Hello everyone, how are you all? i hope you all are doing well and i am too. Today we are going to learn about normal metabolism of bilirubin. We have heard the term jaundice and some may know that there will be yellowish discoloration of eyes and urine and some may know extra about that it is due to increase in bilirubin levels which is result of the breakdown of RBC(red blood cells). So, its a good thing if we learn things which are commonly seen and are related to our health. Without wasting any time we will move into our topic.

Red blood cells(RBC) contains hemoglobin and when red blood cells are engulfed and broken down by reticuloendothelial cells which are macrophages (present in spleen) especially the old red blood cells, the hemoglobin will come outside of the RBC and this hemoglobin is broken down into heme and globin. Globin is a peptide which has amino acids in it. Heme has iron and protoprophyrin and when this iron taken out and it is stored as ferritin but the protoporphyrin is converted to bilverdin and this bilverdin is finally gets converted to bilirubin and this is how we will get bilirubin. This bilirubin is called unconjugated or indirect bilirubin and this all happened in macrophage which is a phagocytic cell.
The unconjugated bilirubin is thrown out of the macrophage into the blood and it combines with albumin and form bilirubin-albumin complex which travels and reach liver where the bilirubin will be taken into hepatocyte(liver cell) and albumin will stay in blood circulation.
Now the unconjugated bilirubin is combined with the glucoronic acid with the help of glucoronyl transferase enzyme and its converted into conjugated bilirubin or direct bilirubin. This conjugated bilirubin is going to come out of the hepatocyte or liver cell into canaliculi and it will reach bile duct and then gall bladder and then into into intestine through common bile duct. This conjugated bilirubin is water soluble and in large intestine because of the bacterial enzymes action it gets converted to urobilinogen. Most of this urobilinogen is excreted in feces as urobilin and it is responsible for brown coloration of feces and a part of urobilinogen is being absorbed into the
circulation from where it goes back to liver and is taken
by the hepatocytes. It is then again re-excreted in the bile
and this is called enterohepatic circulation. A small amount of
urobilinogen is excreted in urine by getting escaped from the
liver clearance.

This is the flow chart of the normal bilirubin metabolism