Digestive System Lecture Notes

Gastric Juice Formation

• Parietal Cells: Secrete hydrochloric acid (HCl) and intrinsic factor.
  • HCl: Contributes to acidic environment in stomach.
  • Intrinsic Factor: Essential for vitamin B12 absorption.
• Chief Cells: Secrete pepsinogen, an inactive enzyme precursor.
  • Pepsin: Activated by HCl, breaks down proteins into smaller polypeptides.

Stomach Structure and Function

• Elastic, J-shaped organ divided into:
  • Fundus: Superior, rounded region.
  • Body: Central region.
  • Pylorus: Inferior region.
• Contains two curvatures and two sphincters:
  • Gastroesophageal/Cardiac Sphincter: Junction of esophagus and stomach.
  • Pyloric Sphincter: Between stomach and duodenum.
• Rugae: Folds in mucosa for elasticity.
• Muscularis Layer: Includes an oblique muscle layer for churning food into chyme.
• Mechanical vs. Chemical Digestion:
  • Mechanical: Churning mixes food with HCl.
  • Chemical: Enzymatic breakdown of food; HCl is not involved in chemical digestion.
• Protective mucus layer prevents stomach corrosion by HCl.

Small Intestine Structure and Digestion

• Length: 20-21 feet, 3/4 to 1 inch in diameter.
• Segments:
  • Duodenum: Initial segment.
  • Jejunum: Intermediate segment.
  • Ileum: Longest segment.
• Absorptive Structures:
  • Plique Circularis: Circular folds.
  • Villi: Microscopic finger-like projections.
  • Microvilli: Surface of epithelial cells, forms brush border.

Enzymes and Digestion

• Peptidase: Completes protein digestion.
• Maltase, Sucrase, Lactase: Break down disaccharides into glucose.
• Primary site of carbohydrate digestion is the small intestine.

Liver and Gallbladder

• Liver: Secretes bile, involved in detoxification, stores vitamins/minerals, breaks down old RBCs.
• Gallbladder: Stores and concentrates bile.
• Bile emulsifies fats, aiding in mechanical digestion.

Pancreas

• Both an exocrine and endocrine gland.
• Pancreatic Enzymes:
  • Amylase: Digests carbohydrates.
  • Lipase: Digests fats.
  • Trypsin: Continues protein digestion.
• Sodium Bicarbonate: Neutralizes stomach acid.

Large Intestine Function

• Length: 5 feet, 2.5 inches in diameter.
• Sections:
  • Cecum: Beginning pouch.
  • Colon: Ascending, transverse, descending, sigmoid.
  • Rectum and Anus: Terminal.
• Absorbs remaining water/nutrients, forms feces.
• Appendix: Vestigial tissue, inflammation leads to appendicitis.

Peritoneum and Mesentery

• Peritoneum: Serous membrane with visceral (covers organs) and parietal (lines cavity) layers.
• Mesentery: Anchors intestines, provides pathway for vessels and nerves.
• Omenta: Fat storage, prevents friction.

Digestive Process Summary

• Mechanical Digestion: Physical breakdown, no chemical change.
• Chemical Digestion: Enzymatic breakdown, alters structure.
• Absorption: Nutrients move from intestines to blood/lymph.
• Enzyme breakdown products for carbohydrates, proteins, and fats reviewed.

Final Checkpoint

• Overview of enzyme functions and digestion processes highlighted throughout the lecture.