Erythropoietin is a natural hormone (protein). It controls erythropoiesis, which is the production of erythrocytes (or red blood cells) by the bone marrow.
Erythropoietin is produced in the kidney and to a lesser extent in the liver. Its secretion correlates with partial oxygen pressure in secretory cells, and therefore indirectly with haematocrit However, secretion is not controlled by the number of erythrocytes present but rather by the amount of oxygen available.
A number of diseases require erythropoietin to be given to the affected patients, particularly those with renal failure who are on dialysis (artificial cleansing of blood).
Unfortunately, while so useful for dialysis patients, erythropoietin is misused by athletes for doping purposes in order to increase their circulating red blood cell count.