The kidneys secrete erythropoietin, an endocrine hormone that stimulates red blood cell synthesis in response to low oxygen levels in the blood. The secretion of erythropoietin is regulated by oxygen levels in the blood. When oxygen levels decrease, it signals the kidneys to increase erythropoietin production and release it into the bloodstream.
Erythropoietin is predominantly produced and secreted by the kidneys in adults. However, during fetal development, erythropoietin is primarily produced in the liver before the kidneys take over this function after birth.