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(Science Trek Music)

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Blood travels around your body

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using arteries and veins called

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blood vessels.

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A blood vessel has three layers.

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The inner layer is smooth so

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blood can flow more easily.

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The middle layer is mostly

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muscle, and most blood vessels

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have a tough outer layer to

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provide structure.

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When thinking about blood

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vessels, imagine a tree.

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A tree has a trunk, limbs, and

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branches, each getting smaller.

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Blood vessels are like that too.

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There are three basic types of

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blood vessels: arteries, veins

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and capillaries.

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Arteries start out about as wide

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as your thumb.

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They branch into smaller

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arterioles.

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These are about as thick as

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dental floss.

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Arterioles branch into

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capillaries.

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Capillaries are the only blood

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vessel with just one thin layer.

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Their walls have to be so thin

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that gases like oxygen can pass

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through to body tissue.

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Capillaries then take in waste

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products like carbon dioxide and

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attach to very thin venules and

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then to bigger veins.

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Veins are closer to the skin's

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surface, which is why you can

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see them in your hands.

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For more information about

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blood, check out the Science

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Trek website.

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You'll find it at

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sciencetrek.org