Blood doping refers to the use of artificial means to increase the number of red blood cells. Erythropoietin, a hormone produced by both the kidney and liver, increases the rate of maturation of erythrocytes in the red bone marrow. A genetically engineered form of erythropoietin (EPO) can be injected into athletes before an event, with a resulting increase of red blood cell count from 45 to 65 percent. With more red blood cells, there is more oxygen available for heavily exercising muscles. However, once an athlete becomes dehydrated, the blood can become too thick, causing clotting, stroke, or serious heart problems.