Intestinal Absorption Physiology
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Direct answer
Two hundred square metres of absorptive surface — built by folding the small intestine three-fold with plicae circulares, ten-fold with villi, and twenty-fold with microvilli, an amplification of roughly six hundred times — absorb about 8.5 of the 9 litres of fluid presented daily, along with virtually all dietary nutrients. Sodium absorption powers much of the transport: the basolateral Na-K ATPase keeps intracellular sodium low so that apical cotransporters can pull in glucose through SGLT1, amino acids through their own carriers, making sodium absorption solvent-coupled — the physiology that oral rehydration solution exploits, coupling sodium and glucose in a 245 mOsm/L solution that continues absorbing even in cholera. Absorption is site-specific: iron and calcium in the duodenum, folate and most sugars and amino acids in the jejunum, vitamin B12-intrinsic factor and conjugated bile salts only in the terminal ileum — which is why ileal resection produces B12 deficiency, bile-salt diarrhoea and steatorrhoea.
What you must remember
- Surface arithmetic: plicae circulares (x3), villi (x10), microvilli (x20) give a total amplification near 600-fold and about 200 square metres of mucosa — a standard two-mark question.
- Water and electrolytes: roughly 9 litres handled daily (2 litres ingested, 7 litres secreted); the small intestine absorbs 7-8 litres, the colon 1-1.5 litres; absorption follows osmotically active solute transport — solvent drag through tight junctions.
- ORS physiology: SGLT1 cotransports sodium with glucose unimpaired in cholera toxin diarrhoea; WHO low-osmolarity ORS (sodium 75, glucose 75, total osmolarity 245 mOsm/L) is the design consequence — a national programme staple (ORS corners) and the most quoted translational fact in GI physiology.
- Carbohydrate absorption: SGLT1 (glucose-galactose, sodium-dependent), GLUT5 (fructose), GLUT2 (basolateral exit); lactase is a brush-border enzyme whose adult non-persistence is the commonest enzyme deficiency worldwide — high prevalence among South Indian adults.
- Fat absorption: micellar solubilisation, passive diffusion of monoglycerides and fatty acids, re-esterification to chylomicrons; bile salt deficiency, pancreatic lipase deficiency (chronic pancreatitis — common in Indian alcohol-related disease) and mucosal disease (coeliac) each break a different step.
- Site map: duodenum — iron (DMT1, hepcidin-regulated), calcium (calcitriol-dependent active transport); jejunum — folate, sugars, amino acids; terminal ileum — B12-intrinsic factor complex and bile salts via specific active transport.
- Ileal resection consequences: B12 deficiency (pernicious-anaemia-like), bile salt loss (secretory diarrhoea, gallstones from depleted bile pool, steatorrhoea) — the integrative one-liner of the topic.
A worked case in malabsorption
A middle-aged woman with coeliac disease presents with diarrhoea, weight loss and a microcytic anaemia that does not respond to iron. Enteropathy has flattened her proximal jejunal villi — exactly where iron, folate and most nutrients absorb — while the terminal ileum, spared by the patchy proximal disease, still handles B12 and bile salts. The result: iron-deficiency anaemia with folate deficiency, but preserved B12 — the pattern that distinguishes proximal from distal small-bowel disease on a haematology report.
Contrast a Crohn patient after ileocaecal resection: B12 falls over years (body stores last 2-5 years), bile salts reach the colon and are dehydroxylated to secretory agents causing bile-salt diarrhoea, and once the bile pool depletes, fat malabsorption with steatorrhoea and oxalate kidney stones follows. Then cholera: the villus absorptive machinery (SGLT1) still works while crypts flood the lumen; ORS keeps pace by riding glucose-coupled sodium absorption — why ORS corners save more lives than any ICU protocol.
Where students slip
Students quote surface area without the fold structure, then cannot answer which factor contributes what — say 3 (plicae), 10 (villi), 20 (microvilli), product near 600. The second slip is calling glucose absorption insulin-dependent: SGLT1 is sodium-dependent and insulin-independent, which is the entire basis of ORS; mixing it up dismantles the flagship clinical link. Third, B12 absorption is terminal ileum plus intrinsic factor plus pancreatic protease release of the complex — a three-factor requirement and a favourite multi-step viva question. Finally, calcium is actively absorbed in the duodenum under calcitriol, passively throughout the rest — losing the hormonal half of the statement costs the distinction.
Frequently asked questions
How is the small intestine's absorptive surface amplified?
Plicae circulares increase it about three-fold, villi ten-fold and microvilli twenty-fold — a combined amplification near 600-fold to roughly 200 square metres.
Why does oral rehydration solution work in cholera?
The SGLT1 glucose-sodium cotransporter remains intact even during toxin-driven secretion, so the glucose and sodium in ORS (75 mmol/L each, 245 mOsm/L total) are absorbed together, dragging water with them and matching losses.
Which nutrients are absorbed specifically in the terminal ileum?
The vitamin B12-intrinsic factor complex and conjugated bile salts, both by active, carrier-mediated transport unique to that segment — hence the deficiency syndromes after ileal resection.
How is iron absorbed and regulated?
Non-haem iron is reduced to the ferrous form and taken up through DMT1 in the duodenum, exported via ferroportin, and the whole pathway is restrained by hepcidin — the hormone central to haemochromatosis and anaemia-of-inflammation physiology.
Why is terminal ileal disease followed by gallstones?
Bile salt reabsorption fails, depleting the bile acid pool; the reduced bile salt content of bile lets cholesterol precipitate — gallstone formation on physiological grounds.