User:Michlwt/sandbox
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Hyperbilirubinemia is a clinical condition referring to an elevation of blood bilirubin level due to the inability to properly metabolize or excrete bilirubin, a product from the breakdown of erythrocytes, and in severe cases is manifested as jaundice, which is yellowing of tissues like skin and the sclera of eyes when excess bilirubin deposits in them.[1] By definition, bilirubin concentration of greater than 3 mg/ml is considered hyperbilirubinemia, following which jaundice progressively develops and becomes clearly visible when plasma levels reach 20 mg/ml.[2]
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Rather than a disease itself, hyperbilirubinemia is indicative of multifactorial underlying disorders that can be traced back to deviations from normal bilirubin metabolism.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Heme_Breakdown.png/640px-Heme_Breakdown.png)
The total average daily production of bilirubin in humans is 4mg/kg. 80% of which is derived from haemoglobin (Hb) upon breakdown of erythrocytes, with the remainder coming from hemp proteins, like myoglobin and cytochrome, turnover.[3] Hb is broken down to globin and haem, which encounters further degradation through a multi-step enzymatic process. [4]. After two sequential reactions converting Hb into bilirubin IX-alpha, this unconjugated bilirubin is released into the bloodstream, where a significant amount binds reversibly with plasma protein albumin and dissociates to be taken up by liver cells through organic anion transport proteins (OATPs) on cell membranes.[1] Inside hepatocytes, bilirubin is conjugated through uridine diphosphate-glucuronosyl-transferase (UGT) to form bilirubin diglucuronide, otherwise termed conjugated bilirubin, which is transported across the plasma membrane by multidrug resistant-associated protein 2 (MRP2) to finally enter bile capillaries.[4] From there, conjugated bilirubin is secreted into the small intestine with bile where a portion is reabsorbed and cycled back to the liver. Unabsorbed bilirubin reaches the colon to be hydrolysed and reduced into urobilinogen, most of which is excreted in faeces.[4]
Hyperbilirubinemia rarely has adverse outcomes. Kernicterus, a type of brain injury caused by unconjugated bilirubin neurotoxicity, occurs only in extreme cases. Adult hyperbilirubinemia, unlike newborn jaundice, does not require therapy.