Arteries and veins both have the same three layers in their vessel walls. The outer layer consists of connective tissue. In arteries, the outer layer has many elastic fibers which give the arteries great elasticity. The middle layer consists of smooth muscle which helps the vessels dilate or constrict. Veins have a much less well developed smooth muscle layer than arteries have. Thus, the walls of veins are easily collapsible which allows the surrounding skeletal muscles to squeeze the blood in the veins forward toward the heart during body movement. This and not the pumping action of the heart is the mechanism by which blood is returned to the heart from the lower extremities since the hydrostatic pressure produced by the heart on the blood is very low by the time the blood reaches the veins; gravity helps blood return from the upper body. The blood is prevented from moving backwards by a system of one-way valves in the veins. The inner layer of the vessel walls of arteries and veins is made of connective tissue lined with a simple squamous cell endothelium. As the arterioles connect with the capillaries, the two outer layers of their vessel walls as well as the connective tissue of the third layer end leaving the capillaries with vessel walls that are only a single cell thick. The complete three layers of the vessel walls reappear as the capillaries join with the venules of the vein system. While the triple-layered walls of the arteries and veins are impermeable to substances in the blood and, in fact, prevent exchange of materials between blood and tissue, the single cell-thick walls of the capillaries allow an easy exchange of materials between blood and tissue.
