Pump, Vessel, Pulse: Precise Words About the Heart

How Your Heart Moves Blood

1Press your hand flat against your chest, just left of the center, and you may feel a steady thump. That thump comes from your heart, a hollow, muscular organ about the size of your fist. Your heart sits in your chest between your lungs, and its job is to pump blood. With each beat, it sends blood through a network of tubes called blood vessels that reach every part of your body. As the blood circulates, or travels in a loop, it delivers oxygen that your cells need to work.

2Inside, the heart is divided into four chambers, or hollow spaces. The two upper chambers, called atria, collect blood as it flows into the heart. The two lower chambers, called ventricles, have thick, strong walls. When a ventricle contracts, or squeezes tightly, it forces blood out of the heart.

3The heart also has four valves. Valves are flaps of tissue that open to let blood through and then snap shut. They keep blood moving in only one direction. When the valves close, they make the "lub-dub" sound that a doctor hears through a stethoscope.

4Blood leaves the heart through arteries, the blood vessels that carry blood away from the heart. The arteries branch into smaller and smaller vessels. The tiniest vessels are capillaries, which are thinner than a hair. Their walls are so thin that oxygen can pass through them into the body's cells. At the same time, waste such as carbon dioxide passes from the cells into the blood. Then the capillaries join together to form veins, the blood vessels that carry blood back to the heart.

5Each side of the heart has its own job. The right side pumps blood to the lungs, where the blood picks up fresh oxygen and gets rid of carbon dioxide. The left side pumps this oxygen-rich blood out to the rest of the body.

6Each time the ventricles contract, a surge of blood stretches the walls of your arteries. You can feel this as a pulse when you press two fingers gently on the inside of your wrist or on the side of your neck. When you run or jump, your muscles need more oxygen, so your heart beats faster and your pulse speeds up.

Theo's First Draft

Your heart is a really important thing in your body. It pushes stuff through a bunch of tubes. Inside, it has some rooms. Little doors open and close so the blood goes the right way. You can feel your heart going in your wrist. When you run around, it goes a lot faster.