Notes on the Digestive System: Swallowing and Stomach Functions

Swallowing

• Swallowing (Deglutition): Involves over 22 muscles; coordinated by the swallowing center in the medulla oblongata.
• Peristalsis: Involuntary wave of muscular contraction that moves the bolus down the esophagus 😃.
  • Timing: Liquid reaches the stomach in 1-2 seconds; food in 4-8 seconds.
  • The lower esophageal sphincter relaxes to allow food into the stomach.

The Stomach

• Structure: Muscular sac in upper left abdomen; volume varies from 50 mL (empty) to 4 L (very full).
• Functions: Food storage, mechanical breakdown, and chemical digestion (produces chyme).

Stomach Anatomy

• Gross Anatomy: Features include the fundus, pyloric sphincter, gastric rugae (longitudinal wrinkles).
• Microscopic Anatomy:
  • Mucosa: Simple columnar epithelium with mucin-secreting cells; gastric pits and glands (cardiac, pyloric, gastric).
  • Gastric secretions: 2-3 L of gastric juice daily, composed of HCl and pepsin.

Gastric Secretions

• Hydrochloric Acid (HCl):
  • pH as low as 0.8, activates pepsin, helps liquefy food, and destroys pathogens.
• Pepsin: Active enzyme formed from pepsinogen; digests proteins into shorter peptide chains.

Gastric Motility

• Swallowing response: Signals stomach to relax; peristaltic contractions churn food every 20 seconds.
• Chyme passage: The antrum releases small quantities into the duodenum; total gastric emptying takes about 4-6 hours depending on contents.

Digestion and Absorption

• Stomach function: Partial digestion of proteins; minimal absorption of nutrients occurs here (e.g., aspirin, some drugs, alcohol).

The Liver, Gallbladder, and Pancreas

• Liver: Largest gland, secretes bile for digestion.
• Gallbladder: Stores bile; concentrates it by absorbing water and electrolytes.
• Bile composition: Contains bilirubin, bile acids (aid in fat digestion), and can form gallstones when concentrated too much.