Small and Large Intestine: Digestion, Absorption, and Waste Elimination

The intestines complete digestion and reclaim nutrients/water.

Small Intestine (6–7 m) — Nutrient Absorption Hub

• Duodenum: Receives chyme + pancreatic enzymes + bile. Neutralizes acid.
• Jejunum/Ileum: Villi/microvilli absorb:
  • Carbohydrates → glucose, galactose, fructose.
  • Proteins → amino acids, di/tripeptides.
  • Fats → fatty acids, monoglycerides → reassembled to chylomicrons → lacteals.
  • Vitamins, minerals, water, electrolytes.

Large Intestine (1.5 m) — Water Reclamation & Microbiome

• Absorbs water, Na⁺, Cl⁻, K⁺ → solidifies waste.
• Microbiota ferment fiber → SCFAs (butyrate fuels colonocytes), synthesize vitamin K, B₁₂, biotin, folate.
• Stores feces until defecation (rectal distension → urge).

Clinical Relevance

• Small bowel disease → malabsorption, weight loss, diarrhea.
• Colonic disease → altered bowel habits, bleeding, pain.
• Microbiome diversity linked to immunity, metabolism, mood.

Protect with fiber, hydration, diverse plants, judicious antibiotics.