 we will discuss fate of hemoglobin in both intravascular and extravascular hemolysis. Now, first let's consider extravascular hemolysis. Loss of deformability of red cell membrane leads to its breakdown in its plane and macrophages phagosatase these red cells. They also separate it into its components. So, what are these components? So, these are lipids, proteins and hemoglobin. Well, hemoglobin is also a protein, but because it has a separate catabolic pathway, so we are keeping it as separate. Anyways, hemoglobin is further broken down to heme and globin and this globin joins this protein catabolic pathway. So, each of these enter its own catabolic pathways. Proteins are catabolized to amino acid and join amino acid pool of body while lipid also are catabolized further. Now, while here we will talk in detail about catabolism of heme only. Now, macrophages further catabolize heme to biliverin. This is done by an enzyme present in the macrophages known as heme oxygenase and in this process iron is released from the heme and it joins its iron pool which can be reused. Also, oxygen is required. See, this is oxygenase and carbon monoxide is produced as one of the metabolic end product and this is the only known metabolic reaction where carbon monoxide is formed. Now, this biliverin is further reduced to bilirubin. Bilirubin is released into the circulation. Now, in intravascular hemolysis, hemoglobin is not captured by macrophages and it is directly released into the circulation. This hemoglobin which is released binds with a protein known as haptoglobin which is present in the plasma and this binding is reversible. This complex of hemoglobin and haptoglobin enters liver where hemoglobin needs the same fate as that of splenic macrophages. So, finally bilirubin is formed from hemoglobin haptoglobin complex which enters the liver. Now, this bilirubin is the end product of catabolism of heme and it needs to be excreted. So, we will see excretion of bilirubin. Now, since bilirubin is water insoluble, it can be transported in circulation only in combination with the water soluble substance. So, bilirubin after being released into the circulation binds with albumin. Now, whenever a substance is transported by binding with the protein, they bind reversibly and loosely. Bilirubin and albumin binding also behaves in the same way. When this complex reaches liver, bilirubin gets separated from albumin and enters liver by a process of facilitated diffusion. So, what is happening at the level of hepatocytes? So, the here is the blood vessel and this complex enter into the blood vessel and is taken up and the level of the hepatocytes by facilitated diffusion. Now, since it is a diffusion, it is important that intracellular bilirubin concentration is kept low and back diffusion of bilirubin is prevented. So, inside the liver cells, bilirubin binds to another protein known as ligandin. Ligandin prevents its back diffusion into the blood. Also, further metabolism of bilirubin ensures that intracellular bilirubin concentration is kept low. Now, what happens in these hepatocytes? So, let me draw a big hepatocyte. So, once bilirubin enters via facilitated diffusion, it binds with ligandin. Now, this bilirubin then combines with glucronic acid in presence of an enzyme that is glucronyl transferase and forms bilirubin di-lucronite. Now, this is also known as conjugated bilirubin because it has been conjugated to a substance. Finally, conjugated bilirubin is excreted into bile. So, this side is bile, here this is blood, bilirubin is excreted into bile by an active transport process requiring energy. So, it's important to remember this is active transport and to facilitate diffusion to continue, bilirubin should bind with glucronic acid and should be excreted. Otherwise, it will build up inside the hepatocytes. Further, bilirubin cannot enter into the cells and cannot be able to be excreted. Now, once we know the fate of hemoglobin, they can actually utilize the process for understanding the mechanism of diseases and also in determination of some clinical parameters. For example, in extravascular hemolysis, we saw that carbon monoxide is produced in this reaction and it is the only reaction in the body in which carbon monoxide is produced. So, by determining carbon monoxide production, we can actually determine the rate of breakdown of hemoglobin and thus rate of breakdown of RBCs. Second, you see that if intravascular hemolysis increases which happens in some diseases, the plasma haptoglobin will be consumed in binding to hemoglobin and any excess hemoglobin will start appearing in urine and may also precipitate in kidneys. Normally hemoglobin can't appear in urine since it's bound to a protein and their combined size is large enough to not filter in urine. Next, we'll see in another lecture on jaundice where any problem in conjugation or excretion of the bilirubin or if too much bilirubin is formed how it can lead to a jaundice.