GI & Hepatic Physiology

Overview & definition

The GI and hepatic physiology — the digestion, the absorption, the motility, the secretion; the liver (the bilirubin, the urea, the glucose, the coagulation, the detoxification, the bile). The splanchnic circulation. The gut barrier.[1]

GI physiology

• The digestion — the carbohydrates (the amylase), the proteins (the pepsin, the trypsin), the fats (the lipase, the bile salts).[1]
• The absorption — the small intestine (the - the - the - the).[1]
• The motility — the peristalsis; the migrating motor complex (the MMC — the fasting).[1]
• The secretion — the gastric acid (the parietal the cell; the H/K ATPase), the pancreatic (the bicarbonate, the enzymes), the bile.[1]
• The gut barrier — the mucosal; the tight junctions; the - the - the - the.[1]
• The splanchnic circulation — the dual supply (the hepatic artery 25 per cent, the portal vein 75 per cent).[1]

Swallowing reflex — the oropharyngeal to oesophageal sequence

Deglutition (swallowing) is a coordinated reflex that transfers a bolus from the mouth to the stomach while protecting the airway. It has three phases, only the first being truly voluntary: [1]

[1]

The sphincters — the UES (cricopharyngeus, striated muscle, baseline high pressure ~40-100 mmHg) and the LES (smooth muscle, ~10-30 mmHg) — are tonically CLOSED and relax only during swallowing (LES relaxation = transient lower oesophageal sphincter relaxation, TLESR, the mechanism of reflux). The crural diaphragm provides an external pinch on the LES (the anti-reflux barrier), which is why a hiatus hernia (loss of the diaphragmatic crural support) predisposes to gastro-oesophageal reflux.[1]

[1]

Gastric acid secretion — the three phases and the H⁺/K⁺-ATPase

Gastric acid (HCl, pH ~1-2, up to ~2 L/day) is secreted by the parietal (oxyntic) cell of the gastric body and fundus. The final common effector is the H⁺/K⁺-ATPase (the proton pump) — an electroneutral ATP-driven exchanger on the apical (canalicular) membrane that pumps H⁺ into the lumen in exchange for K⁺. The proton pump is the target of proton pump inhibitors (PPIs, omeprazole) — irreversible (covalent) binders of the pump activated in the acidic canaliculus — and is the rationale for their near-complete acid suppression.[1]

Three secretagogues converge on the parietal cell: [1]

[1]

Somatostatin (D cell of the gastric antrum and body, released when luminal pH < 3) is the master INHIBITOR — it suppresses gastrin (G cell), histamine (ECL cell) and the parietal cell directly (sst₂ receptor) — the negative-feedback brake that prevents acid over-secretion. This is why loss of the antrum (surgery) or achlorhydria (PPI/pernicious anaemia) causes hypergastrinaemia, and why somatostatin analogues (octreotide) reduce acid (and splanchnic) output.[2]

[1]

The cellular activation cascade: stimulation → tubulovesicles (holding H⁺/K⁺-ATPase) fuse with the apical canalicular membrane → expansion of the secretory canaliculus → the proton pump inserts and secretes H⁺. At rest the pumps are sequestered intracellularly; with stimulation they translocate to the membrane — the basis for PPIs only inactivating ACTIVATED pumps (hence give PPIs ~30 min before a meal, when pumps are maximally active).[1]

[1]

Pancreatic enzyme secretion and activation — the zymogen safeguard

The exocrine pancreas secretes ~1.5 L/day of a bicarbonate-rich (pH ~8), enzyme-laden juice that is the principal digestive engine for carbohydrates, fats and proteins. Secretion is driven by two hormones: [1]

• Secretin (S cells of the duodenum, released by acidic chyme pH < 4.5) → duct cells secrete bicarbonate-rich, watery juice (neutralises acid, brings duodenal pH to ~6-7 for enzyme function).
• Cholecystokinin / CCK (I cells of the duodenum, released by amino acids, fatty acids, monoglycerides) → acinar cells secrete enzyme-rich juice; also contracts the gallbladder and relaxes the sphincter of Oddi.
• Vagal input (ACh, M₃) potentiates both (the cephalic and gastric phases of pancreatic secretion). [1]

The end-result is a suite of enzymes: amylase (starch → maltose/maltotriose/α-dextrins, secreted active), lipase (triglycerides → 2-monoglyceride + free fatty acids, requires colipase to function in the presence of bile salts), phospholipase A₂ (phospholipids → lysophospholipids + fatty acids), trypsin/chymotrypsin/carboxypeptidase/elastase (proteins → peptides and amino acids), and the nucleases (ribonuclease, deoxyribonuclease).[1]

Bile acid circulation — the enterohepatic loop

Bile has two functions — digestion (emulsification of dietary fat and micelle formation) and excretion (the route for conjugated bilirubin, cholesterol, and lipophilic xenobiotics). Hepatocytes synthesise the primary bile acids — cholic acid and chenodeoxycholic acid — from cholesterol (rate-limiting enzyme 7α-hydroxylase, CYP7A1; bile acid synthesis is the major pathway of cholesterol catabolism, and is feedback-inhibited by bile acids returning via the portal vein). They are conjugated with glycine or taurine (lowering their pKa so they remain soluble at intestinal pH as bile salts) → secreted across the canalicular membrane by BSEP (ABCB11) → bile flows to the gallbladder for storage and concentration → released post-prandially when CCK (released by I-cells in response to fat/amino acids) contracts the gallbladder and relaxes the sphincter of Oddi.[4]

In the duodenum, bile salts act as detergents: their amphipathic structure (hydrophobic steroid nucleus + hydrophilic hydroxyl/side-chain) emulsifies dietary triglycerides into small droplets, then forms mixed micelles (bile salts + fatty acids + monoglycerides + fat-soluble vitamins) that ferry lipids to the enterocyte brush border for absorption.[4]

[1]

The clinical corollaries: (1) terminal ileal disease/resection (Crohn's, surgical) disrupts reabsorption → bile acid loss in stool → depletion of the pool → fat malabsorption (steatorrhoea) and cholesterol gallstone formation (a depleted pool cannot keep cholesterol solubilised in bile); (2) bile acid malabsorption causes bile-salt diarrhoea (choleretic enteropathy, the colon irritated by secretory bile acids) — treat with a bile acid sequestrant (cholestyramine).[4][6]

Intestinal absorption — carbohydrates, proteins, fats, B₁₂, iron, calcium

The small intestine (surface area ~300 m² with villi and microvilli) is the principal absorptive organ. Each macronutrient class and several key micronutrients have specific transport mechanisms that are common exam material. [1]

[1]

The fat pathway is distinctive: because lipids are membrane-permeable, they diffuse into the enterocyte, are re-esterified in the smooth ER, packaged with apolipoproteins into chylomicrons, and secreted by exocytosis into the lymphatic lacteals (NOT the portal vein) — bypassing the hepatic first-pass. This is why medium-chain triglycerides (MCTs, C6-C12), which are water-soluble and absorbed directly into the portal vein (no micelle/chylomicron needed), are useful in fat malabsorption (pancreatic insufficiency, cholestasis, terminal ileal resection).[6]

The two classic terminal-ileum dependencies — vitamin B₁₂ and bile acids — are the most examined absorption facts: any disease or resection of the terminal ileum causes B₁₂ deficiency (megaloblastic anaemia ± neuropathy) and bile acid malabsorption (diarrhoea, steatorrhoea, gallstones).[7][4]

Gut motility — the migrating motor complex and peristalsis

Gut motility serves two distinct modes — the fed pattern (mixing and slow propulsive movement to allow absorption) and the fasting pattern (housekeeping to clear residual content and bacteria). The switch between them is hormonally controlled (motilin initiates fasting activity). [1]

[1]

The MMC has three phases that recur every ~90-120 minutes in the fasted state: Phase I (quiescence), Phase II (irregular contractions of increasing amplitude), Phase III (the intense, short burst of regular, high-amplitude peristaltic contractions — the "housekeeper" wave that does the clearing), and Phase IV (declining activity back to Phase I). The cycle is driven by motilin and the interstitial cells of Cajal (the gut pacemakers, generating slow waves). The MMC is abolished by feeding and by opioids, and is restored by motilin agonists (erythromycin) and the acetylcholinesterase inhibitor neostigmine.[3]

Peristalsis — the coordinated reflex contraction of circular smooth muscle behind a bolus and relaxation ahead of it — is mediated by the enteric nervous system (the intrinsic primary afferent neurons, interneurons, and motor neurons of Auerbach's/myenteric plexus). It functions even after extrinsic denervation (the gut "brain"). The law of the intestine (Bayliss and Starling): distension evokes contraction proximal (oral) and relaxation distal (anal) to the bolus, propelling it aborally. The gastrocolic reflex (mass colonic movement after a meal, mediated by gastrin/CCK and vagal reflexes) is the physiological basis for post-prandial defaecation and for the post-prandial urgency of irritable bowel syndrome.[1]

Splanchnic circulation — the dual supply and the "canary" organ

The splanchnic bed receives ~25% of cardiac output and is the only circulation with two capillary beds in series joined by a portal vein — gut capillaries → portal vein → hepatic sinusoids. The liver's dual blood supply is the central exam fact:[13]

[1]

The gut is a sacrificial circulation: in any shock state, sympathetic (α₁), angiotensin II, vasopressin (V₁) and endothelin preferentially constrict the splanchnic arterioles to divert cardiac output to heart and brain. The gut therefore becomes ischaemic EARLY and remains so LONG after systemic perfusion is restored (the splanchnic debt repaid last) — the physiological basis for mesenteric ischaemia complicating any shock, and for the persistent lactate that signals ongoing splanchnic hypoperfusion despite a "normal" MAP. The colonic watershed areas (splenic flexure / Griffith's point at the SMA-IMA junction; rectosigmoid / Sudeck's point at the IMA-internal iliac junction) have the poorest collaterals and are most vulnerable to ischaemic colitis.[13]

The gut barrier — the gatekeeper that fails in critical illness

The gut barrier (~300 m² surface area) must simultaneously absorb nutrients and exclude ~10¹³ luminal organisms. It has four integrated layers: (1) the mucus layer (goblet-cell mucin-2 + secretory IgA); (2) the epithelium + tight junctions (claudins, occludin, ZO-1; Paneth-cell antimicrobial peptides — defensins, RegIIIγ; turnover every 3-5 days, the fastest-renewing epithelium); (3) GALT (Peyer's patches, lamina propria lymphocytes, intraepithelial lymphocytes — ~70% of body immune cells); and (4) the microbiome (~10¹³ organisms, dominated by anaerobes, providing colonisation resistance). The barrier is disrupted in critical illness by ischaemia-reperfusion, antibiotics (dysbiosis), PPIs (bacterial overgrowth), ileus, and parenteral nutrition → bacterial/endotoxin translocation → Kupffer-cell cytokine cascade → MODS — the "gut motor" of MODS. The cycle is broken by restoring perfusion early, early enteral nutrition (maintains mucosal integrity and trophic flow), and antibiotic/PPI stewardship.[12][14]

Hepatic physiology

• The bilirubin metabolism — the heme (the - the - the) → the biliverdin → the unconjugated the bilirubin (the lipid-soluble; the albumin-bound) → the liver → the conjugation (the glucuronic acid; the water-soluble) → the bile → the gut → the urobilinogen → the stercobilin (the stool) and the - the - the.[1]
• The urea cycle — the ammonia (the toxic) → the urea (the - the - the - the).[1]
• The glucose homeostasis — the glycogen the storage; the gluconeogenesis; the glycogenolysis.[1]
• The coagulation — the factors II, VII, IX, X (the vitamin K dependent); the factor V; the albumin.[1]
• The detoxification — the phase I (the cytochrome P450) and the phase II (the conjugation).[1]
• The bile — the bile salts (the emulsify); the cholesterol; the bilirubin.[1]

The liver as a metabolic factory — overview

The liver weighs ~1.5 kg and performs >500 functions. For the intensivist, six are central: the dual blood supply, glucose homeostasis, protein synthesis, lipid metabolism, drug metabolism, bilirubin metabolism, the urea cycle / ammonia detoxification, and regeneration. Each is examined in detail below.[1]

Glucose homeostasis — the liver as glycostat

The liver maintains plasma glucose in the narrow 4-7 mmol/L range by balancing storage, mobilisation and new synthesis. It stores ~100 g glycogen (enough for ~12-24 h of fasting) and is the only organ (with the kidney) that can release free glucose into the blood (it expresses glucose-6-phosphatase; muscle does not, so muscle glycogen feeds only itself via lactate).[1]

[1]

In critical illness the liver is pushed to maximal gluconeogenesis by the counter-regulatory surge (cortisol, catecholamines, glucagon, GH overriding insulin) — the stress hyperglycaemia of sepsis/trauma/burns, fuelled by substrate (lactate from hypoperfused tissues, amino acids from muscle catabolism, glycerol from lipolysis). Two failure modes matter in ICU: (a) hypoglycaemia in acute liver failure (exhausted glycogen + failed gluconeogenesis + impaired insulin clearance → hyperinsulinaemia) — check glucose hourly, it is masked by encephalopathy; (b) refeeding syndrome — when nutrition restarts after starvation, the insulin surge drives phosphate, magnesium and potassium intracellularly for glycolysis/ATP phosphorylation → dangerous hypophosphataemia.[1]

Protein synthesis — the liver makes most plasma proteins

The liver synthesises the majority of circulating proteins. The clotting factors are the most exam-relevant because their synthesis is the basis for monitoring liver synthetic function: [1]

[1]

The synthesis of ALL clotting factors EXCEPT factor VIII and von Willebrand factor (both endothelial) is hepatic. The synthesis sequence that fails in liver disease is clinically useful: clotting (factor VII, half-life 6 h) first, then ammonia clearance (encephalopathy), then albumin (half-life 20 days) last — a graded window onto the severity and chronicity of hepatic synthetic failure.[1]

Lipid metabolism — β-oxidation, ketogenesis, lipoprotein synthesis

The liver is the hub of lipid metabolism: it (1) β-oxidises free fatty acids (released from adipose tissue by glucagon-sensitive hormone-sensitive lipase in the fasted state) → acetyl-CoA → ATP (energy) and ketone bodies (acetoacetate, β-hydroxybutyrate) — ketogenesis, the brain's alternative fuel in prolonged fasting; (2) synthesises fatty acids and triglycerides in the fed state; (3) synthesises and secretes the apolipoproteins and packages VLDL (endogenous triglyceride transport), HDL, and accepts chylomicron remnants; and (4) synthesises cholesterol and converts it to bile acids (the major cholesterol disposal route).[1]

The fasted/stressed ICU patient runs on hepatic β-oxidation and ketogenesis: lipolysis (driven by counter-regulatory hormones) floods the liver with free fatty acids → β-oxidation → acetyl-CoA → ketones exported to brain, heart and muscle. In fatty liver of critical illness / sepsis (and in alcoholic and non-alcoholic fatty liver disease), triglyceride accumulation outstrips β-oxidation/export → steatosis → mitochondrial dysfunction → progression to steatohepatitis and cirrhosis.[1]

Drug metabolism — Phase I (CYP450) and Phase II (conjugation)

The liver converts lipophilic drugs (which would accumulate in fat and resist renal excretion) into polar, water-soluble metabolites for elimination. This occurs in two phases: [1]

[1]

First-pass (presystemic) metabolism: orally absorbed drugs pass via the portal vein through the liver BEFORE reaching the systemic circulation — a fraction is metabolised before it reaches its target (bioavailability < 100%). In cirrhosis, portosystemic shunts bypass the liver → first-pass metabolism is LOST → bioavailability of high-extraction drugs (morphine, midazolam, propranolol, verapamil) rises dramatically → a normal oral dose causes overdose. The rule in liver failure: BOTH phases are impaired but Phase I falls earlier and faster than Phase II — prefer drugs cleared by conjugation, start LOW, go SLOW.[1]

Paracetamol hepatotoxicity is the classic Phase I mechanism: paracetamol is mostly glucuronidated/sulphated (Phase II), but a small fraction undergoes CYP2E1 oxidation → NAPQI, a reactive intermediate normally detoxified by glutathione. In overdose (saturating Phase II) or with chronic alcohol (CYP2E1 induction + glutathione depletion), NAPQI accumulates → centrilobular (zone 3) hepatocellular necrosis → acute liver failure. The antidote N-acetylcysteine (NAC) restores glutathione. The King's College Criteria for paracetamol-induced ALF use pH, INR, creatinine and grade of encephalopathy.[1]

Bilirubin metabolism — unconjugated → conjugated → enterohepatic

Bilirubin is the yellow-orange end-product of haem catabolism. Its metabolism is a classic exam cascade, and derangement at each step produces a recognisable clinical-biochemical pattern. [1]

[1] [1]

The two exam pearls: (1) bilirubinuria (dark urine with conjugated hyperbilirubinaemia) precedes clinical jaundice — CB is water-soluble and filtered before skin threshold is reached; (2) pale stool + dark urine = obstructive cholestasis (conjugated bilirubin cannot reach the gut → no stercobilin → pale stool — and backs up into blood → spills into urine → dark urine).[1]

Ammonia → urea cycle — hepatic detoxification

Ammonia (NH₃) is a neurotoxic by-product of nitrogen metabolism. The gut generates most of it (bacterial deamination of dietary protein, urease splitting of urea), and the liver detoxifies it via the urea cycle (Krebs-Henseleit cycle) — the reason portal blood ammonia is high but systemic ammonia is low.[1]

Liver regeneration — the unique capacity for compensatory hyperplasia

The liver is the only human solid organ with a clinically meaningful regenerative capacity — following injury or resection, the remaining hepatocytes re-enter the cell cycle and restore mass (compensatory hyperplasia, not true regeneration of lobular architecture). Up to 70% of the liver can be resected with full functional recovery in weeks. This is the basis of live-donor liver transplantation (a partial graft regenerates in both donor and recipient) and the recovery potential of acute hepatic injury.[9]

[1]

Regeneration FAILS in chronic injury (cirrhosis, fibrosis — the stellate-cell collagen scar distorts the architecture and blocks regeneration), in senescence, and in massive acute injury beyond the replicative reserve → acute liver failure. The regenerative drive also explains hepatocellular carcinoma (chronic injury → chronic regenerative cycling → mutational accumulation → carcinoma in cirrhotic livers).[9]

Hepatic encephalopathy mechanism — the ammonia-astrocyte-oedema axis

Hepatic encephalopathy (HE) is the reversible spectrum of neuropsychiatric disturbance in liver failure, ranging from inattention/mild confusion through asterixis (flapping tremor) and somnolence to coma. Ammonia is the central, most actionable toxin, but NOT the sole mechanism.[10][11]

The core mechanism: hyperammonaemia → NH₃ crosses the BBB → astrocyte glutamine synthetase → glutamine → osmotic astrocyte swelling → cerebral oedema + astrocyte dysfunction. In ACUTE liver failure this is the leading cause of death (intracranial hypertension, uncal herniation) — an arterial ammonia > 150 µmol/L with high-grade encephalopathy predicts cerebral oedema/herniation. In CIRRHOSIS (chronic, with portosystemic shunting), chronic low-grade ammonia exposure produces the Alzheimer type II astrocyte change and a predominantly GABA-ergic (increased inhibitory) neurotransmission picture (hence the historical response to flumazenil).[11]

Additional contributors beyond ammonia: systemic inflammation (cytokines increase BBB permeability and ammonia sensitivity), benzodiazepine-like compounds (from the gut), manganese (basal ganglia deposition in chronic portosystemic shunting), and mercaptans. The clinical management follows the physiology: lactulose (acidifies the colon → traps NH₃ as non-absorbable NH₄⁺ → laxation expels it) and rifaximin (a minimally-absorbed antibiotic that suppresses urease-producing bacteria) — ammonia-lowering is the backbone of therapy. Precipitants of HE in cirrhosis (GI bleed → protein load; infection; constipation; electrolyte disturbance; sedatives; TIPSS → increased portosystemic shunting) all share the mechanism of acutely increasing ammonia or BBB permeability.[10]

Clinical correlations

• The jaundice (the pre-hepatic — the haemolysis; the hepatic — the hepatocellular; the post-hepatic — the obstruction).[1]
• The hepatic encephalopathy (the ammonia).[1]
• The coagulopathy (the factors).[1]
• The portal hypertension (the cirrhosis; the varices; the ascites).[1]

[1] [1]

Exam practice — SAQs

[1]

Clinical pearls

Red flags

Prognosis and integrated physiology

[1]

The unifying principle is that the gut-liver axis is BOTH a target of, and a driver of, critical illness. In shock the sacrificed splanchnic circulation becomes ischaemic, the gut barrier fails, and bacterial/endotoxin translocation activates the Kupffer-cell cytokine cascade that propagates inflammation to distant organs — the 'gut motor' of MODS. The failing liver loses its synthetic (clotting factors, albumin), metabolic (glucose, ammonia, drug clearance), and excretory (bilirubin, bile) functions in a predictable sequence — clotting (factor VII, half-life 6 h) first, then ammonia/encephalopathy, then albumin (half-life 20 days) last. Understanding this physiology — the dual blood supply and hepatic arterial buffer response, the three phases of gastric acid secretion, the zymogen activation safeguard, the enterohepatic circulation, the terminal-ileum dependencies (B₁₂ and bile acids), the migrating motor complex, the phases of drug metabolism, the bilirubin cascade, the urea cycle, liver regeneration, and the ammonia-astrocyte-oedema axis of encephalopathy — is the foundation for every ICU intervention from early enteral feeding and antibiotic stewardship to PPI selection, ammonia-lowering therapy, neuroprotection in liver failure, and cholestasis management.[1][12][11]

Key trials and evidence

[1] [1] [1] [1] [1]

References

1. [1]Schubert ML Physiologic, pathophysiologic, and pharmacologic regulation of gastric acid secretion Curr Opin Gastroenterol, 2017.PMID 28787289
2. [2]Schubert ML, Rehfeld JF Gastric Peptides-Gastrin and Somatostatin Compr Physiol, 2019.PMID 31853955
3. [3]Takahashi T Interdigestive migrating motor complex -its mechanism and clinical importance J Smooth Muscle Res, 2013.PMID 24662475
4. [4]Hofmann AF, Hagey LR Key discoveries in bile acid chemistry and biology and their clinical applications: history of the last eight decades J Lipid Res, 2014.PMID 24838141
5. [5]Hofmann AF The enterohepatic circulation of bile acids in mammals: form and functions Front Biosci (Landmark Ed), 2009.PMID 19273221
6. [6]Hegyi P, Maleth J, Walters JR Guts and Gall: Bile Acids in Regulation of Intestinal Epithelial Function in Health and Disease Physiol Rev, 2018.PMID 30067158
7. [7]Miret S, Simpson RJ, McKie AT Physiology and molecular biology of dietary iron absorption Annu Rev Nutr, 2003.PMID 12626689
8. [8]Weiss FU, Halangk W, Lerch MM New advances in pancreatic cell physiology and pathophysiology Best Pract Res Clin Gastroenterol, 2008.PMID 18206809
9. [9]Michalopoulos GK Advances in liver regeneration Expert Rev Gastroenterol Hepatol, 2014.PMID 24964729
10. [10]Thabut D, Bouzbib C, Meunier L Diagnosis and management of hepatic encephalopathy: The French recommendations Liver Int, 2023.PMID 36625084
11. [11]Blei AT Brain edema in acute liver failure Crit Care Clin, 2008.PMID 18241781
12. [12]Deitch EA Gut-origin sepsis: evolution of a concept Surgeon, 2012.PMID 22534256
13. [13]Landow L, Andersen LW Splanchnic ischaemia and its role in multiple organ failure Acta Anaesthesiol Scand, 1994.PMID 7839769
14. [14]Balzan S, de Almeida Quadros C, de Cleva R, Zilberstein B, Cecconello I Bacterial translocation: overview of mechanisms and clinical impact J Gastroenterol Hepatol, 2007.PMID 17376034