Artery walls contain muscle primarily in the form of smooth muscle fibers. These smooth muscle cells are located mainly in the tunica media, the middle layer of the artery wall. Along with smooth muscle fibers, artery walls also contain elastic fibers and collagen fibers. The smooth muscle fibers in the artery walls are responsible for regulating the diameter of the vessel lumen, allowing arteries to contract (vasoconstriction) or relax (vasodilation) to control blood flow and pressure. Elastic fibers, mainly found in the tunica media, provide elasticity, allowing the artery to stretch and recoil with the pulsatile flow of blood. Collagen fibers provide structural support and strength, limiting the extension of the artery wall under pressure. Thus, artery walls contain smooth muscle fibers along with elastic fibers and collagen fibers, which together provide the artery with muscle function, elasticity, and strength.