What is it?
It's a big name for one of the most important systems in the body. Made up of the heart, blood and blood vessels, the circulatory system is your body's delivery system. Blood moving from the heart, delivers oxygen and nutrients to every part of the body. On the return trip, the blood picks up waste products so that your body can get rid of them.

Your Heart
About the size of your clenched fist, your heart is a muscle. It contracts and relaxes some 70 or so times a minute at rest -- more if you are exercising -- and squeezes and pumps blood through its chambers to all parts of the body. And it does this through an extraordinary collection of blood vessels.

Your Blood Stream
Your blood travels through a rubbery pipeline with many branches, both big and small. Strung together end to end, your blood vessels could circle the globe 2 1/2 times! The tubes that carry blood away from your heart are called arteries. They're hoses that carry blood pumped under high pressure to smaller and smaller branched tubes called capillaries. The tubes that more gently drain back to the heart are veins.

How does your blood get oxygen?
When you inhale, you breathe in air and send it down to your lungs. Blood is pumped from the heart to your lungs, where oxygen from the air you've breathed in gets mixed with it. That oxygen-rich blood then travels back to the heart where it is pumped through arteries and capillaries to the whole body, delivering oxygen to all the cells in the body -- including bones, skin and other organs. Veins then carry the oxygen-depleted blood back to the heart for another ride.

What's blood, anyway?
Most of your blood is a colorless liquid called plasma. Red blood cells make the blood look red and deliver oxygen to the cells in the body and carry back waste gases in exchange. White blood cells are part of your body's defense against disease. Some attack and kill germs by gobbling them up; others by manufacturing chemical warfare agents that attack. Platelets are other cells that help your body repair itself after injury.

FACTOIDS:

• The body of an adult contains over 60,000 miles of blood vessels!
• An adult's heart pumps nearly 4000 gallons of blood each day!
• Your heart beats some 30 million times a year!
• The average three-year-old has two pints of blood in their body; the average adult at least five times more!
• A "heartbeat" is really the sound of the valves in the heart closing as they push blood through its chambers.