Final answer:
Erythropoietin (EPO) is released by the kidneys in response to low blood oxygen levels. It stimulates the production of red blood cells in the bone marrow.
Step-by-step explanation:
Erythropoietin (EPO) is released by the kidneys in response to low oxygen levels in the blood. When blood flow and/or oxygen levels decrease, the kidneys sense the shortage of oxygen and release erythropoietin.
This hormone then stimulates the production of erythrocytes (red blood cells) in the bone marrow.