Erythropoiesis is the process of red blood cells formation or production (erythro = red, poiesis = to form). It occurs almost exclusively in the red bone marrow. When blood oxygen drops, kidneys produce ERYTHROPOIETIN, a hormone which stimulates erythropoiesis, and speeds red blood cells formation. Presence of more red blood cells = more oxygen-carrying capacity. It has been seen that athletes use commercial erythropoietin or EPO to stimulate RBC production and improve their performance.