The level of glucuronyl transferase is initially low in the newborn and any increase in the rate of bilirubin formation can overwhelm the capacity to conjugate, thus resulting in elevated bilirubin levels. Most unconjugated bilirubin is excreted in the stool, but some is reabsorbed and returned to the liver for re-conjugation (enterohepatic circulation). In the small bowel, some of the bilirubin is hydrolyzed to yield unconjugated bilirubin and glucuronic acid. Conjugated bilirubin is secreted into the bile and enters the duodenum. Unconjugated bilirubin is bound to albumin in the plasma and transported bound to albumin to the liver and is conjugated with glucuronic acid in the hepatocytes the conjugation is catalyzed by glucuronyl transferase. Heme is catabolized to unconjugated bilirubin in the reticuloendothelial system. Excessive bruising from birth trauma or abnormal blood collections such as in a cephalohematoma may further add to the rate of RBC destruction and bilirubin formation. The rate of neonatal RBC destruction is higher than in adults resulting in greater quantity of hemoglobin release. In newborns, the normal hemoglobin level is 15-18 mg/dl. Red cell hemoglobin accounts for approximately 85% of all bilirubin. Bilirubin Metabolismīilirubin is a product of heme catabolism. Although neonatal hyperbilirubinemia is usually a benign and physiologic condition, very high bilirubin levels occur in certain pathologic conditions and are potentially injurious to the central nervous system. ![]() ![]() Jaundice is seen in approximately half of all newborns.
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