The heart.

Your heart is just a little bigger than your fist. Yet it beats more than 2,000,000,000 (that's two billion!) times in your life. It pumps nine tons of blood each day.

How does the heart "pump" blood?

You've probably squeezed a water balloon before. Remember how you could push the water from the top to the bottom of the balloon? Imagine two connected water balloons on one side of your heart and two connected water balloons on the other.

Blood is squeezed by the heart muscle from the top balloon (the right atrium) into the bottom balloon (the right ventricle). Then the muscle gives the ventricle a hard squeeze and pushes the blood into a tube called the pulmonary (PULL-muhn-air-ee) artery.

The pulmonary artery carries blood to the lungs. Other tubes called the pulmonary veins carry blood back to the left atrium. From there it flows into the left ventricle. The left ventricle squeezes hard and pushes the blood on a 60,000 mile journey through the body.

All this happens very quickly. Both atria squeeze first. Then the ventricles squeeze. And it all takes less than one second.

Why does blood flow only one direction?

The heart had trapdoors called valves. These "doors" open only one way. They keep the blood from blowing backward.

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When the valves close, they make a noise. The valve between each atrium and ventricle makes the loud noise of the heartbeat. The valve between the left ventricle and the aorta (ay-OR-tuh) and the valve between the right ventricle and the pulmonary artery make the soft sound.

Doctors can hear this very clearly with a stethoscope. The next time you visit your doctor, ask if you can use the stethoscope and listen to your own heart at work!