(bĭl-ĭ-roo′bĭn)
[Pronunciation]
[″ + ruber, red]

C₃₃H₃₆O₆N₄; the orange-colored or yellowish pigment in bile.
It is derived from hemoglobin of red blood cells that have completed their life span and are destroyed and ingested by the macrophage system of the liver, spleen, and red bone marrow. When produced elsewhere, it is carried to the liver by the blood. It is changed chemically in the liver and excreted in the bile via the duodenum. As it passes through the intestines, it is converted into urobilinogen by bacterial enzymes, most of it being excreted through the feces. If urobilinogen passes into the circulation, it is excreted through the urine or re-excreted in the bile. The pathological accumulation of bilirubin leads to jaundice in many cases, such as physiological jaundice of the newborn.

Figure: BILIRUBIN CRYSTALS (×400)

Subentries:
direct bilirubin
indirect bilirubin