The kidneys and liver produce erythropoietin when oxygen levels in the cells are low. The hormone then stimulates the bone marrow, which, in turn, makes more red blood cells. It is normal for a small amount of erythropoietin to circulate in the blood, as this helps replenish the body's natural blood stores.
An erythropoietin (EPO) test may be ordered when you have anemia that does not appear to be caused by iron deficiency, vitamin B12 or folate deficiency, decreased lifespan of red blood cells (RBCs; hemolysis), or by excessive bleeding.
Erythropoiesis is stimulated by eEpo, and under conditions of severe hypoxia (low O2 concentration) eEpo levels can increase up to 1000-fold (Erslev, 1997). Epo is initially synthesized as a 193 amino-acid precursor.
Epo secretion is stimulated by hypoxia, which is detected by an oxygen sensor located in the kidney. This oxygen sensor has been recently shown to be an heme protein.