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Edward Kaplan, age 15, of Bronx, Now York, for his question:

How do red and white corpuscles differ?

The blood stream teems with busy traffic. The stream itself is watery plasma. The traffic is countless billions of little boats called corpuscles or blood cells. Each has its own job to do and never rests. The red corpuscles act as postmen. There are three types of white corpuscles. They act as soldiers, doctors or garbage men.

The liquid plasma keeps the traffic circulating throughout the body. The beating heart pumps it through a network of branching blood vessel so large and small. The round trip takes the blood through the lungs and through the heart. The plasma carries dissolved food and healing ingredients. It also removes certain wastes from the body tissues. The rest of the work is done by the floating cells.

Each drop of healthy blood is about 55 percent watery plasma. That drop of blood teems with about 300 million red cells and about 428,570 white cells. So you can imagine how small a single corpuscle must be. Each red cell looks like a fat coin with a swollen rim. The white cells are pale blobs with dark centers. And they are slightly larger than the red cells.

The mail carried by the red cells is oxygen. They got it from the main post office, which is the lungs. Here the red cells flow past spongy pockets of air. Each red cell is a sack of hemoglobin, a magic stuff made of iron and protein. The hemoglobin grabs oxygen from the air in the lungs. The red postmen are then whisked on their rounds carrying bags of oxygen.

Somewhere along the way, the oxygen will be delivered to living cells. The postmen will pick up a return load of waste carbon dioxide. This will be left in the lungs when a now load of oxygen is collected.

For every 700 red cells, there is one white cell in the blood stream. About three quarters of the white cells are called leucocytes. These fellows are warriors. When bacteria invade the body, the white leucocytes attack and devour them. Countless numbers of these soldiers perish in the fray. Their worn bodies form part of the pus from an infected wound.

When the warriors we is done, the flesh is a t‑‑^,‑n battlefield. Then the doctor cells move in to do healing and repair work. Those are the lymphocytes and they make up about 22 percent of all the white cells. The rest of the white cells are monocytes. These fellows act as garbage men and health officers. They prowl the blood stream seeking alien bacteria and foreign bodies which might harm the tissues. They pounce and devour these invaders.

The life of the red postman cells is busy, but not dangerous. A red blood cell may live through 17 to 18 weeks of fetching and carrying. Then it is broken down and the thrifty body uses its iron in making new red cells. The white cells live a hazardous and mach shorter life. A white cell may lose its life after a few hours of heroism. At most, it can live only ten days or so.

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