Reticuloendothelial haem breakdown produces biliverdin and then bilirubin.
← LCCH Physiology
Module 04
Where does bilirubin go wrong in critical illness?
Follow bilirubin from haem breakdown to albumin-bound transport, hepatocyte uptake, conjugation, canalicular export and biliary drainage — then layer on cholestatic physiology.
Clinical frame
The organising problem.
Bilirubin is both a metabolite and a marker of transport failure. Hyperbilirubinaemia can arise before the hepatocyte, within hepatocyte uptake/conjugation/export, or downstream in the biliary system; critical illness often produces mixed mechanisms.
Mechanism · Production
Haem is converted to unconjugated bilirubin
01Haem
02Biliverdin
03Unconjugated bilirubin
04Albumin-bound transport
Unconjugated bilirubin is poorly water soluble and circulates tightly bound to albumin.
Increased production from haemolysis raises unconjugated bilirubin if hepatic handling is exceeded.
Step 1 of 6
Locate the bottleneck in bilirubin handling.
Select a disturbance and see why jaundice is a shared output of different processes.
Increase haemolysis. Haem breakdown increases while hepatic transport remains relatively preserved.
- 01
Bilirubin production
ReferenceUsual haem turnover
Selected scenarioUnconjugated load rises
- 02
Conjugation
ReferenceUGT1A1 capacity available
Selected scenarioGreater substrate burden
- 03
Canalicular export
ReferenceMRP2 export maintained
Selected scenarioCan remain intact
- 04
Duct drainage
ReferenceBile reaches intestine
Selected scenarioCan remain patent
Why this happens
The initial disturbance is increased production. If production exceeds handling capacity, the unconjugated fraction rises without requiring a primary duct problem.
What to look for
Look for a compatible haemolysis pattern alongside the bilirubin fractions.
Limit of the model Critical illness often creates mixed patterns; a bilirubin fraction alone does not diagnose haemolysis.
Sources and context: Bilirubin transporter mechanisms ↗Sepsis-associated cholestasis: mechanisms ↗
Go deeper
Mechanisms beneath the model.
Open the topics you need. Keep several explanations visible to compare mechanisms.
Evidence key: Established: broadly supported. Nuanced: context affects interpretation. Evolving: evidence or definitions remain unsettled. These labels describe physiological certainty, not a formal GRADE rating.
01PolarityThe hepatocyte is a polarised transport cellEstablished
Sinusoidal uptake and canalicular export occur at different membrane domains, allowing vectorial movement from blood to bile.
- Basolateral and canalicular transporter repertoires differ.
- ATP-dependent export creates steep concentration gradients.
- Loss of polarity is one mechanism of severe hepatocellular dysfunction.
02Bile acidsBilirubin and bile acids share bile but not the same transport physiologyEstablished
BSEP is primarily a bile-acid export pump, whereas MRP2 handles bilirubin glucuronides and other organic anions.
- Bile acids are actively recycled through enterohepatic circulation.
- FXR-linked signalling helps regulate bile-acid synthesis and transport.
- Cholestasis exposes hepatocytes to potentially toxic retained bile constituents.
03ICUCholestasis of critical illness is usually multifactorialNuanced
Inflammation, drugs, nutrition, hypoperfusion and biliary pathology may overlap. The laboratory phenotype does not identify mechanism by itself.
- Sepsis can produce transporter-mediated cholestasis.
- Prolonged shock can damage the biliary tree.
- Imaging answers a different question from transporter physiology.
04PolarityThe hepatocyte has a blood side and a bile sideEstablished
Bilirubin handling is directional. Unconjugated bilirubin travels bound to albumin; hepatocytes conjugate it through UGT1A1, increasing water solubility. MRP2 exports bilirubin conjugates at the canalicular membrane. BSEP exports bile salts and is a different transport system.
- The canaliculus is formed between adjacent hepatocytes. Tight junctions maintain separation between bile and the blood-facing compartment.
- Bile-salt secretion contributes to bile flow, but bilirubin is not exported by BSEP.
- When canalicular handling fails, basolateral transport can return conjugates to blood. Conjugated hyperbilirubinaemia therefore need not imply a blocked large duct.
05PatternA bilirubin fraction localises part of a processNuanced
Predominantly unconjugated hyperbilirubinaemia suggests excess production or impaired conjugation. A conjugated pattern points towards impaired excretion or altered transport, but does not by itself distinguish sepsis-associated cholestasis from obstruction or drug injury.
- Haemolysis, transfusion, drugs, infection and pre-existing disease can contribute simultaneously in ICU.
- Interpret fractions with the enzyme pattern, haemolysis assessment, medication exposure, imaging and time course.
- Bilirubin can lag behind clinical improvement. A concentration trend alone cannot identify which transport step is recovering.
Interventions
What are we trying to change?
Therapy makes more sense when its physiological target is explicit. These are mechanism summaries, not prescribing guidance.
TargetDownstream ductal resistance
Relieve obstruction
Restores bile flow when a mechanical blockage is the driver.
TargetInflammatory and microcirculatory drivers
Treat sepsis / restore perfusion
May reverse critical-illness cholestasis as the systemic insult resolves.
TargetIatrogenic contributors
Review drugs / nutrition
Removes potentially cholestatic or hepatotoxic exposures where relevant.
Apply the mechanism.
What not to conclude.
- Using ‘cholestasis’ and ‘obstruction’ as synonyms.
- Assuming all bilirubin elevation reflects loss of synthetic function.
- Confusing BSEP physiology with bilirubin export through MRP2.
- Over-interpreting a single bilirubin value without its trajectory and context.
Four things to keep.
- Bilirubin handling is a vectorial transport process from blood to bile.
- Conjugation and canalicular export are distinct steps.
- Critical illness can impair bile transport without mechanical obstruction.
- Bilirubin trajectory reflects production, transport, excretion and illness severity — not one mechanism.
Evidence & current guidance
Where the physiology meets current practice.
Clinical criteria and treatment recommendations change faster than core physiology. This layer keeps the module tied to current consensus and primary evidence.
Bilirubin is a vectorial transport problem, not simply a marker of 'liver function'
Bilirubin handling requires sequential production, albumin-bound delivery, hepatocyte uptake, UGT1A1-mediated conjugation and canalicular export. Hyperbilirubinaemia can therefore arise before the liver, within hepatocytes or downstream in the biliary tree. Serum bilirubin is clinically useful, but it is not a direct measure of hepatic synthetic capacity.
- Most bilirubin arises from haem catabolism: haem oxygenase generates biliverdin and biliverdin reductase generates unconjugated bilirubin.
- Unconjugated bilirubin circulates tightly bound to albumin and is poorly water soluble.
- UGT1A1 conjugates bilirubin in the hepatocyte, making bilirubin glucuronides suitable for active excretion.
- The molecular route responsible for initial unconjugated bilirubin uptake is less completely resolved than the downstream conjugation and export steps; avoid presenting OATP transport as the entire uptake mechanism.
Bilirubin export and bile-salt export are related — but not the same transporter system
The canalicular membrane contains multiple ATP-dependent export systems. MRP2/ABCC2 is the major canalicular exporter of conjugated bilirubin and other organic anions. BSEP/ABCB11 is the major canalicular bile-salt export pump and a key driver of bile-acid-dependent bile flow. Conflating the two obscures the physiology of cholestasis.
- MRP2 exports bilirubin glucuronides into bile.
- BSEP exports monovalent bile salts and is the rate-limiting canalicular step in bile-salt secretion.
- NTCP is a major basolateral uptake system for conjugated bile acids returning from the portal circulation.
- Different diseases and drugs can impair these transporters selectively, so conjugated hyperbilirubinaemia and bile-acid retention need not move in perfect parallel.
When canalicular export fails, the hepatocyte tries to send potentially toxic solutes back to blood
Cholestasis triggers adaptive transporter changes intended to limit intracellular accumulation of bile acids and organic anions. Canalicular MRP2 activity may fall while basolateral efflux pathways such as MRP3 increase, allowing conjugated bilirubin to regurgitate into sinusoidal blood rather than remain trapped inside the hepatocyte.
- MRP3/ABCC3 can export conjugated bilirubin across the basolateral membrane during impaired canalicular excretion.
- OATP1B1 and OATP1B3 participate importantly in hepatic reuptake of conjugated bilirubin from blood.
- Bile-acid uptake can also be reduced during cholestasis, limiting further hepatocyte bile-acid loading.
- Conjugated hyperbilirubinaemia can therefore represent an adaptive escape route as well as failed biliary excretion.
Sepsis-associated cholestasis is transporter and microcirculatory dysfunction — not necessarily obstruction
Systemic inflammation can impair bile formation even when the extrahepatic ducts are patent. Endotoxin and inflammatory signalling alter transporter expression and localisation, cytoskeletal architecture, tight junctions and hepatobiliary perfusion. The result can be predominantly conjugated hyperbilirubinaemia with variable alkaline phosphatase and GGT changes.
- Sepsis can reduce hepatocyte bile-acid uptake through NTCP/OATP pathways and impair canalicular BSEP and MRP2 function.
- Canalicular cytoskeletal and tight-junction disruption further impairs effective bile secretion.
- Critical-illness cholestasis can coexist with hypoxic hepatocellular injury, haemolysis, drug injury or mechanical obstruction.
- A rising bilirubin in ICU therefore describes a phenotype; it does not identify the mechanism by itself.
Mechanical cholestasis and intrahepatic cholestasis can look biochemically similar while requiring completely different treatment
Both downstream obstruction and hepatocellular transporter failure can raise conjugated bilirubin. The physiology diverges at the point of failure: obstruction raises resistance to bile flow beyond the hepatocyte, whereas inflammatory or drug-related cholestasis may impair uptake, canalicular transport and bile formation without a removable blockage.
- Relieving a stone, stricture or blocked stent can restore downstream flow when obstruction is the driver.
- Drainage cannot correct sepsis-mediated transporter dysfunction when the ducts are patent.
- Fractionated bilirubin, biochemical pattern, imaging and clinical context are therefore complementary rather than interchangeable.
- Cholangitis adds infection to obstruction; critical-illness cholestasis adds inflammation to a non-obstructed transport system.
The bilirubin trajectory can remain abnormal after the acute physiology begins to improve
Serum bilirubin reflects production, conjugation, canalicular export, basolateral escape and downstream drainage. It can therefore change more slowly than haemodynamics or aminotransferases. In prolonged conjugated hyperbilirubinaemia, a fraction of bilirubin can become covalently albumin-bound ('delta bilirubin'), further slowing the fall measured by routine direct-reacting assays.
- A persistent bilirubin rise does not necessarily mean that the initiating shock or inflammatory insult is still worsening.
- Conversely, improving haemodynamics do not immediately prove restored canalicular transport.
- Trend interpretation should be integrated with AST/ALT, ALP/GGT, INR, haemolysis markers, imaging and the clinical course.
- This is one reason bilirubin trajectory is useful prognostically while remaining mechanistically non-specific.
Ask which step has failed before calling every jaundiced ICU patient 'cholestatic'
A useful bedside framework is to localise the dominant failure: excess bilirubin production, impaired uptake/conjugation, impaired canalicular export, inflammatory cholestasis, or downstream obstruction. More than one mechanism is common in critically ill patients.
- Predominantly unconjugated hyperbilirubinaemia should refocus attention on haemolysis, production, uptake and conjugation.
- Predominantly conjugated hyperbilirubinaemia points toward impaired excretion, adaptive basolateral efflux or obstruction, but does not distinguish them alone.
- Bilirubin is not synonymous with bile acids, hepatocyte necrosis or synthetic failure.
- The physiology becomes most useful when paired with trajectory and clinical context rather than a single threshold.
Key sourcesPrimary guidance and landmark evidence
Reviewed 12 September 2026 · next scheduled review September 2027Bilirubin Hepatic and Intestinal Transport and Catabolism: Physiology, Pathophysiology, and BenefitsReview published 2025; PubMed PMID 41300483.
Recent comprehensive review of bilirubin production, hepatic transport, intestinal handling and modern bilirubin physiology.
View source ↗Pathophysiology of sepsis-induced cholestasis: A reviewGhenu MI, et al. JGH Open. 2022;6:378–387. PMID 35774351.Mechanistic review of inflammatory, transporter, cytoskeletal and microcirculatory changes underlying cholestasis during sepsis.
View source ↗The roles of MRP2, MRP3, OATP1B1, and OATP1B3 in conjugated hyperbilirubinemiaKeppler D. Drug Metab Dispos. 2014;42:561–565. doi:10.1124/dmd.113.055772.Key mechanistic review explaining canalicular MRP2 export, basolateral MRP3 escape and OATP-mediated reuptake of conjugated bilirubin.
View source ↗Regulatory mechanisms of the bile salt export pump (BSEP/ABCB11) and its role in related diseasesSystematic review. PMID 33581308.Detailed review of BSEP as the major canalicular bile-salt exporter and its role in bile flow and cholestatic disease.
View source ↗Dubin-Johnson and Rotor Syndromes: A Review and Update of Pathophysiological MechanismsReview published 2026. doi:10.14218/JCTH.2025.00560.Recent molecular review illustrating how selective defects in MRP2 versus OATP1B1/1B3 generate distinct conjugated-bilirubin transport phenotypes.
View source ↗Review policy: each module is reviewed at least annually and sooner after a major guideline, consensus statement or practice-changing study. A review date indicates editorial review, not that every linked source is open access.