Healthy individuals at sea level continuously generate red blood cells in a processknown as “basal erythropoiesis” that is essential to life.Erythropoiesis increases by up to 10-fold its basal rate in response to hypoxicstress, as may occur at high altitude, or in response to anemia or hemorrhage.Erythropoietic rate is regulated by the hormone Erythropoietin (Epo), whoseconcentration in blood spans a remarkable, three orders of magnitude range, from≈0.01 U/ml in the basal state to 10 U/ml in extreme stress. Here, EPO is linked to anemia.