Intestinal Motility and Absorption
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Direct answer
Ten litres of fluid pass through the gut daily — about 2 litres ingested plus 7-8 litres of secretion — and the small intestine absorbs 8-9 litres of it, the colon a further 1.4-1.5 litres, leaving only 100-200 mL in stool. Between meals the fasting gut runs the migrating motor complex, a sweeping wave every 90-120 minutes driven by motilin (the reason erythromycin, a motilin agonist, is a prokinetic); after food, segmentation mixes at 11-12 contractions per minute in the duodenum falling to 8 in the ileum, while peristalsis propagates over the law of the intestine. Absorption is regional to a fault: iron and folate favour the duodenum and proximal jejunum, bile salts and vitamin B12 are reclaimed only in the terminal ileum, glucose and galactose enter through sodium-coupled SGLT1 and leave through GLUT2, fructose uses GLUT5, and dietary fat crosses as micelles and exits the enterocyte as chylomicrons into lacteals, bypassing the portal vein entirely. Cholera toxin locks Gs in its active state, producing unrelenting cyclic AMP-driven chloride and water secretion — yet SGLT1 keeps working, which is the physiological licence for oral rehydration salts.
What you must remember
- Fluid ledger: 10 L handled (2 ingested, 7-8 secreted: saliva 1.5 L, gastric 2 L, bile 0.5-1 L, pancreatic 1.5 L, intestinal 1-2 L); jejunum absorbs the bulk, colon recovers 1.4-1.5 L, stool 100-200 mL — colon loss above its capacity means diarrhoea.
- Motility numbers: migrating motor complex every 90-120 min in fasting (motilin-mediated); segmentation 11-12/min duodenum, 8/min ileum; small bowel transit 3-5 hours; colonic transit 8-15 hours; mass movements follow meals via the gastrocolic reflex.
- Transporter map: SGLT1 — sodium-glucose (and galactose) cotransport, apical; GLUT2 basolateral exit; GLUT5 fructose; PepT1 peptide uptake; DMT1 iron at the duodenum; chloride secretion through CFTR at the crypt base.
- Site specialisation: iron, folate, calcium — duodenum and jejunum; vitamin B12-intrinsic factor and bile salts — terminal ileum exclusively; water and electrolytes throughout with colonic salvage.
- Fat path: micellar solubilisation, enterocyte uptake, re-esterification, chylomicron assembly, exocytosis into lacteals — no portal route, so chyle carries the fat; pancreatic lipase with colipase is the rate-limiting digestion step.
- Secretory diarrhoea mechanism: cholera toxin ADP-ribosylates Gs, persistent adenylate cyclase, raised cAMP opens CFTR chloride channels, water follows; Escherichia coli heat-labile toxin does the same — and glucose-coupled sodium absorption stays intact.
- Enzyme deficiency: lactase is the most common brush-border enzyme deficiency worldwide, lactose non-persistence being the ancestral default in most South Asian adults — osmotic diarrhoea, bloating, and a positive hydrogen breath test.
Cholera, understood physiologically
A patient with rice-water stool can lose a litre an hour, yet the mucosa is architecturally intact and the villous absorptive machinery hums. The toxin binds GM1 receptors and ADP-ribosylates the Gs alpha subunit, freezing it active; adenylate cyclase runs unregulated, cyclic AMP rises, and protein kinase A phosphorylates CFTR — crypt cells pour chloride into the lumen, water follows, and villous absorption is overwhelmed. The therapeutic genius is metabolic: SGLT1 couples sodium and glucose uptake regardless of cAMP, so glucose-salt solution is absorbed even while secretion continues. The WHO reduced-osmolarity ORS (245 mOsm/L) exploits exactly this carrier — a national-programme fact the exam expects — cutting stool output compared with the original formula. Antibiotics shorten the illness, but it is the glucose-sodium cotransporter that has saved more lives in the subcontinent's cholera wards than any antimicrobial.
Where students slip
Two slips recur. First, the B12 site: vitamin B12 is absorbed in the terminal ileum only, and it needs intrinsic factor — so ileal resection or Crohn's disease causes deficiency years before iron or folate problems from more proximal disease; students misplace it in the jejunum. Second, the fat path: chylomicrons enter lymphatics, not the portal vein, so fat malabsorption shows as bulky pale stools with fat-soluble vitamin loss; separately, congenital glucose-galactose malabsorption (SGLT1 defects) improves on fructose. Also distinguish osmotic from secretory diarrhoea: osmotic (lactose, sorbitol) stops with fasting and shows a stool osmotic gap; secretory (cholera, VIPoma) continues through the night regardless of intake.
Frequently asked questions
How much fluid does the gastrointestinal tract handle daily and how much reaches the stool?
About 10 litres — 2 ingested and 7-8 secreted — of which the small intestine absorbs 8-9 litres and the colon 1.4-1.5 litres, leaving 100-200 mL in formed stool.
Why does oral rehydration solution work in cholera?
The SGLT1 glucose-sodium cotransporter is intact despite toxin-driven secretion, so glucose-coupled water absorption continues — the basis of WHO reduced-osmolarity ORS.
Where is vitamin B12 absorbed?
In the terminal ileum, only in complex with intrinsic factor — ileal disease or resection causes B12 deficiency, as does gastric intrinsic factor failure.
What is the migrating motor complex?
A recurring fasting motility pattern every 90-120 minutes that sweeps residual contents toward the colon, motilin-driven and interrupted by feeding.
How does fructose differ from glucose in absorption?
Fructose enters enterocytes through GLUT5 by facilitated diffusion without sodium coupling, whereas glucose and galactose use SGLT1 sodium cotransport — the reason fructose tolerates SGLT1 defects.