Janelle Spain, age 13, of Sioux City, Iowa, for her question:

Why doesn't the hard working heart get tired?

There are many kinds of work and they all require efforts or energy. We can measure energy and compare the effort used to do different jobs. For example, the heart uses energy to pump and we can compare this effort with a lifting machine. During an average day, your hard working heart uses enough energy to lift your body a mile above the ground.

The heart starts to beat before birth and continues day and night until the last moment of life. The muscles and eyes, the stomach and other organs require reasonable rest periods and every night the whole body requires seven or eight hours of restful sleep. But through all this loafing, the busy heart continues its beat, drum drumming the streaming blood throughout all the living tissues. It seems to take no rest at all, but this is not true. Actually it does as much loafing as the rest of the body. But it rests in its own special way.

Some people get through very long, busy days by taking short catnaps now and then. Well, the heart is a highly talented catnapper. Instead of taking one long rest period every day, it takes about 72 short naps every minute day and night.. Every time it beats lubb dupp, it pauses to rest for maybe three fifths of a second. Minute by minute, these miniature catnaps tot up to quite a lot of loafing., Al¬together, the heart rests just as long or longer than the rest of the body. This is why, provided you treat it sensibly, the heart can carry on its day and night pumping duties without becoming tired.

Of course, the heart can be overtaxed and strained beyond its endurance. It does its best to cope with extra exertion and works so hard that finally it may become tired and fail to beat altogether. However, these are extreme conditions that do not apply to normal daily work or to the exercise we need to keep our bodies in proper condition. Healthy people who loaf around with the excuse that they do not want to overwork their hearts are kidding themselves. The heart is built to speed up to supply extra oxygen to the muscles during periods of exercise. It

seems to enjoy a spell of fast pumping. In any case, if a healthy body fails to take a few daily spells of brisk exercise, the heart forgets how to pump faster and gets sadly out of condition.

During a brisk run, : heart can double its normal m    but after every pulsing beat it always takes a brief pause. When the exercise is finished, the pulse  rate remains fast for a few minutes. So you pause to catch your breath and allow your still racing heart to slow down. This is important. It continues to slow down and reaches the normal pulse rate of 70 to 75 pumps per minute. Then it may continue to beat even slower than normal for a while. This is another way the heart uses to rest itself. It uses another method while you sleep. When your body is quietly resting, it takes advantage of the situation to beat a little slower. And always, day and night, resting or exercising, every heartbeat ends with a brief catnap.

Let's figure the average heartbeat to be 72 pulses per minute. Each pulse is somewhat like the squeezing of a fist. The upper and lower chambers of the heart contract in two flowing operations. The upper chambers contract in 0.1 seconds, the lower chambers in 0.3 seconds. This completes a pulse. Then the heart muscles loaf for 0.4 seconds while blood fills the chambers for the next pulsing beat.