The walls of arteries and veins consist of three tunics, or layers:

(1) the tunica_externa, also called the tunica_adventitia;

(2) the tunica_media; and

(3) the tunica_interna, also known as the tunica_intima.

The tunica externa (tunica adventitia) is the outermost layer and is composed of areolar or fibrous CT. The tunica media is composed primarily of smooth_muscle and elastin. The tunica interna (tunica intima) lines the lumen of the vessel. The tunica interna is composed of a special class of epithelium called endothelium, which consists of squamous_epithelium with a small underlying layer of basal_lamina.

The three tunics are present in the walls of veins, but they are not as extensive as in the arteries. Veins also have valves, and the valves help return blood to the heart against the force of gravity.