3000 mg of bilirubin is formed daily, 75% of which is from heme metabolism and 25% of which is from other proteins (ex. cytochrome P450)
Normal Bilirubin Levels
- Total: 0.1-1.9 mg/dL (depending on the source)
- Conjugated: 0-0.4 mg/dL (also depending on the source)
What is bilirubin? – bile pigment, a physiological product of heme metabolism – not all jaundice indicates liver dz – metabolized in the liver and excreted into bile ducts • Except for liver diseases, HEMOLYSIS, will also present with jaundice.
Metabolism of bilirubin: Bilirubin–>heme and globin –>iron and biliverdin–>bilirubin –>binds tightly with albumin in the blood stream, and is separated just before being uptaken into liver cells–>conjugates with glucuronic acid –>conj. Bilirubin–>free unconjugated bilirubin in the blood stream and is not soluble in water. After being taken into hepatocytes, it is converted to soluble conjugated form and is able to be excreted into bile ducts.
About 15 ~ 20 % of the urobilinogen is reabsorbed from the intestine into portal veins and finally
90 % of them return to the liver and is re-excreted in the bile, it is called entero-hepatic circulation of bilirubin. The remaining 10 % gets into the systemic circulation and finally exreted in the urine through kidney. Thus the urine urobilinogen increases in the case of hemolytic disease, hepatocellular disease and porto-systemic shunt.
Excretion of bilirubin: bilirubin–> urobilinogen(colorless) by intestinal pathogens–>urobilin (aka, stercobilin, colored)–>excreted via kidneys most of the reabsorbed urobilinogen return to the liver and re-excreted into bile