What Are Veins Made Out Of?

The tunica adventitia is the strong outer covering of arteries and veins. It is composed of connective tissue as well as collagen and elastic fibers. These fibers allow the arteries and veins to stretch to prevent overexpansion due to the pressure that is exerted on the walls by blood flow. The tunica media is the middle layer of the walls of arteries and veins. It is composed of smooth muscle and elastic fibers. This layer is thicker in arteries than in veins. The tunica intima is the inner layer of arteries and veins. In arteries this layer is composed of an elastic membrane lining and smooth endothelium that is covered by elastic tissues.
Veins do not contain the elastic membrane lining that is found in arteries. In some veins the tunica intima layer also contains valves.