Animated video showing fate of RBC/hemoglobin
Hemoglobin is a purple-colored, iron-containing respiratory pigment of cytoplasm of RBC. Hemoglobin is a conjugated protein and is formed of 4 heme groups and a protein called globin. Each heme group contains an iron-containing porphyrin ring formed of 4 pyrrole rings. Globin is formed of 4 polypeptide chains—2a and 2p chains. One RBC possess about 280 million hemoglobin molecules.
When the RBCs become old, their wall become weak and the shape is changed due to slowing of enzyme activities. Some of these RBCs are then broken down by the force of the circulation. The hemoglobin released combines with haptoglobin (a plasma protein) and is carried to the RE cells. But most of the RBCs are phagocytosed by the RE cells and broken down. The Hb is released inside the RE cells. Th RE cells are situated mainly in the spleen and also the liver, bone marrow, etc. and these cells do not phagocytose the normal RBCs as the normal RBCs are flexible and can pass easily through the splenic filter. The old RBCs are deformed and stiff hence fail to pass through the filter and are phagocytosed by the RE cells situated near the filter.
There may be other mechansims for the detection of senile RBCs by the RE cells.
Some Hb in plasma is split into haem and globin. This haem is carried to the RE cells by another plasma protein known as haemopexin.
First step of the process is opening of tetrapyrrole ring. This is catalysed by the enzyme haem oxidase with liberation of one molecule of carbon monoxide (only reaction on the body producing CO). Then the globin is separated and added to the amino acid pool. THe Fe liberated is either stored in the RE cells as ferritin or used again. The other portion i.e. the opened up tetrapyrrole structure, forms the biliverdin which is reduced to bilirubin and is carried to hepatocytes (liver cells).
Inside the hepatocytes the billirubin is bound to the cytoplasmic proteins called ligandins. There, with the help of the enzyme gluconyl transferase it is conjugated to form bilirubin mono or diglucuronides in the smooth endoplasmic reticulum. Deficiency of this enzyme in neonates leads to physiological jaundice. Now this is called conjugated bilirubin (soluble) and is excreted through the bile in intestine. A small amount of bilirubin sulphate, etc. are also formed. If bilirubin accumulates inside the body, jaundice occurs and jaundice may occur due to increased destruction on RBCs, due to decreased glucuronyl transferase activity or other problems in the liver, or due to obstruction of the bile flow to the intestine.
In the intestine the bilirubin is acted upon by bacteria to form urobilinogen. SOme of the urobilinogen which passes through stool is called stercobilinogen which gives yellow color to the stool. This stercobilinogen on oxidation, outside the body, is converted into stercobilin which then gives stool a brown color.
Part of the urobilinogen formed in the intestine is reabsorbed and re-excreted through bile (entero-hepatic circulation). A part of this urobilinogen is also excreted through urine. On oxidation tis is converted to urobilin.
It is clear thar from e above discussion that bilirubin is formed from the dead RBCs and is excreted with bile, so bilirubin is purely an excretory product. But as stated before, the bilirubin level in the body increases either due to increased hemolysis or due to decreased excretion by the liver, hence its plasma level is a good indicator of liver disease and also hemolysis.
Source: Essentials of Medical Physiology By Mahapatra