Process of bilirubin formation and excretion in neonate

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The formation and excretion of bilirubin are very essential in adults as well as in the neonate. But, there is a higher risk of hyperbilirubinemia present in neonates. And that’s the main cause of neonatal jaundice.

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To get more clarification on this subject, we have to go through the concept. Like where the formation of bilirubin takes place, how they circulate, and at last where they go, etc.


process of bilirubin formation in the fetus

  • The abnormal or damaged red blood cells of the fetus are removed from the normal circulation. It has mainly done by the reticuloendothelial cells. 
  • These aged heme are converted into bilirubin and go to hepatocytes(the liver cells).
  • In the liver, the glucuronyl transferase (liver enzyme) conjugates the bilirubin with the bilirubin conjugating enzyme called uridine diphosphoglucuronic acid (UDPGA).
  • The above process can form bilirubin diglucuronide. It is the conjugated bilirubin. Further, it secrets actively in the bile duct.
  •  Through the bile duct, the bilirubin diglucuronide which is the conjugated bilirubin passes to the gastrointestinal tract. But, it cannot be eliminated from the fetal body. As the fetus does not normally pass stool. 


What happens when the bilirubin does not excrete in the fetus?


  • The enzyme called beta-glucuronidase, present in the small bowel luminal brush border released into the intestinal lumen. Here the conjugated bilirubin(bilirubin diglucuronide) becomes deconjugated.
  • After deconjugation, the bilirubin glucuronide reabsorbs in the intestinal tract and again enters the fetal circulation.
  • The fetal bilirubin then goes to the maternal blood by the placenta. From that, it goes to the maternal liver and becomes excreted after conjugation.


How does the bilirubin cause neonatal jaundice in the neonate?


  • At birth, the placental connection terminates between fetus and mother. In the extrauterine life, the liver of the neonate is not fully matured. But, somehow able to conjugate the bilirubin and excreted into the bile.
  • In the neonate, there are also insufficient intestinal bacteria to oxidize the bilirubin into urobilinogen.
  • Like that the unconjugated bilirubin is present in the stool of neonates. it forms the yellowish coloration of the stool.


Why does the bilirubin cause yellow skin in the neonate?


  • After birth, the gastrointestinal tract of neonate contains a beta-glucuronidase enzyme.
  • It helps to deconjugate some of the bilirubin.
  • The unconjugated bilirubin reabsorbs and gets back to the circulation. It has also mainly come from the intestinal lumen.
  • At last, This can cause neonatal jaundice in most the neonate.



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