Ron Walker, age 9, of Huntsville, Alabama, for his question:

How many miles of blood vessels are in the body?

Suppose we could separate all the blood vessels from a grows man and arrange them in one long tube. It would be long enough to reach all the way around the equator twice    and almost half the way around a third time. Naturally this has never been done and nobody would think of trying. But scientists figure that a grown man has about 60,000 miles of blood vessels in his body.

Some are the big veins and arteries that run to and from the heart. The big artery that leaves the heart branches into smaller arteries and still smaller vessels called arterioles. The arterioles branch into fine networks of skinny little capillaries. The capilliaries join skinny little venules. Networks of venules join to form the larger and larger veins that carry the blood back to the heart. Most of the 60,000 miles of blood vessels in a man's body are tiny capilliaries and venules    and they are too fine to be seen.