--- module: 021-01 language: en chapter: 21 title: "Digestion, Absorption, Liver Function, and Nutrition" module_title: "Foundations" source_sha256: 39045fdea18b65c48e6b06e275b7af73f895a4d4d8b425393c229b0160f84498 --- # Digestion, absorption, liver, and nutrition ## Organisation and control ### Mucosa, submucosa, muscle, serosa ### Folds, villi, microvilli amplify surface ### Tight junctions and selective transporters ### Rapid renewal repairs injury #### Vulnerable to antimetabolites, radiation, ischaemia ### Enteric plexuses coordinate local function ### Parasympathetic promotes digestion ### Sympathetic inhibits and constricts vessels ### Cajal cells generate slow waves ### Hormones coordinate regions #### Gastrin: acid and mucosal growth #### Secretin: bicarbonate for duodenal acid #### Cholecystokinin: enzymes, gallbladder, slow emptying #### Incretins and motilin ## Motility from mouth to colon ### Swallowing reflex protects airway ### Impaired sphincter relaxation causes achalasia ### Inappropriate reflux exposes squamous epithelium ### Proximal stomach accommodates, distal grinds ### Duodenal signals slow emptying ### Vomiting loses chloride, potassium, volume ### Segmentation mixes, peristalsis advances ### Migrating complex clears fasting residue ### Haustra absorb water, mass movements propel ### Defecation combines reflex and voluntary control ## Secretion and digestion ### Saliva lubricates, buffers, starts digestion ### Parietal cells: acid and intrinsic factor #### Histamine, acetylcholine, gastrin converge #### Somatostatin, prostaglandins restrain ### Mucosal defences prevent self-digestion ### Pancreas: bicarbonate and enzymes #### Inactive protease precursors limit autodigestion #### Insufficiency causes steatorrhoea ### Bile acids emulsify and form micelles #### Reclaimed in terminal ileum #### Broken cycle: diarrhoea, fat malabsorption ## Absorption ### Sugars as glucose, galactose, fructose ### Sodium-glucose coupling explains oral rehydration ### Long-chain fats: chylomicrons via lymph ### Shorter fatty acids enter portal blood ### Vitamin B12 needs intrinsic factor and ileum ### Iron proximal, regulated by hepcidin ### Colon salvages water and sodium ### Diarrhoea mechanisms #### Fasting improves osmotic, not secretory #### High volume: hypovolaemia, acidosis, kidney injury ## Barrier, microbiome, and splanchnic flow ### Mucus, epithelium, antimicrobials, immune cells, microbes ### Microbiome ferments and modifies bile acids ### Association does not prove causation ### Countercurrent flow leaves villus tips hypoxic ### Ischaemia from hypotension, occlusion, thrombosis #### Pain out of proportion to examination #### Later infarction: peritonism, collapse ## Liver architecture and zonation ### Portal and hepatic arterial inflow ### Sinusoids drain toward central veins ### Bile flows opposite toward ducts ### Hepatocytes see oral drugs first ### Periportal: oxygen, oxidation, urea ### Centrilobular: low oxygen, drug metabolism #### Vulnerable to hypoxia and toxins ### Stellate activation drives fibrosis ## Metabolic, synthetic, and detoxifying roles ### Glycogen and gluconeogenesis buffer glucose ### Albumin, clotting factors, complement #### Albumin supports oncotic pressure, binds drugs ### Phase one exposes, phase two conjugates #### Inactivate, activate prodrugs, or form toxins ### Exposure altered by induction, genetics, shunting ### Bilirubin from haem breakdown #### Unconjugated: overproduction or poor conjugation #### Conjugated: impaired excretion or obstruction ## Nutrition and clinical assessment ### Energy balance: intake to losses ### Malnutrition reduces muscle, immunity, reserve ### Obesity with deficiency and sarcopenia ### Assessment integrates trajectory and function #### Albumin not a standalone marker ### Refeeding after prolonged undernutrition #### Phosphate, potassium, magnesium enter cells #### Replace thiamine, start cautiously, monitor ### Enteral feeding preserves mucosa ### Parenteral: lifesaving but catheter risks