How does your heart pump?

Your heart is a strong muscle about as big as your fist. It sits in the middle of your chest, a little to the left.

Every time your heart squeezes, it pushes blood through tubes called blood vessels. Blood carries oxygen, a gas from the air that your body needs, to every part of you.

The heart has two sides. One side sends blood to your lungs to pick up fresh oxygen. The other side sends that blood out to your whole body.

Inside are little doors called valves. They open one way only, so blood always moves forward and never slips back.

Squeeze, relax, squeeze, relax. Each squeeze is one heartbeat. You can feel your pulse by gently pressing two fingers on the inside of your wrist.

When you run, your muscles need more oxygen, so your heart beats faster. When you rest, it slows down again.