Since heme oxygenase is also present in renal tubular cells, the renal epithelium is capable of converting hemoglobin to bilirubin. However, this only occurs when there is intravascular hemolysis with hemoglobinuria (i.e. the renal epithelium does not take up unconjugated bilirubin or hemoglobin from blood!). The renal epithelium absorbs the filtered hemoglobin from the urine, converting it to unconjugated bilirubin and then conjugating it for excretion into the urine. This may be responsible for some of the bilirubinuria seen in animals with intravascular hemolysis, however in most of these animals, there is concurrent cholestasis that is responsible for the bilirubinuria (which is conjugated).
