Erythropoietin or EPO is a hormone secreted by the interstitial cells of the kidney.
It is responsible for stimulating the stem cells in the bone marrow to produce red blood cells and prevent the destruction of red blood cells.
When oxygen levels drop, the interstitial cells of the kidney respond by increasing the production of erythropoietin.
For example, at high altitudes, oxygen levels are low; this increases the production of erythropoietin to produce more red blood cells for oxygen transport to the tissues.
Erythropoietin levels can be tested in a laboratory and are commonly done for athletes to check for blood doping.