Bone marrow, particularly in larger bones, is where stem cells give rise to red blood cells (erythrocytes) as well as white blood cells (leukocytes) and blood clotting agents (platelets). Specialized cells in the kidney, which are sensitive to low oxygen levels, produce a hormone called erythropoietin (EPO), which in turn promotes the production of red blood cells (erythropoiesis). EPO travels from the kidneys to the bone marrow, where it binds to receptors in the walls of stem cells. The boost in red blood cell production in turn increases the oxygen-carrying capacity of the blood.