<P> Adult humans have roughly 20--30 trillion red blood cells at any given time, constituting approximately 70% of all cells by number . Women have about 4--5 million red blood cells per microliter (cubic millimeter) of blood and men about 5--6 million; people living at high altitudes with low oxygen tension will have more . Red blood cells are thus much more common than the other blood particles: there are about 4,000--11,000 white blood cells and about 150,000--400,000 platelets per microliter . </P> <P> Human red blood cells take on average 60 seconds to complete one cycle of circulation . </P> <P> The blood's red color is due to the spectral properties of the hemic iron ions in hemoglobin . Each human red blood cell contains approximately 270 million of these hemoglobin molecules . Each hemoglobin molecule carries four heme groups; hemoglobin constitutes about a third of the total cell volume . Hemoglobin is responsible for the transport of more than 98% of the oxygen in the body (the remaining oxygen is carried dissolved in the blood plasma). The red blood cells of an average adult human male store collectively about 2.5 grams of iron, representing about 65% of the total iron contained in the body . </P> <P> Red blood cells in mammals anucleate when mature, meaning that they lack a cell nucleus . In comparison, the red blood cells of other vertebrates have nuclei; the only known exceptions are salamanders of the genus Batrachoseps and fish of the genus Maurolicus . </P>

Number of hemoglobin molecules in a red blood cell