BIG: The ongoing flow of blood from the heart to the body and back to the heart again is called circulation. Blood vessels carry blood throughout your body. Veins are the vessels that return blood to the heart.
Anatomy of a vein
Veins have cuplike flaps called valves spaced along their inside walls. Valves open upward, so blood can move up the vein. Valves close to keep blood from falling back down the vein.
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