BILIRUBIN
Sources of bilirubinFrom the breakdown of red cells at the end of their life in the reticuloendothelial system mainly in the spleen.
From the breakdown of immature red cells in the bone marrow.
From Hb – related compounds such as myoglobin and cytochromes.
The released Hb is split into globin, which enters the general protein pool, and haem, which is converted to bilirubin after removal of iron. The iron is reutilised, 80% of bilirubin is metabolised daily by this process.
Unconjugated bilirubin (free or indirect)
That is bilirubin which has not yet been made water-soluble by conjugation with glucoronate) is carried to the liver bound to plasma albumin. At the hepatic cell membrane, bilirubin is removed from the albumin, then conjugated with glucoronic acid forming bilirubin diglucoronide. The reaction is catalysed by the enzyme uridyl diphosphate glucoronyl transferase (UDP).The unconjugated (indirect or free) bilirubin is water insoluble and lipid-soluble and can enter and damage brain cells.
Conjugated or direct bilirubin
The conjugated or direct bilirubin is water soluble. Normally, about 300 mg of bilirubin reaches the liver and is conjugated daily.Conjugated bilirubin is secreted into bile canaliculus and further to gall bladder for storage, concentration and excretion into small intestine. In the intestine, bacteria deconjugate the bilirubin glucuronate and reduce bilirubin to urobilinogen. Most of this urobilinogen is excreted in the faeces as such or as urobilin after oxidation in air. A small portion is reabsorbed through the enterohepatic circulation to be excreted in the urine. Therefore, normal urine contains urobilinogen but not bilirubin. However, the disappearance of urobilinogen and appearance of bilirubin (direct) in the urine is an abnormal condition that indicate the presence of obstructive jaundice. P.235
Jaundice
Jaundice is due to an increase in the concentration of bilirubin in blood and is, a common sign in hepatic or biliary tract disorders.Types of jaundice
A- Pre-hepatic jaundice (hemolytic) ( unconjugated).1- Acute and chronic hemolytic anemia.
2- Neonatal physiological jaundice.
B- Hepatic jaundice ( mainly unconjugated).
1- Conjugation failure.2- Transport disturbances.
3- Hepatocellular damage or necrosis, viral and toxic hepatitis and cirrhosis.
4- Intrahepatic obstruction (e.g. edema) ( conjugated and unconjugated).
C- Post-hepatic jaundice (obstruction of common bile duct due to
stones, tumor or spasm and stricture (mainly conjugated).
Normal values
Total bilirubin up to 17 μmol/L (1 mg/dl)Direct bilirubin up to 5 μmol/L (0.3 mg/dl)
To convent mg/dl to μmol/L each 1 mg/dl of bilirubin=17 μmol/L