Debbie Rub1e, age 11, of Muncie, Ind., for her question:

When does the heart rest?

The heart is a organ that beats a quiet rhythm from the beginning to the very end of life. It takes no vacations from duty and its steady drumming does not stop even when the body falls as1eep. Nevertheless, it takes regular rest periods.

With a steady lub dub, the beating heart pulses blood through the circulatory system of the entire body. Fresh blood from the lungs carries oxygen, and the living tissues need oxygen to carry on their work. Certain fragile cells cannot survive a minute without oxygen. The pulsing blood stream also carries away carbon dioxide, and if this waste gas is not removed the tissues soon choke and perish.

The drumming heart is a living pump that keeps this vital circulation going. Its lub dub pulsing cannot stop for even a few minutes. But the heart itself is a muscle, and even sturdy muscle tissue must have rest. It accomplishes this with many short rest periods. The day and night drumming never stops throughout life, but with every beat, the heart manages to take a brief moment of restful relaxation.

The average beat is 70 times a minute, so each throbbing pulse takes less than a second. But in that short time the four chambers of the muscular heart perform an orderly series of motions. A strong wall divides the heart, separating the two upstairs atria and the two downstairs ventricles. There are trapdoor valves between the upper and lower chambers and complex valve systems to connect the atria with che blood vessels outside the heart.

The pumping or systole phase of each pulse begins when blood from major veins and arteries enters the atria and pushes down to fill the ventricles. The muscular ventricles contract like fists, pulsing used blood from the right side to the lungs where it exchanges its carbon dioxide for oxygen, and fresh blood from the lungs into the arteries. The systole lub beat is followed by a dub sound as these exit valves close and others open.

Now the diastole phase of the pulse begins and the heart can relax. Valves admit fresh blood from the lungs into the left side and used blood from the veins into the right side. While the empty atria and ventricles refill, all the muscles relax. The heart rests during the quiet diastole phase of every pulse, right after the lub dub pumping work of the systole phase.

The heart rests a mere fraction of a second, but it takes an average of 70 of these short relaxation periods every minute. This is enough to keep itself in good condition  and the heart is a marvelous housekeeper. It renews its worn cells and often repairs injured parts. It beats faster during stress or exercise to meet the body's need for more oxygen  and after every lub dub drum beat, the wonderful living pump takes a rest for itself.