To Estimate Total and Direct Bilirubin in Serum
Bilirubin originates from breakdown of heme. It is a waste product and the body eliminates it through bile. The heme present in erythrocytes contributes approximately 85% of total bilirubin which is destroyed in the reticuloendothelial cells. The destruction of red blood cell precursors in the bone marrow and catabolism of other heme-containing proteins like myoglobin, cytochromes, and peroxidases release the remaining 15% of bilirubin. Biliverdin is first formed from the porphyrin part of heme in reticuloendothelial cells and is reduced to bilirubin called unconjugated bilirubin, which is then transported to the liver in association with albumin. This unconjugated bilirubin is insoluble in water. Bilirubin then enters to microsomes of hepatocytes and conjugated by the action of glucuronyl transferase to produce bilirubin monoglucuronide and diglucuronide (conjugated bilirubin). The bilirubin conjugated with glucuronic acid is water soluble and it exerted through bile. The conjugated bilirubin present in bile passes along the bile ducts into the intestine. Here it is reduced by bacterial action and is also deconjugated, mainly in the colon to “urobilinogens.” Urobilinogen is recycled through the body and a part of it is excreted through the urine.