Digestive System Lecture Notes

Lower Gastrointestinal (GI) Tract Overview

• Lower GI tract organs are responsible for the continued digestion and absorption of food.
• Functions include the elimination of indigestible and unabsorbable materials.

Small Intestine

• Divided into three sections: duodenum, jejunum, and ileum.
  • Duodenum: Receives chyme from the stomach, mixed with gastric juice.
    • Major site for chemical digestion with accessory organ secretions (pancreas, liver).
  • Jejunum: Main site for absorption.
  • Ileum: Absorbs remaining nutrients and connects to the large intestine via the ileocecal valve.

Accessory Organs

• Liver: Produces bile, which is concentrated and released by the gallbladder.
• Pancreas: Secretes pancreatic juice containing digestive enzymes and bicarbonate to neutralize stomach acid.

Large Intestine

• Absorbs water, electrolytes, and vitamins.
• Converts chyme into feces for elimination via the anus.

Disorders

• Inflammatory Bowel Disease (IBD): Includes Crohn's disease and ulcerative colitis.
  • Crohn's Disease: Can affect both small and large intestines; involves chronic inflammation.
  • Ulcerative Colitis: Affects only large intestine, primarily the colon.
• Irritable Bowel Syndrome (IBS): Involves diarrhea, cramps, bloating; common in women.

Structural Features of Small Intestine

• Designed to maximize absorption through:
  • Circular Folds: Increase surface area, slow chyme movement, more prominent in duodenum and jejunum.
  • Villi: Finger-like projections that contain capillaries and lacteals for nutrient absorption.
  • Microvilli: Further increase surface area on epithelial cells' apical surface.

Movement in the Small Intestine

• Segmentation: Mixes chyme with digestive secretions.
• Peristalsis: Propels chyme through the intestines.
• Migrating Motility Complex: Periodic sweeping of contents through the GI tract.

Liver Functions

• Produces bile, stores carbohydrates as glycogen, detoxifies absorbed substances.
• Synthesizes blood proteins like albumin and clotting factors.
• Processes nutrient-rich blood from the GI tract via the hepatic portal vein.

Bile

• Contains no digestive enzymes but includes bile salts for emulsification.
• Helps neutralize acid chyme in the duodenum with bicarbonate content.

Gallbladder

• Stores and concentrates bile produced by the liver.
• Gallstones can form from bile components causing pain and blockage.

Pancreas

• Endocrine Function: Produces insulin and glucagon.
• Exocrine Function: Produces pancreatic juice with enzymes (amylase, lipase, proteases, nucleases) and bicarbonate.

Regulation of Bile and Pancreatic Juice

• Cholecystokinin (CCK): Released in response to fatty chyme to stimulate bile and enzyme secretion.
• Secretin: Stimulates bicarbonate release in response to acidic chyme, inhibits gastric secretions.

Additional Notes

• Cirrhosis: Liver damage due to excess alcohol, drugs, or infections, leading to scar tissue formation.
• Pancreatic Cancer: Often develops without early symptoms; pain between scapulae can be a sign.
• Absorption in the small intestine is highly efficient, with the large intestine being even more efficient for water and electrolytes.