Bilirubin Metabolism - Pathway Made Simple [Erythrocytes, Conjugation, Urobilinogen etc.]
This video will clearly explain the pathway of Bilirubin Metabolism.
Hopefully this video will be helpful for you and if you have any questions, comments or feedback: please comment below!
Some important background information:
Red blood cells, also called erythrocytes, are located in the blood and transport oxygen through our body. The oxygen is bound to an iron-containing oxygen-transport protein, called hemoglobin. When the red blood cells are are old, they are degraded by specialized macrophages called reticuloendothelial cells. This initiates the so called heme-catabolism.
First hemoglobin is degraded, it splits into heme and globin. Globin will be reused for the production of red blood cells (erythropoiesis). Iron is extracted from the heme and that leaves a structure of four pyrrole nuclei. From this, Biliverdin is formed.
Biliverdin is then reduced to bilirubin, or better known as unconjugated bilirubin. This is released into the plasma and there, much of the bilirubin associates with plasma albumin.
When this arrives in the liver, the bilirubin dissasociates and is taken up by hepatocytes, the cells of the liver, via facilitated diffusion. In hepatocytes, 80% is conjugated to form bilirubin glucuronide; and the rest conjugates with other substances.
The hepatocytes secrete this conjugated bilirubin into bile to be excreted. In the intestines, bacterial action converts conjugated bilirubin, which is too big to be absorbed, into urobilinogen, which is highly soluble. A portion of the urobilinogen is participating in a so called ‘enterohepatic urobilinogen cycle’. Which means that some of the urobilinogen is reabsorbed. Most of this is re-excreted by the liver and 5% by the kidney into the urine, in a form that is yellow and called urobilin, which exists after exposure to air.
In feces it also alters and is oxidized to form stercobilin.
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