CAROL & MIKE WERNER / VISUALS UNLIMITED, INC. / SCIENCE PHOTO LIBRARY CAROL & MIKE WERNER / VISUALS UNLIMITED, INC. / SCIENCE PHOTO LIBRARY
Blood vessels. Artery on the left, vein on the right, with small arterioles and venules or capillaries branching off. Note the valves in the vein to control the direction of blood flow.
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