MIKKEL JUUL JENSEN / SCIENCE PHOTO LIBRARY MIKKEL JUUL JENSEN / SCIENCE PHOTO LIBRARY
Vein valve. Computer illustration of a section through a vein, showing a closed valve (centre) and the layers that make up the vein's wall: (from outside in) adventitia, smooth muscle, basement membrane, endothelium. Veins contain valves to prevent blood from flowing back in the wrong direction (arrows).
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