Randy bem, age 14, of Spokane, Wash., for his question:

What makes the heart beat?

Lub dub, lub dub, lub dub, lub dub, the heart beats like a drum, slower when you sleep, faster when you. Run. It beats more than 4000 times an hour, and each minute it sends fiye quarts of blood pulsing through the Veins and arteries. Nerves connected with the brain and spinal column can slow down or speed up the pulse. But each beat is started by a built in system of nerves and fibers within the heart itself.

The heart of a grown man is about as big as a fist and weighs little more than half a pound. During a quiet minute, it beats about 70 times. The nerves and fibers which cause each pulsing beat are embedded in the muscular walls of the heart, and they are not connected with the rest of the nervous system. The stem heart beat, then, is an automatic operation which does not depend upon the rest of the body.

There are, however, other nerves which change the pace of the heart beat, and they are connected with the nervous system.. Two pairs of nerves go from the heart to the spinal column. They are the accelerators which make the heart beat faster when the body needs a faster supply of blood. Another pair goes to the brain. These are the inhibitors which make the heart beat slower while you sleep.

The heart itself is a small miracle of tough muscle. The hollow center is divided into two separate parts by a wall of muscle. Each side has a small upper chamber called the atrium or the auricle and a larger lower chamber called the Ventricle. The heart relaxes while the chambers fill with blood. Then it contracts and squeezes the blood out to circulate through the body

Each beat is started by a wad of tissue called the pacemaker or the sinoauricular node. It causes the upper chambers to contract and triggers off a second node. The impulses from these built in nerves and fibers start at the top and slither down to the ventricles at the bottom of the heart. Then, with one mighty squeeze, the ventricles push up the blood into the vessels of the circulatory system. For a moment, the heart rests. Then blood flows down to fill the chambers., and the little pacemaker gets ready to trigger off the impulses which start another pulsing beat.

The left side of the heart receives fresh blood from the lungs and pulses it out through a great artery called the aorta. From there, it travels through the circulatory system, delivering food and oxygen and collecting waste materials. It returns through the veins to the right side of the heart, and each pulse sends it streaming to the lungs to be refreshed. The fresh blood then returns to the left side of the heart to be sent on another trip through the circulatory system.