Erythropoietin is a hormone produced primarily by the kidneys. Its main function is to stimulate the production of red blood cells in the bone marrow. This process is crucial for maintaining adequate oxygen levels in the blood, especially during times of low oxygen availability, such as at high altitudes or in cases of anemia. When oxygen levels in the body decrease, the kidneys release more erythropoietin into the bloodstream. This hormone then signals the bone marrow to increase red blood cell production. Erythropoietin is also used as a medication to treat certain types of anemia, particularly in patients with chronic kidney disease or those undergoing chemotherapy.