Make a fist. That is about the size of your heart. The heart is an organ that pumps blood through your body. It is made of muscle, and it sits in your chest between your lungs, a little to the left of the middle. With each heartbeat, the heart sends blood all through your body. Put a hand on your chest and think about that.

Now look inside. The heart has four chambers. A wall of tissue called the septum divides the right and left sides of the heart. The two lower chambers are the ventricles, and they pump blood out of the heart. Valves inside the heart keep blood moving in the right direction. A heartbeat makes a "lub-DUB" sound.

Next, let us follow the blood on its way around you. 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 contains about 60,000 miles of blood vessels.

The left ventricle pumps oxygen-rich blood to the whole body. That blood carries oxygen to every cell. After the blood delivers oxygen, it returns to the heart, and the heart sends it to the lungs to pick up more oxygen. Your lungs add oxygen to your blood.

You can feel your pulse on the inside of your wrist. Your pulse is the rate your heart beats, also called your heart rate. To find your heart rate, count the beats you feel in 30 seconds and double the number. At rest, a heart rate of 60 to 100 beats per minute is normal for adults. Try it, and count slowly.

Here are some ways to look after your heart. 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 exercises that are good for your heart, so pick one and try it. Which one will you try?

Words to know

• septum: dividing wall
• ventricles: lower chambers
• capillaries: tiny vessels
• pulse: heart rate
• valves: one-way gates

The short version

The heart is an organ about the size of your fist that pumps blood through your body. It is made of muscle and has four chambers. With each heartbeat, it sends blood carrying oxygen to every cell, and blood returns to the heart and goes to the lungs to pick up more oxygen.

Quick facts

• The heart is an organ about the size of your fist, in the chest.
• A wall of tissue called the septum divides the right and left sides of the heart.
• Your pulse is the rate your heart beats, also called your heart rate.
• At rest, a heart rate of 60 to 100 beats per minute is normal for adults.
• Your body contains about 60,000 miles of blood vessels.