Shelly Barnes, age 12, of Tulsa, Oklahoma, for her question:

What do red and white blood cells do?

The red cells are more or less alike and their main duties are to fetch and carry oxygen. The white blood cells come in several different shapes and sizes. Some are designed to fight germs and various enemy invaders. Not all of their many duties are fully understood. Also pres¬ent in the flowing bloodstream are special little cells that help to seal and heal open wounds.

Each drop of wondrous blood in your body is a watery straw colored liquid, teeming with a variety of separate floating cells. Most of them are red blood cells, shaped like tiny saucers with thick rims. They con¬tain a substance called hemoglobin, which is capable of remarkable chem¬ical activities. Its molecules can grab and loosely hold atoms of oxygen. They release this fuel to the body’s busy cells—in exchange for mole¬cules of their waste carbon dioxide.

Day and night, the bloodstream carries its floating red cells on a ceaseless parade between the lungs and the busy cells. On each return trip to the lungs, they exchange their waste carbon dioxide molecules for supplies of fresh oxygen. These cells are vivid red when they carry oxygen and bluefish when they carry carbon dioxide. When blood flows from a wound, it takes oxygen from the air and becomes red.

There are enough teeming red cells to color the pale liquid and conceal the white cells. There is one white cell of some sort for about 700 of the reds. Some of the white cell types are called granulocytes because microscope slides reveal that they contain little granules. There are several types of these grainy white cells, all employed in some way by the body’s defense systems.

When an enemy bacterium enters the body, it is pursued and engulfed by one of the white blood cells. In some cases, the white cell oozes itself through the wall of a blood vessel and pursues the enemy through the cell tissues. When the body is attacked by a serious infection, enormous numbers of white cells are lost in the fray. This triggers the manufacture of more and still more white cells to carry on the good work. Other white cells rally to cope with pockets of infected tissue.  Others specialize in attacking only certain enemy agents. They may be remodeled to cope with this or that enemy should it attack again at a later date. When a doctor suspects an infection, he takes a count of the white blood cells. If the number is higher than normal, he knows that the body is manufacturing extras to replace those lost in combat.

Also present in the bloodstream are mini mini cells called platelets. Their special duty is to cope with blood flowing from an open wound. When they come in contact with the air, in some mysterious way they create fibrous material which causes the blood to thicken, or clot. The clot plugs up a broken vessel, stops the flow of blood and also forms the scab that helps to heal the wound.