Digestion in the Small Intestine

Overview

• Chyme from the Stomach: Contains partially digested carbohydrates and proteins; fats remain largely undigested.
  • Chyme is radically different from ingested food.
• Time in Small Intestine: 3 to 6 hours depending on fat content.
  • Higher fat content = closer to 6 hours
  • Higher carbohydrate content = closer to 3 hours

Functions and Adaptations

• Chemical Digestion and Absorption: Major role in digestion, not in ingestion or defecation.
• Water and Nutrient Absorption: Majority of water and almost all nutrients absorbed.

Chyme Delivery and Adaptations

• Slow Delivery of Acidic and Hypertonic Chyme:
  • Nutrient-dense chyme can disrupt circulatory water balance if delivered too quickly.
• Mitigation of Acidity:
  • Accessory Glands:
    • Pancreas releases bicarbonate-rich pancreatic juice to neutralize low pH.
    • Fluid in pancreatic juice reduces hypertonicity.
  • Bile:
    • Emulsifies fats, aiding pancreatic enzymes in lipid breakdown via lipases.

Mechanical Digestion: Segmentation

• Primary Motion in Small Intestine:
  • Intrinsic rate set by pacemaker cells.
  • Alternating contraction and relaxation leads to chyme mixing.
  • Intensity modulated by parasympathetic activity.
  • Sympathetic (fight-or-flight) response decreases motility.

Peristalsis

• Housekeeping Role:
  • Clears out small intestine post-meal.
  • Initiated by hormone motilin during the intestinal phase (90-120 minutes after reaching the stomach).
  • Peristalsis progresses distally to remove meal remnants, bacteria, debris.
  • Known as the Migrating Motor Complex.

Control of Motility

• Local Enteric Neurons:
  • Parasympathetic system dominant.
  • Acetylcholine-based neurons activate contraction.
  • Uses circular and longitudinal muscles to move chyme along the GI tract.

Ileal Sphincter and Valve

• Default Position: Ileal sphincter contracted to prevent movement into the large intestine.
• Relaxation Conditions:
  • Gastroileal Reflex: Enhances ileum segmentation to move material into the large intestine.
  • Gastrin Hormone: Released in intestinal phase, increases ileum motility.
• Closure of Ileal Valve:
  • Backward pressure from large intestine causes flap to close, preventing backflow into the small intestine.