| Small intestine (1–3 cm diameter; length, duodenum 0.4 m, jejunum 2.5 m, ileum 3.5 m) | Absorbs > 95% of essential dietary nutrients (e.g., water, ions, amino acids, peptides, glucose, fructose, lipids, iron, vitamins). Peristalsis mixes and moves chyme distally. Secretions neutralize stomach acid and chemically digest food (e.g., bile salts). Endocrine feedback blocks stomach acid production and stimulates pancreatic insulin secretion. Supports local immunity against pathogens. | Surface of the luminal epithelium is covered with microvilli, which contain digestive enzymes and membrane nutrient transporters. Aerobic environment with pH of 6-7. Partial pressure of oxygen (pO2) in terminal ileum tissue, 33 mmHg.
Blood supplied by superior mesenteric artery. At rest, intestinal capillaries receive ~20% of total cardiac output, but ~75% of these capillaries are not perfused when unfed (i.e., without glucose or other nutrients). | Aerobic species and facultative anaerobes (with bimodal metabolism) predominate. Gene activity analysis indicates: amino acids > carbohydrates > cofactors/vitamins > lipid metabolism. | Duodenum and jejunum, <103; Ileum, 103–107. |
| Large intestine (6 cm diameter; length, cecum 0.2 m, colon 1.4 m, rectum 0.1 m) | Absorbs unabsorbed water and vitamins. Secretions neutralize acidic pH. Compacts waste for elimination. Rectum stores feces temporarily. Supports local immunity against pathogens. | Microvilli are absent. Anaerobic environment (pO2 of sigmoid colon tissue, 3 mmHg; rectum, <1 mm Hg) with a pH of 5–7. The lumen center has a pO2 < 0.1 mmHg. | Caecum has greatest diversity (500–1,000 species); facultative anaerobes dominate. Distal colon has smallest species diversity; obligate anaerobes dominate. | Colon, 109–1012; Feces, 1010–1012 |
