In this article, we will discuss the Sites of Hemolysis. So, let’s get started.
- Intravascular hemolysis: When RBC are rapidly destroyed within circulation, free hemoglobin is released into the plasma. Free hemoglobin is toxic to cells and the body has evolved a binding mechanism so as to minimise its toxicity. Haptoglobin, an alpha globulin produced by the liver is first to bind free hemoglobin to form haptoglobin-hemoglobin complex which is too large to be excreted by the kidneys, hence, is degraded by the liver (RE cells). Haptoglobin is used for binding to hemoglobin, hence, its levels are reduced in intravascular hemolysis. Once haptoglobins are saturated, free hemoglobin is oxidised to form methaemalbumin which is detected on spectrophotometry of the plasma (Schumm’s test). Methaemalbumin is degraded and free haem formed which binds to a second binding protein called hemopexin to form a complex. When all the protective mechanisms are overloaded, free hemoglobin appear in the urine called hemoglobinuria, a characteristic feature of intravascular hemolysis. When hemolytic process is fulminant as seen in malaria, this gives rise to black urine. In small amounts renal tubular cells absorb the hemoglobin, degrade it and store the iron as hemosiderin. When the renal tubular cells containing hemosiderin are shed off into the urine, hemosiderinuria results which is always indicative of intravascular hemolysis.
- Extravascular hemolysis: In most hemolytic conditions, red cells destruction is extravascular and little or no depletion of haptoglobin occurs- a differentiating feature from intravascular hemolysis. The red cells are removed from the circulation by macrophages in RE system of liver and spleen There is no hemosiderinuria.