Erythropoietin (EPO) is a glycoprotein produced by the kidney and, to a lesser extent, by the liver. EPO is also a hormone, which is a compound that is secreted into body fluids to be transported to another organ, where it causes an alteration in some aspect of metabolism.
Specifically, EPO is transported via the blood to the bone marrow, where it binds to receptors. This binding stimulates the bone marrow to produce more red blood cells (erythrocytes). Medically, EPO has a therapeutic benefit in the treatment of certain forms of anemia, a condition where the numbers of red blood cells are abnormally low. Red blood cells contain hemoglobin, a molecule that transports oxygen. By increasing the red blood cell content and, consequently, the content of hemoglobin, the oxygen-carrying capacity of blood is elevated.
The athletic boost provided by EPO can be produced naturally (and permissibly) by training at higher altitudes, which stimulates the production of more red blood cells in response to the decreased content of oxygen in the air. However, EPO content can also be boosted by directly injecting the compound into the bloodstream, or via transfusion with blood that has been previously removed from an athlete. These latter strategies, which are termed blood doping, are illegal in athletics.
Because aerobic activity relies on the availability of oxygen and its use by muscles, the ability of EPO to increase the oxygen in blood for the muscles has been recognized and exploited by some athletes.
The illegal athletic use of EPO has not deterred its use in sports such as marathon running, speed skating, Nordic (cross-country) skiing, and cycling, where endurance is a critical element of success.
The enhanced athletic performance produced by EPO comes with health risks. The associated increased thickness (viscosity) of the blood due to the presence of the compound and the loss of water from dehydration during vigorous exercise can make an athlete more prone to a heart attack or stroke, due to the lodging of blood in a constricted region of an artery.