Katrina Rayburn, age __ of Louisville, Ky., for her question:

Where do the veins begin and end?

Almost every nine year old girl has played with a hoop. But she cannot tell you where the round ring begins and ends. The hoop is a circle or cycle that has no end and no beginning. The veins in the body also go around and around in a never ending cycle. The bigger veins branch this way and that way in a network of tiny tunnels. But all these blood vessels link together in a cycle that has no end or beginning.

The blood stream flows through its endless cycle of blood vessels. There are stations along the way, but the flow never stops. At one point the veins lead the blood to the lungs where they get fresh supplies of oxygen. At another point the blood vessels lead blood from the heart. But the network of veins and blood vessels has no end or beginning, for the blood must flow without stopping day and night.