|Red Blood Cells|
An adult person has about five to six liters of blood. In a portion of blood about the size of two pinheads (1 cubic millimeter) there are normally about five million red blood cells. The total number in the body is probably about thirty-five trillion. Men ordinarily have a few more red cells than woman, and newborn baby has about one million more than a grown person. It is possible, by counting in the way described above, to determine whether a person has the proper number of red blood cells for health.
Red blood cell (Wikipedia)