function of the cardiovascular system.

There are 5 different blood vessels in your body.

1) Arteries and Arterioles.
Arteries and Arterioles are similar to each other. They both take blood away from the heart under extreme pressure. for the Arteries and Arterioles to survive the pressure they have a thick layer of muscle. They're elastic, because they need to expand to deal with the high pressure. Arterioles are small version of Arteries, Arterioles still have the same structure as Arteries.

2) Capillaries.
Capillaries are so small. They are only one cell thick. In the capillaries the exchange of gasses and waste products takes place. They turn waste into urine. Oxygen  gets off and C02 gets in. They're only one cell thick to allow for an easy exchange.

3) Veins and Venules.
These vessels carry blood back to the heart under low pressure. they're muscle layer is thin, because for them to have large lumin. The large lumin allows blood to flow with as little resistance as possible. They contain valve, because to prevent back flow.

Arteries are away from your heart and veins are to your heart. Arteries take blood away from the heart and the capillaries put oxygen on to it, after the veins take it to the heart to pump it and it starts again.