METABOLISM OF BILIRUBIN

METABOLISM OF BILIRUBIN

 
 

After 120 days of lifespan RBC's are broken down to release Hemoglobin

The hemoglobin then broken down to form Haem and Globin

            Globin being a protein is used as such or converted to amino acids and taken to amino acid pool

The Haem is then broken down to release a porphyrin compound and Iron

            The iron is taken to Iron pool and re utilized

The porphyrin ring opens and is converted to Biliveridin a green pigment

The biliveridin is immediately reduced to Bilirubin an yellow pigment

The formation of Bilirubin is taking place in the reticulo endothelial cells of  Spleen, Bone marrow and

 
 

The Bilirubin thus formed being water insoluble is then attached with the transport protein albumin and is then transported to liver through the blood. This albumin Bilirubin complex is known as the Unconjugated Bilirubin or Indirect Bilirubin or Free Bilirubin

 
 

Liver.

In liver there are three processes taking place

1. Uptake of Bilirubin in to liver cells (hepatocytes)
   
2. Conjugation of Bilirubin with Glucuronic acid
   
3. Excretion of Conjugated Bilirubin in to Bile
   

The albumin is removed form the Bilirubin and is taken in to the  hepatocytes where they are conjugated.

            The Bilirubin in the hepatocyte is conjugated with two molecules of the UDP Glucuronic acid to form the Bilirubin Di Glucuronide and is known as the Conjugated Bilirubin or Direct Bilirubin.

            The conjugated Bilirubin being water soluble is excreted in to the bile and it reaches the small intestine

Small Intestine:

In small intestine the glucuronides are removed and the Bilirubin is converted to urobilinogen by the intestinal bacteria.

A part of the urobilinogen is reabsorbed back in to the liver through the portal circulation and reaches the liver and thus in to the general circulation. Then it is excreted in to the urine via kidneys. The urobilinogen in urine is the cause of yellow coloration to urine and when it is exposed to oxygen is converted to dark Urobilin.

The remaining urobilinogen in the small intestine is excreted in to the feces as Stercobilinogen. This Stercobilinogen in the stool is the cause of yellow coloration to the stool and when it is exposed to the oxygen it is converted to dark colored stercobilin.