Blood vessels. Light micrograph of a transverse section through an artery (centre left) and a vein (upper right). The different functions of these blood vessels are reflected in their structures. The artery distributes blood from the heart to the capillaries. The numerous concentric layers of smooth muscle fibres (purple) in its wall ensure that a high arterial blood pressure is maintained between heart beats. The vein is part of a low- pressure collecting system; it returns blood from the capillaries to the heart. There are therefore fewer smooth muscle layers in the wall of the vein. Magnification: x20 at 35mm size.