Updated: 2/8/2022

Bilirubin Production and Excretion

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Overview
bilirubin
  • 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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