Light microscopy of an artery (left) and vein (right). For comparable physical size the artery shows a far thicker wall compared with the vein. The arterial wall is comprised of smooth muscle and elastic fibres (black/indigo wavy lines) that serve to control blood pressure and flow via vessel constriction and relaxation/elastic recoil. In contrast, veins function as reservoirs holding blood with capacity for expansion and elasticity but limited contractile properties hence a scarcity of smooth muscle in the smaller-sized veins. Magnification x180 when printed at 10 cm.