What Are Bile Pigments, Formation & Excretion And Conjugated Bilirubin

Bile Pigments:

Bile pigments are the excretory products in bile. Bilirubin and biliverdin are the two bile pigments and bilirubin is the major bile pigment in human beings. Bile pigments are formed during the breakdown of hemoglobin, which is released from the destroyed RBCs in the reticuloendothelial system.

Formation And Excretion of Bile Pigments:

Stages of formation and circulation of bile pigments:

  • Senile erythrocytes are destroyed in reticuloendothelial system and hemoglobin is released from them.
  • Hemoglobin is broken into globin and heme.
  • Heme is split into iron and the pigment biliverdin.
  • Iron goes to iron pool and is reused.
  • First formed pigment biliverdin is reduced to bilirubin.
  • Bilirubin is released into blood from the reticuloendothelial cells.
  • In blood, the bilirubin is transported by the plasma protein, albumin. Bilirubin circulating in the blood is called free bilirubin or unconjugated bilirubin.
  • Within few hours after entering the circulation, the free bilirubin is taken up by the liver cells.
  • In the liver, it is conjugated with glucuronic acid to form conjugated bilirubin.
  • Conjugated bilirubin, is then excreted into intestine through bile.

Fate Of Conjugated Bilirubin:

Stages of excretion of conjugated bilirubin:

  • In intestine, 50% of the conjugated bilirubin is converted into urobilinogen by intestinal bacteria. First the conjugated bilirubin is deconjugated into free bilirubin, which is later reduced into urobilinogen.
  • Remaining 50% of conjugated bilirubin from intestine is absorbed into blood and enters the liver through portal vein (enterohepatic circulation). From liver, it is re-excreted in bile.
  • Most of the urobilinogen from intestine enters via enterohepatic circulation. Later, it is re-excreted through bile.
  • About 5% of urobilinogen is excreted by kidney through urine. In urine, due to exposure to air, the urobilinogen is converted into urobilin by oxidation.
  • Some of the urobilinogen is excreted in feces as stercobilinogen. In feces, stercobilinogen is oxidized to stercobilin.

Normal Plasma Levels of Bilirubin:

Normal bilirubin (Total bilirubin) content in plasma is 0.5 to 1.5 mg/dL. When it exceeds 1 mg/dL, the condition is called hyperbilirubinemia. When it exceeds 2 mg/dL, jaundice occurs.

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