Transport  of Bilirubin

• Unconjugated bilirubin (UCB), the principal mammalian bile pigment, is the end intravascular product of heme catabolism.
• Bilirubin is toxic to tissues; therefore, it is transported in the blood bound to albumin.
• Bilirubin in the bloodstream is usually in a free, or unconjugated, state.
• Bilirubin leaves the site of production in the reticuloendothelial system and is transported in plasma bound to albumin .
• The capacity of serum albumin to bind bilirubin is known as the binding capacity, and the strength of the bilirubin-albumin bond is referred to as the binding affinity.
• In  100 ml of plasma,  approximately 25  mg of  bilirubin can  be  tightly  bound  to  albumin  at its  high-affinity  site.
• The conjugation  of bilirubin  is  catalyzed  by a specific enzyme called glucuronyltransferase.
• proteins Ligandin  (a member  of  the  family  of  glutathione S-transferases) and Protein Y help in  intracellular  binding
• In  the  liver  the  bilirubin is  removed  from albumin, Taken  up at the  sinusoidal surface  of the  hepatocytes by  a carrier-mediated  saturable system  (facilitated transport  system). It is then concentrated to about 1,000 times the strength found in blood plasma.
• Much bilirubin leaves the liver and passes to the gallbladder, where it is further concentrated and mixed with the other constituents of bile. 
• conjugated bilirubin passes from the gallbladder or liver into the intestine. There, it is reduced by bacteria to mesobilirubinogen and urobilinogen.
• Some urobilinogen is reabsorbed back into the blood; the rest goes back to the liver or is excreted from the body in urine and fecal matter. In humans, bilirubin is believed to be unconjugated until it reaches the liver.
• 80-90%  of the  urobilinogen stercobilinogen  and  stercobilin and  excreted  through  feces.
• 70-20 %  enterohepatic  circulation reaches  the  liver, This  is  called  enterohepatic urobilinogen  cycle.
• A small  fraction  < 3 mg/dl escape  hepatic  uptake, filters  across  renal  glomerulus  and  is  excreted through  urine.

Don’t Forget to Solve all the previous Year Question asked on Transport of Bilirubin