# How many red cells are there in human blood? How are they counted?

 Red Blood Cells
The liquid part of our blood is called plasma. In the plasma there are millions of tiny cells, too small to be seen except under a microscope. Of these cells, the red ones (picture, left) are by far the most numerous. In spite of their small size it is still possible to figure out how many of them there are. To do this scientists take very small, measured quantity of blood, count the red cells in it and from the figure they can calculate the number in the whole body.

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.