Erythropoietin (EPO) is a Hormone that stimulates Red Blood Cell (RBC) production and is normally produced in the kidney or liver, but primarily in the kidney. A Hormone is a chemical messenger released by cells into the blood that affects a distant part of the body. Erythropoietin travels through plasma to bone marrow increasing Red Blood Cell production that acts on an enzyme. An enzyme is a protein molecule that acts as a catalyst (something that speeds up reactions) for chemical reactions in living systems. EPO increases Red Blood Cell production, in theory, causing more oxygen to be put into active muscles resulting in increased muscle endurance.
             EPO can be increased through natural causes in the human body or through a method called 'Blood Doping'. Blood Doping is usually related to athletics. Athletes can do one of two main ways to increase Red Blood Cells in the body. They could remove a fair amount of their blood well before a competition, separate the red blood cells from plasma, and then re-insert the red blood cells prior to the competition. Another way to increase Red Blood Cells is to inject the drug Erythropoietin. Artificial EPO is produced by recombinant DNA techniques originally developed with chronic renal failure (kidney failure). The more legal way to increase erythropoietin is to train when there is a lack of oxygen, usually at high altitudes. This first became evident during the 1968 summer Olympics in Mexico City. Athletes that lived and trained in higher altitudes generally did better in long distance competitions. Sometimes tumors can produce Erythropoietin, or even excessive blood loss can activate production of Erythropoietin in the kidneys. Erythropoietin is stimulated by both Androgens (Steroid which produces secondary male sexual characteristics) and growth hormone. However, it is primarily produced when there is a low concentration of oxygen
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