Gastroenterology

Digestive System

From mouth to anus — a 9-meter assembly line for nutrients

The digestive tract is a 9-meter tube from mouth to anus that mechanically and chemically extracts nutrients. Mouth (mastication, salivary amylase). Esophagus (peristalsis, no absorption). Stomach (HCl, pepsin, churning). Small intestine — duodenum (pancreatic enzymes, bile), jejunum and ileum (~90% of absorption). Colon (water reclamation, microbiome fermentation). Liver makes bile; pancreas makes lipase, amylase, trypsin, insulin. Total transit time: 24-72 hours. Surface area of small intestine: ~250 m² thanks to plicae, villi, microvilli — about a tennis court.

  • Total length~9 meters
  • Small intestine surface area~250 m² (with villi)
  • Stomach pH1.5-3.5
  • Daily saliva1-1.5 L
  • Gut microbiome~10^14 organisms; 1.5 kg
  • Transit time24-72 hours

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Why digestive physiology matters

  • Drug pharmacokinetics. First-pass metabolism, gastric emptying, enterohepatic recycling all affect dosing.
  • Bariatric surgery. Roux-en-Y, sleeve gastrectomy alter anatomy and produce dramatic weight loss.
  • Cancer screening. Colorectal cancer is the second-leading cancer killer; screening prevents most cases.
  • IBD biologics. Anti-TNF (infliximab), anti-integrin (vedolizumab), anti-IL-23 (ustekinumab).
  • Gut-brain axis. Probiotics, vagal stimulation, microbiome implicated in mood disorders.
  • GLP-1 agonists. Slow gastric emptying drives satiety effects beyond glucose lowering.
  • Malabsorption workup. Steatorrhea, weight loss, deficiencies suggest pancreatic insufficiency or celiac.

Common misconceptions

  • Stomach acid digests most food. Most digestion happens in small intestine via pancreatic enzymes.
  • Spicy food causes ulcers. H. pylori and NSAIDs cause >90% of peptic ulcers.
  • "Detox cleanses" help the liver. Liver detoxifies continuously; juice cleanses don't enhance it.
  • Celiac is a wheat allergy. It's autoimmune; gluten-free is lifelong, not optional.
  • Probiotics treat all digestive ills. Evidence is strongest for antibiotic-associated diarrhea and IBS in select strains.
  • Appendix is useless. It harbors microbiome reservoir and lymphoid tissue; not vestigial.

Frequently asked questions

How does digestion start?

Mouth: chewing breaks food mechanically; salivary α-amylase begins starch digestion (limited time before stomach acid denatures it); lingual lipase starts fat. Bolus passes pharynx via swallowing reflex (cranial nerves IX, X). Esophagus: ~25 cm, peristaltic waves push bolus to stomach in ~7 seconds. Lower esophageal sphincter prevents reflux; failure causes GERD.

What does the stomach do?

Stores ~1-1.5 L meal. Parietal cells secrete HCl (pH 1.5-3.5) via H+/K+ ATPase — proton pump inhibitors block this. Chief cells secrete pepsinogen, activated to pepsin in acid. Mucous cells protect lining with bicarbonate-rich mucus. G cells produce gastrin. Intrinsic factor binds B12 for ileal absorption. Churning produces chyme; pyloric sphincter releases gradually to duodenum.

Where is most absorption?

Small intestine — particularly jejunum. Duodenum: iron, calcium, glucose. Jejunum: most carbs, proteins, fats, water-soluble vitamins. Ileum: bile salts (recycled via enterohepatic circulation), B12 (with intrinsic factor), fat-soluble vitamins. Resection of ileum impairs B12 and bile salts. Colon absorbs ~1.5 L water and electrolytes daily; failure causes diarrhea.

What do liver and pancreas contribute?

Liver: bile (bile salts, bilirubin, cholesterol) emulsifies fats; stored in gallbladder, released by CCK after fatty meal. Pancreas: exocrine — lipase, amylase, trypsin/chymotrypsin (zymogens activated in duodenum), bicarbonate to neutralize gastric acid. Endocrine — insulin (β-cells), glucagon (α-cells). Pancreatitis: trypsin auto-activation digests the gland.

How does the microbiome contribute?

~100 trillion bacteria, predominantly Firmicutes and Bacteroidetes. Functions: ferment fiber to short-chain fatty acids (butyrate fuels colonocytes), synthesize vitamin K and biotin, modulate immune system, exclude pathogens, metabolize bile acids and drugs. Dysbiosis linked to IBD, obesity, C. difficile, depression. Fecal microbiota transplant cures recurrent C. difficile in >85%.

What controls motility?

Enteric nervous system (~500 million neurons — "second brain") coordinates locally. Vagus and sympathetic provide central modulation. Migrating motor complex sweeps unabsorbed material between meals. Hormones: motilin, ghrelin (stimulate); CCK, secretin, GLP-1 (inhibit gastric emptying). Gastroparesis (often diabetic) delays emptying — early satiety, nausea.

What goes wrong?

GERD (reflux): PPIs and H2 blockers. Peptic ulcer: H. pylori (eradicate with triple/quadruple therapy) or NSAIDs. Celiac: gluten-driven autoimmune small bowel injury. IBD (Crohn's, ulcerative colitis): biologics revolutionized care. IBS: functional disorder, ~10% of population. Colon cancer: screen with colonoscopy at 45+, FIT yearly. Diverticulitis, appendicitis, gallstones.