||Most of the heme which is degraded comes from hemoglobin.
Since in the steady state 6-8 grams of hemoglobin are synthesized daily, 6-8 grams must also be degraded.
This gives rise to about 300 milligrams of heme. Heme is not reutilized, so it must be degraded and excreted.
Although heme is not recycled, its iron is conserved.
- Normally, senescent and damaged erythrocytes are sequestered by the spleen, which processes them in a manner that preserves their iron content.
- If hemolysis occurs, hemoglobin (with its iron) is released into the plasma.
- Possible causes of hemolysis include
- erythrocyte fragility
- thermal burns
- erythroblastosis fetalis.
- In the plasma, oxyhemoglobin dissociates into a-b dimers, which can escape through the glomerular filtration system of the kidney to appear in the urine.
To prevent this, there is a plasma protein, haptoglobin, which binds the dimer and
- delivers it to the reticuloendothelial system for processing
- activates the heme to prepare it for degradation.
Any free heme is bound to another plasma protein, hemopexin, which then transports it to the liver for degradation.