Bilirubin Production and Excretion

Overview

Overview bilrubin is a yellow-colored byproduct of hemoglobin (heme) metabolism elevated levels of bilirubin causes jaundice, characterized by yellow skin and sclerae  in the newborn, short-lived RBCs results in increased production of unconjugated bilirubin as heme breakdown products, decreased conjugation due to immature UDP-glucuronosyltransferase, and increased enterohepatic circulation due to decreased gastrointestinal activity

Pathway

Steps from bilirubin production to excretion (see above) (1) reticuloendothelial system (RES) macrophages phagocytose sensescent erythrocytes hemoglobin metabolism yields bilirubin pathway: heme → biliverdin (green-colored) → bilirubin (yellow-colored) (2) bloodstream albumin binds bilirubin and complex is carried to liver bilirubin-albumin complex = indirect bilirubin (water insoluble) (3) liver hepatocytes take up bilirubin hepatic microsomes conjugate bilirubin with glucoronic acid conjugation via UDP glucuronyl transferase enzyme is synthesized slowly after birth, sometimes causing newborn jaundice conjugated bilirubin = direct bilirubin aka water soluble a portion of conjugated bilirubin is excreted in urine remainder is secreted into bile and then into small intestine (4) gastointestinal tract in terminal ileum and colon, bilirubin is deconjugated by bacterial enzymes and metabolized to urobilinogen 18% of urobilinogen is absorbed via enterohepatic circulation and delivered back to liver 80% of urobilinogen is converted to stercobilin and excreted in feces stercobilin gives characteristic color of feces 2% of urobilinogen is converted to urobilin and excreted in urine urobilin gives characteristic color of urine

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