Editing Jaundice (section)

==== Prehepatic metabolism ====
When [[red blood cell]]s complete their lifespan of about 120 days, or if they are damaged, they rupture as they pass through the [[reticuloendothelial system]], and cell contents including [[hemoglobin]] are released into circulation. Macrophages phagocytose free hemoglobin and split it into [[heme]] and [[globin]]. Two reactions then take place with the heme molecule. The first [[oxidation]] reaction is catalyzed by the microsomal enzyme [[heme oxygenase]] and results in [[biliverdin]] (green color pigment), [[iron]], and [[carbon monoxide]]. The next step is the reduction of biliverdin to a yellow color [[tetrapyrrole]] pigment called bilirubin by cytosolic enzyme [[biliverdin reductase]]. This bilirubin is "unconjugated", "free", or "indirect" bilirubin. Around 4&nbsp;mg of bilirubin per kg of blood are produced each day.<ref name="PedRev">{{cite journal|vauthors=Pashankar D, Schreiber RA |date=July 2001|title=Jaundice in older children and adolescents|journal=Pediatrics in Review|volume=22|issue=7|pages=219–226|doi=10.1542/pir.22-7-219|pmid=11435623}}</ref> The majority of this bilirubin comes from the breakdown of heme from expired red blood cells in the process just described. Roughly 20% comes from other heme sources, however, including ineffective [[erythropoiesis]], and the breakdown of other heme-containing proteins, such as muscle [[myoglobin]] and [[cytochrome]]s.<ref name="PedRev" /> The unconjugated bilirubin then travels to the liver through the bloodstream. Because this bilirubin is not soluble, it is transported through the blood bound to [[serum albumin]].{{citation needed|date=August 2020}}