: Three layers for mechanical digestion (unlike other GI tract areas with two layers)
• Mucosa: Specialized with goblet/mucous cells
• Rugae: Folds of mucosa when stomach is empty

Pyloric Sphincter Abnormalities in Newborns

• Pyloric Stenosis: Narrowing causing blockage
• Pyloric Spasm: Muscle fibers don’t relax
• Symptoms: Projectile vomiting, inability to keep food down

Anatomy

• Curvatures:
  • Greater curvature connects to greater omentum
  • Lesser curvature connects to lesser omentum
• Sphincters:
  • Lower esophageal sphincter (top)
  • Pyloric sphincter (bottom)

Digestive Process

• Mechanical Digestion:
  • Churning like a washing machine
  • Sphincters closed during churning
• Chyme Formation:
  • Gradual release into duodenum

Gastric Histology

• Gastric Pits and Glands:
  • Located in the mucosa
  • Contain different cell types:
    • Mucous neck cells: Secrete mucus
    • Chief cells: Secrete pepsinogen and gastric lipase
    • Parietal cells: Secrete hydrochloric acid and intrinsic factor
• Enteroendocrine Cells:
  • G cells secrete gastrin

Conditions and Disorders

• Zollinger-Ellison Syndrome: Excess hydrochloric acid production due to too much gastrin

Chemical Processes

• Hydrochloric Acid:
  • Converts pepsinogen to active pepsin
  • Creates acidic environment for digestion
• Intrinsic Factor:
  • Needed for vitamin B12 absorption

Gastric Juice and Hormones

• Gastrin Hormone:
  • Stimulates gastric juice release
  • Increases gastric motility
  • Regulates sphincters

Stomach Wall and Absorption

• Impermeable to most substances
• Exceptions: Water, electrolytes, alcohol, certain drugs

Alcohol Metabolism

• Alcohol Dehydrogenase:
  • Found in stomach lining and liver
  • Converts alcohols to aldehydes/ketones
  • Varies by sex, age, and race

Gastric pH Regulation

• Influences on pH:
  • Food intake
  • Gastric juice acidity
• Homeostasis:
  • Submucosal plexus and parasympathetic nervous system regulate acidity and muscular contractions

This lecture covered the anatomy, function, and histology of the stomach, along with disorders and the underlying chemical processes vital for digestion.