Absorption and Defecation
High-Yield Summary
- Most absorption occurs in the jejunum and ileum. Villi + microvilli massively increase surface area.
- Sugars and amino acids enter capillaries → hepatic portal circulation. Lipids enter lacteals → lymphatic system.
- Glucose/galactose: secondary active transport via SGLT1 (Na+ cotransport). Fructose: facilitated diffusion via GLUT5 (no ATP/ion coupling). Amino acids: sodium-dependent active transport (di/tripeptides absorbed intact too).
- Lipid absorption: micelles deliver fatty acids/monoglycerides to epithelium → reassembled into triglycerides → packaged into chylomicrons → lacteals → lymphatic system. Fat-soluble vitamins (A, D, E, K) follow this route; water-soluble vitamins go straight into capillaries.
- Ileum handles 2 special cases: vitamin B-12 absorption (needs intrinsic factor) and bile salt reabsorption (enterohepatic circulation).
- Large intestine: reabsorbs water/electrolytes, forms feces, eliminates waste. Ileocecal valve, appendix, 2 anal sphincters (internal = involuntary, external = voluntary). Gut bacteria produce vitamin K and biotin.
Key Terms
- Villi / microvilli
- Finger-like projections (villi) and even smaller projections on epithelial cells (microvilli) that maximize small intestine surface area.
- Lacteal
- Central lymphatic vessel inside a villus that absorbs lipids as chylomicrons.
- Chylomicrons
- Large lipoprotein particles (triglycerides + cholesterol + phospholipids + fat-soluble vitamins) that carry lipids through the lymphatic system.
- Enterohepatic circulation
- Recycling loop where bile salts are reabsorbed in the ileum and returned to the liver for reuse.
- Ileocecal valve
- Controls passage of material from the ileum into the cecum and prevents backflow.
Capillary Route vs. Lacteal Route
| Capillaries (Blood) | Lacteals (Lymph) |
|---|---|
| Sugars and amino acids | Lipids (as chylomicrons) |
| Delivered to hepatic portal circulation → liver | Enter lymphatic system before reaching bloodstream |
| Water-soluble vitamins | Fat-soluble vitamins (A, D, E, K) |
Nutrient Transport Mechanisms
- SGLT1 (glucose/galactose)
- Secondary active transport, cotransported with Na+; indirectly ATP-dependent (via Na+/K+ ATPase gradient).
- GLUT5 (fructose)
- Facilitated diffusion down its concentration gradient — no ATP, no ion coupling.
- Amino acids
- Sodium-dependent active transport; small di/tripeptides can also be absorbed intact.
Common MCAT Trap
- Fructose (GLUT5) uses facilitated diffusion — NOT active transport like glucose/galactose (SGLT1). A common trap groups all monosaccharides together.
- Chylomicrons are too large to enter capillaries directly — they MUST go through lacteals/lymphatics first, unlike every other absorbed nutrient class.
- Internal anal sphincter = involuntary (smooth muscle); external anal sphincter = voluntary (skeletal muscle) — same pattern as the urinary sphincters, don't mix them up.
Quick Recall
How does fructose absorption differ from glucose absorption?
Why do lipids need to travel via lacteals instead of capillaries?
What 2 things happen specifically in the ileum?
What vitamins does gut bacteria in the large intestine produce?