the kidneys produce the hormone erythropoietin, which stimulates red bone marrow to increase production of oxygen-carrying red blood cells. The production of erythropoietin is stimulated by a decrease in the oxygen content of the blood, a condition called hypoxia. As the production of red blood cells increases in response to the erythropoietin, the increased oxygen level in the blood feeds back to the kidneys and slows the production of erythropoietin. This helps maintain the long-term homeostasis of the blood's oxygen-carrying ability.
