EPO is naturally occurring in the body and is produced by the kidneys. EPO stimulates the production of red blood cells in bone marrow and regulates the concentration of red blood cells and hemoglobin in the blood. This is useful for athletes since red blood cells shuttle oxygen to the cells, including muscle cells, enabling them to operate aerobically. By injecting EPO, athletes aim to increase their concentration of red blood cells and, consequently, their aerobic capacity by injecting EPO.  If EPO levels are too high the body will produce too many red blood cells, which can lead to blood clotting, heart attack and stroke. In fact, EPO has been implicated in the deaths of several athletes.