Put a hand on your chest. Your heart is in there. It is an organ about the size of your fist. It pumps blood through your body. It is made of muscle. The heart has four chambers. The two lower ones are called ventricles. They pump blood out of the heart.

Now let us follow the blood on its trip through you. With each heartbeat, the heart sends blood throughout your body. The blood carries oxygen to every cell in your body. After the blood delivers oxygen, it returns to the heart. Then the heart sends it to the lungs to pick up more oxygen.

Blood travels through three main kinds of blood vessels: arteries, veins and capillaries. Arteries carry blood away from the heart, and veins carry blood back to the heart. Capillaries are tiny blood vessels that connect small arteries to small veins. Your body has about 60,000 miles of blood vessels.

Next, count your beats. You can feel your pulse on the inside of your wrist. Your pulse is the rate your heart beats. It is also called your heart rate. Put two fingers there and try it. Can you feel it?

Here is how to find your heart rate. Count the beats you feel in 30 seconds and double the number. You can do this any time you like. Your heart pumps more or less blood depending on what your body needs. When you exercise, your heart beats faster to get more oxygen to your muscles. Running and swimming are good for your heart.

Words to know

• ventricles: lower chambers
• chambers: heart rooms
• capillaries: tiny vessels
• pulse: heart rate
• arteries: outgoing vessels