This picture of a blood vessel in the spleen looks like a large, fuzzy looking wall with big holes, through which oddly shaped cells with smooth surfaces try to squeeze. These are red blood cells, but they don't really look like normal red blood cells. When they pass through capillaries, our narrowest blood vessels, where the life-sustaining oxygen is exchanged for carbon dioxide, our red blood cells must be able to change their shape in order to hug the capillary wall so tightly that the gasses can go through the membranes.

After about 120 days, the red blood cell membrane becomes too rigid for the cell to pass these holes. So they are trapped and picked up by garbage collecting cells (scavenger cells, or "macrophages"), which then recycle most of their contents for the production of new red cells.

Some things about blood are well known and manageable, such as the blood groups; some are amazingly simple and elegant, such as the slight change in cell membrane flexibility described above that helps eliminate outdated red cells from the circulation; but most are complex, largely unknown, and in need of research. At the Jerome H. Holland Laboratory of the American Red Cross in Rockville, Maryland we research how blood works, and we're always discovering neat new things.