Small Intestine Function: Mechanical and Chemical Digestion, Nutrient Absorption, and Water Balance

The small intestine is the primary site of digestion and absorption:

1. Mechanical digestion – Peristalsis mixes and propels chyme.
2. Chemical digestion – Pancreatic and brush-border enzymes break down:
   • Carbohydrates → glucose (amylase)
   • Proteins → amino acids (trypsin, chymotrypsin)
   • Fats → glycerol + fatty acids (lipase; bile emulsifies)
3. Nutrient absorption – Villi and microvilli vastly increase surface area:
   • Glucose/amino acids → blood capillaries
   • Fats → lacteals (lymphatic) → blood
4. Water/electrolyte absorption – Most water reclaimed here.
5. Motility – Peristalsis and segmentation optimize mixing and absorption.