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Gastrointestinal ` gastrointestinal—PHysiology Gastrointestinal ` gastrointestinal—PHysiology seCtion iii 375

Bilirubin Heme is metabolized by heme oxygenase to biliverdin, which is subsequently reduced to bilirubin.
Unconjugated bilirubin is removed from blood by liver, conjugated with glucuronate, and excreted
in bile.
Direct bilirubin: conjugated with glucuronic acid; water soluble (dissolves in water).
Indirect bilirubin: unconjugated; water insoluble.

Excreted in urine as
urobilin ( yellow color) Kidney
90% 10%
Enterohepatic
circulation

20%
Macrophages Bloodstream Liver Gut
Albumin
Unconjugated Unconjugated bilirubin- Conjugated
RBCs Heme Urobilinogen
bilirubin albumin complex bilirubin
UDP-
glucuronosyl- Gut bacteria
transferase 80%

Indirect bilirubin Direct bilirubin
(water insoluble) (water soluble) Excreted in feces as
stercobilin ( brown
color of stool)

FAS1_2019_09-Gastrointestinal.indd 375 11/7/19 4:42 PM