<P> Erythropoiesis (from Greek' erythro' meaning "red" and' poiesis' meaning "to make") is the process which produces red blood cells (erythrocytes). It is stimulated by decreased O in circulation, which is detected by the kidneys, which then secrete the hormone erythropoietin . This hormone stimulates proliferation and differentiation of red cell precursors, which activates increased erythropoiesis in the hemopoietic tissues, ultimately producing red blood cells . In postnatal birds and mammals (including humans), this usually occurs within the red bone marrow . In the early fetus, erythropoiesis takes place in the mesodermal cells of the yolk sac . By the third or fourth month, erythropoiesis moves to the liver . After seven months, erythropoiesis occurs in the bone marrow . Increased level of physical activity can cause an increase in erythropoiesis . However, in humans with certain diseases and in some animals, erythropoiesis also occurs outside the bone marrow, within the spleen or liver . This is termed extramedullary erythropoiesis . </P> <P> The bone marrow of essentially all the bones produces red blood cells until a person is around five years old . The tibia and femur cease to be important sites of hematopoiesis by about age 25; the vertebrae, sternum, pelvis and ribs, and cranial bones continue to produce red blood cells throughout life . </P>

Which substance increases the production of rbc’s