Your heart is a kind of pump which sends blood to all parts of your body, since none of your cells can work or grow without it.
Blood has three main functions: it carries food and oxygen to permit your body’s cells to work and grow; it carries wastes from these cells to organs which take them out of your body; and it helps fight germs that make you sick.
As your blood travels throughout your body doing these important jobs, it follows a definite route through tubes called blood vessels.
There are three main kinds of blood vessels: arteries, veins, and capillaries. When the blood is pumped out of your heart, it goes into your largest arteries, then moves into the smaller arteries, and into the capillaries. Capillaries are like bridges between your arteries and veins. They are such tiny blood vessels that you can’t see them without a microscope. But these capillaries serve a very important purpose. They are like “traders,” for it is through their thin walls that blood “trades,” or exchanges food and oxygen for the waste materials that these body cells do not need.
Blood travels from the capillaries into tiny veins and then into larger and larger veins. Finally, the largest veins take the blood back to your heart. The round trip that blood makes is a continuous one which goes on and on while a person is alive. Each round trip from the heart, through the body, and back to the heart again takes less than one minute. And this round trip is made thousands of times each day.
If all the blood vessels in your body were straightened out and placed end to end, they would be 100,000 miles long, or long enough to go around the equator four times!