Absorption of Fluid and Electrolytes

Fluid delivery to the GI tract is 8-10 L/d, including 2 L/d ingested; most is absorbed in small bowel. About 2 L/d is delivered to the colon; about 0.2 L/d is excreted in the stool. Colonic absorption is normally 0.05-2 L/d, with capacity for 6 L/d if required. Intestinal water absorption passively follows active transport of Na⁺ , Cl^- , glucose, and bile salts. Additional transport mechanisms include Cl^- /HCO₃^- exchange, Na⁺ /H⁺ exchange, H⁺ , K⁺ , Cl^- , and HCO₃^- secretion, Na⁺ -glucose cotransport, and active Na⁺ transport across the basolateral membrane by Na⁺ ,K⁺ -ATPase.

Nutrient Absorption

1. Proximal small intestine: iron, calcium, folate, fats (after hydrolysis of triglycerides to fatty acids by pancreatic lipase and colipase), proteins (after hydrolysis by pancreatic and intestinal peptidases), carbohydrates (after hydrolysis by amylases and disaccharidases); triglycerides absorbed as micelles after solubilization by bile salts; amino acids and dipeptides absorbed via specific carriers; sugars absorbed by active transport
2. Distal small intestine:vitamin B₁₂, bile salts, water
3. Colon: water, electrolytes

Intestinal Motility

Allows propulsion of intestinal contents from stomach to anus and separation of components to facilitate nutrient absorption. Propulsion is controlled by neural, myogenic, and hormonal mechanisms; mediated by migrating motor complex, an organized wave of neuromuscular activity that originates in the distal stomach during fasting and migrates slowly down the small intestine. Colonic motility is mediated by local peristalsis to propel feces. Defecation is effected by relaxation of internal anal sphincter in response to rectal distention, with voluntary control by contraction of external anal sphincter.