Your heart, blood, veins, and arteries make up the circulatory system. The job of the circulatory system is to circulate, or move, blood through your body.

The heart is a muscle. Your heart is about the size of your clenched fist. Each heart has a left side and a right side.

The left side of your heart fills with blood that has oxygen from your lungs. The heart squeezes and pumps the blood filled with nutrients and oxygen all over your body.

The right side of the heart does the opposite. It fills with blood that has carbon dioxide and waste from your body. The heart pumps this blood to the lungs. Your lungs remove the carbon dioxide when you exhale, or breathe out.

Your blood carries oxygen to all the parts of your body by going through little tubes called blood vessels. There are two types of blood vessels.

The first type of blood vessel is called an artery. This blood vessel is blue. It carries blood away from the heart. The artery works with the left side of the heart. An artery carries oxygen to all parts of your body.

The second type of blood vessel is called a vein. This blood vessel is red. It carries blood back to the heart. The vein works with the right side of the heart. It brings carbon dioxide from the body to the heart.

If you put all of your blood vessels end to end, they could go around the world 2 ½ times!