Arteries and veins have three layers in their walls. Connective tissue composes the tunica externa or the outer layer, smooth muscle and elastic tissue composes the tunica media or the middle layer, and the smooth tunica interna or the inner lining is composed of endothelium. Veins have valves to keep blood flowing in the correct direction. Capillaries have walls that are the thickness of only one endothelial cell; this permits diffusion to occur across the wall.