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Absorption of digested nutrients in the small intestine is maximized by the three-level surface amplification of mucosal folds, villi, and microvilli (brush border), collectively increasing absorptive surface ~600-fold. Monosaccharides (glucose, galactose) and amino acids enter enterocytes via Na⁺-coupled cotransporters (SGLT1, neutral amino acid transporters) on the apical membrane and exit into portal capillaries via facilitated diffusion on the basolateral side. Fatty acids and monoglycerides are reassembled into triglycerides inside enterocytes, packaged into chylomicrons, and exported via lacteals into the lymphatic system — bypassing portal circulation and initial hepatic metabolism. Fat-soluble vitamins (A, D, E, K) follow the lipid pathway; vitamin B12 requires intrinsic factor for receptor-mediated endocytosis in the terminal ileum.
Map the three macronutrient absorption routes separately, following each from the intestinal lumen to the bloodstream: glucose (SGLT1 apical → GLUT2 basolateral → portal vein → liver), amino acids (cotransporter → basolateral → portal vein), fats (micelle → enterocyte → re-esterification → chylomicron → lacteal → thoracic duct → bloodstream). Ask: why do fat-soluble drug overdoses take longer to reverse than water-soluble drug overdoses?
Digestion breaks food into small molecules, but those molecules are useless until they cross the intestinal wall and enter the body. Absorption is the process by which digested nutrients move from the intestinal lumen, through the enterocyte epithelium, and into either the blood or the lymph. The small intestine is spectacularly designed for this task: its inner surface is folded into circular folds (plicae circulares), which bear finger-like projections called villi, which in turn are covered with microvilli (the brush border) on each enterocyte. This three-tiered amplification increases the absorptive surface area roughly 600-fold compared to a smooth tube — to approximately 200 square meters, about the area of a tennis court.
The absorption route depends on whether the nutrient is water-soluble or fat-soluble. Monosaccharides like glucose and galactose enter the enterocyte through SGLT1 (sodium-glucose linked transporter 1) on the apical membrane — a secondary active transporter that harnesses the sodium gradient you studied in active transport. Inside the cell, glucose exits through GLUT2 facilitated diffusion transporters on the basolateral membrane and enters capillaries within the villus. These capillaries drain into the portal vein, carrying glucose directly to the liver for first-pass metabolism. Amino acids follow a similar pattern: Na⁺-coupled cotransporters on the apical side, facilitated diffusion on the basolateral side, and portal venous delivery to the liver. Fructose is an exception — it uses GLUT5 (facilitated diffusion, not sodium-coupled) on the apical membrane, which is why fructose absorption is slower and capacity-limited.
Lipid absorption follows a completely different route. Long-chain fatty acids and monoglycerides are poorly soluble in the aqueous lumen, so bile salts package them into micelles — tiny aggregates with hydrophobic interiors and hydrophilic surfaces. Micelles ferry lipids to the enterocyte surface, where fatty acids and monoglycerides diffuse across the apical membrane (they are small and hydrophobic enough to cross the lipid bilayer directly). Inside the enterocyte, they are reassembled into triglycerides in the smooth endoplasmic reticulum, coated with apolipoproteins, and packaged into large lipoprotein particles called chylomicrons. Chylomicrons are too large to enter blood capillaries, so they are exocytosed into lacteals — lymphatic capillaries within each villus — and travel through the lymphatic system to the thoracic duct, entering the bloodstream at the left subclavian vein. This lymphatic route bypasses the liver entirely, which is why dietary fat is not subject to first-pass hepatic metabolism.
Fat-soluble vitamins (A, D, E, K) dissolve in micelles and follow the lipid pathway into chylomicrons. Water-soluble vitamins generally use specific transporters, with one notable exception: vitamin B12 requires intrinsic factor, a glycoprotein secreted by gastric parietal cells, for its absorption. The B12-intrinsic factor complex binds to receptors in the terminal ileum and is absorbed by receptor-mediated endocytosis. This explains why gastric surgery or autoimmune destruction of parietal cells (pernicious anemia) causes B12 deficiency even when dietary intake is adequate — without intrinsic factor, the ileum cannot absorb B12 regardless of how much is present in the lumen.