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Updated: Feb 8 2022

Bilirubin Production and Excretion

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  • 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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