About one-fourth of all the cells in your body (not including bacteria and other microbes) are red blood cells. The lifespan of a single red blood cell is short—only about 120 days. But your body makes new red blood cells all the time—they are produced in the spongy area of your bones called the marrow. This image was taken with a scanning electron microscope. (Image via Mustafa Mir, Sam Copeland and Gabriel Popescu) Read More