(Science Trek Music) Blood travels around your body using arteries and veins called blood vessels.
A blood vessel has three layers.
The inner layer is smooth so blood can flow more easily.
The middle layer is mostly muscle, and most blood vessels have a tough outer layer to provide structure.
When thinking about blood vessels, imagine a tree.
A tree has a trunk, limbs, and branches, each getting smaller.
Blood vessels are like that too.
There are three basic types of blood vessels: arteries, veins and capillaries.
Arteries start out about as wide as your thumb.
They branch into smaller arterioles.
These are about as thick as dental floss.
Arterioles branch into capillaries.
Capillaries are the only blood vessel with just one thin layer.
Their walls have to be so thin that gases like oxygen can pass through to body tissue.
Capillaries then take in waste products like carbon dioxide and attach to very thin venules and then to bigger veins.
Veins are closer to the skin's surface, which is why you can see them in your hands.
For more information about blood, check out the Science Trek website.
You'll find it at sciencetrek.org