Right now, your heart is pumping blood all through your body.
There are about five quarts of blood in you. It is moving through more than 60,000 miles of blood vessels. That includes arteries, veins, and capillaries. That’s pretty far. It’s more than two trips around the whole Earth! A heart works constantly. In an average life, it beats 3 billion times without stopping once. Pretty good for a muscle the size of a fist and lighter than two baseballs!
Why does it work so hard? You! It keeps you alive. Your heart’s main job is to pump blood through your body all the time. That blood contains cells and chemicals. Without them, you would die. If certain chemicals are low or missing, your body systems go out of whack. If a virus invades, blood brings an army of white blood cells to fight it. Red blood cells carry oxygen. Muscles and organs—especially the brain—need a steady supply of oxygen. After just a few minutes without oxygen, the brain begins to die.
Treat your heart right, and it will keep going and going.

◀ “If I only had a heart,” sings the Tin Man from The Wizard of Oz. With a heart, he thinks he’d be tender and gentle. “And awfully sentimental regarding love and art,” he says. The heart doesn’t create feelings. Even so, the Tin Man knows it helps people “register emotion—jealousy, devotion.”
What’s In Your Heart of Hearts?
In a way, you have two hearts. The right heart handles only used, low-oxygen blood. The left heart deals only with oxygen-rich blood.
“Right” means the side where your right arm and leg are. ▼

Pulmonic Valve
Right Atrium
Pericardium (slimy skin that keeps the heart well lubricated)
Tricuspid Valve
Right Ventricle (receives blood from the right atrium and forces blood into the pulmonary artery)
Coronary Arteries (feed oxygen-rich blood to the heart muscle)
Aorta (or main artery, carries blood from the heart’s left side to the organs of the body)
Pulmonary Artery (carries low-oxygen blood from the heart to the lungs)
Pulmonary Vein (carries oxygen-rich blood from the lungs to the heart)
Left Atrium
Mitral Valve
Left Ventricle
Check It Out!
Arteries take blood away from the heart. Veins bring blood into the heart. Why do arteries have to be bigger and thicker?
The powerful heart pumps blood directly into the arteries. They have to be strong enough to handle this strong flood of blood.
Animal Hearts

▲ The hearts of reptiles and fish cannot separate oxygen-rich blood and oxygen-low blood. They don’t have enough chambers to do it.

▲ Round, thick earthworms have hearts. But thin flatworms don’t. Why? They are so thin that they can absorb nutrients directly through the skin. So they don’t need a pump to spread the nutrients around. Because earthworms are thicker, they do need a pump.

▲ A grasshopper doesn’t have any chambers at all. It just has one long blood vessel (a tube for blood) running down its back!

▲ Brachiosaurus had a heart the size of a pickup truck. It pumped blood three stories high to the brain!