Lecture on Digestion and the Gastrointestinal System

Overview

• Focus on the structure and function of the stomach and small intestine
• Digestion involves both chemical and mechanical processes

The Stomach

Location and Structure

• Lies in the upper left quadrant of the abdomen, below the diaphragm
• Composed of three muscle layers, including a diagonal muscle layer

Functions

• Secretes 2-3 liters (quarts) of gastric juices daily
• Gastric juice pH: 1.5-2 (very acidic)

Components of Gastric Juices

• Hydrochloric Acid (HCL): Provides acidity
• Mucus: Protects stomach lining
• Enzymes:
  • Pepsin: Protein-splitting enzyme
  • Pepsinogen: Inactive form of pepsin, activated by HCL
  • Intrinsic Factor: Important for Vitamin B12 absorption

Digestion Process

• Chemical Digestion: Liquefies food into a paste called chyme using HCL and enzymes
• Mechanical Digestion: Muscles contract in different directions to mix food

Phases of Gastric Juice Secretion

1. Cephalic Phase: Initiated by sight, smell, taste of food; mediated by the vagus nerve
2. Gastric Phase: Triggered by food presence, involves secretion of gastrin hormone
3. Intestinal Phase: Duodenum releases hormones to regulate chyme entry from the stomach

Anatomy

• Cardia: Inlet of the stomach
• Fundus: Area above the cardia, often contains gas
• Body: Main part
• Pyloric Antrum: Distal end, leads to pylorus
  • Pylorus: Gatekeeper, allows passage to the small intestine

Microscopic Structure

• Rugae: Longitudinal folds in the stomach lining
• Gastric Pits: Contain glandular cells
• Chief Cells: Secrete pepsinogen
• Parietal Cells: Secrete hydrochloric acid

Small Intestine

Structure and Function

• Length: 3-5 meters (yards)
• Begins at the pylorus; ends at the ileocecal valve

Sections

1. Duodenum:

   • First part, 12 finger-width long
   • Receives pancreatic enzymes and bile
   • Major site for chemical digestion

2. Jejunum:

   • Middle section
   • High surface area due to folds, villi, and microvilli
   • Primary site for nutrient absorption

3. Ileum:

   • Final section
   • Absorbs remaining nutrients and water

Absorption Process

• Villi and Microvilli: Increase surface area
• Enterocytes: Simple columnar cells that facilitate nutrient absorption
• Blood and Lymphatic Transport:
  • Water-soluble nutrients (proteins, carbohydrates) enter the bloodstream, routed to the liver
  • Fatty acids travel through lymphatic vessels before entering the bloodstream

Key Takeaways

• Digestion involves both chemical and mechanical processes
• The stomach and small intestine play crucial roles in breaking down food and absorbing nutrients
• Various phases and structures ensure efficient digestion and absorption