Absorption and Excretion Unit: Digestive System Overview

Introduction to the Digestive System

• The digestive system converts food into energy and raw materials for the body.
• Breaks down food mechanically and chemically into simpler forms for absorption.
• Themes: Efficiency in using and storing nutrients, avoiding waste.

Digestive Process Overview

1. Ingestion
   • Involves eating and the use of accessory structures like teeth and salivary glands.
   • Mechanical breakdown begins with chewing.
2. Propulsion
   • Movement of food through the GI tract via swallowing and peristalsis.
   • Involves voluntary and involuntary muscle actions.
3. Mechanical Breakdown
   • Physical breakdown to increase surface area for better nutrient absorption.
4. Chemical Digestion
   • Use of enzymes to break down food into monomers for nutrient absorption.
5. Absorption
   • Transport of digested nutrients into blood or lymph.
6. Defecation
   • Elimination of indigestible substances as feces.

Key Structures of the Digestive System

• Alimentary Canal (GI Tract): Mouth, pharynx, esophagus, stomach, small and large intestines.
• Accessory Organs: Teeth, tongue, salivary glands, gallbladder, liver, pancreas.
• Length: Approximately 9 meters (30 feet) if extended.

Digestive System Histology

• Layers of the GI Tract:
  • Mucosa: Secretes mucus, absorbs nutrients, protects against disease.
  • Submucosa: Provides elasticity.
  • Muscularis externa: Smooth muscle for propulsion.
  • Serosa: Outer connective tissue layer.
• Epithelial Cells:
  • Stratified squamous: Protection against abrasion in mouth, esophagus, anus.
  • Simple columnar: Absorption and secretion in stomach, intestines.

Digestive Enzymes and Their Functions

• Amylase: Begins starch digestion in the mouth.
• Pepsin and HCl: Start protein digestion in the stomach.
• Pancreatic Enzymes: Digest proteins, fats, and starches.
• Intestinal Enzymes: Break down peptides, sucrose, lactose, maltose into simpler sugars and amino acids.
• Bile Salts: Assist in fat digestion.

Regulation of the Digestive System

• Enteric Nervous System: Independent system regulates digestive activities, communicates with the CNS via the vagus nerve.
• Endocrine System: Hormones like gastrin, serotonin, and histamine regulate digestion.
• Phases of Gastric Fluid Secretion:
  • Cephalic Phase: Brain response to food stimuli.
  • Gastric Phase: Stomach response to food presence.
  • Intestinal Phase: Intestinal response to chyme.

Conclusion

• Overview of digestive processes, structures, and regulation.
• Class activity: Modeling the digestive system.