The diameter of a typical human erythrocyte is 6-8 µm. Adult humans have roughly 2-3 × 1013 red blood cells at any given time (women have about 4-5 million erythrocytes per cubic millimeter of blood and men about 5-6 million). Red blood cells are thus much more common than the other blood particles: about 4-11 thousand white blood cells per cubic millimeter, and about 150-400 thousand platelets per cubic millimeter. The red blood cells store collectively about 3.5 grams of iron; that's more than five times the iron stored by all the other tissues combined. The blood types of humans are due to variations in surface glycoproteins of erythrocytes. The process by which red blood cells are produced is called erythropoiesis. Erythrocytes are continuously being produced in the red bone marrow of large bones. (In the embryo, the liver is the main site of red blood cell production.) The production can be stimulated by the hormone erythropoietin (EPO), which is used for doping in sports. Erythrocytes develop in about 7 days and live a total of about 120 days. The aging cells swell up to a sphere-like shape and are engulfed by phagocytes, destroyed and their materials are released into the blood. The main sites of destruction are the liver and the spleen. The heme constituent of hemoglobin is eventually excreted as bilirubin. Red blood cells can be separated from blood plasma by centrifugation. During plasma donation, the red blood cells are pumped back into the body right away, and the plasma is collected. Some athletes have tried to improve their performance by doping their blood: first about 1 liter of their blood is extracted, then the red blood cells are isolated and frozen, and they are reinjected shortly before the competition. (Red blood cells can be conserved for 5 weeks at -78° Celsius.) This practice is hard to detect but may endanger the human cardiovascular system because it is not equipped to deal with blood of the resulting higher viscosity.