Three tissue layers called tunicae, or coats, surround the lumen of an artery. The adventitia (outer coat) is the outermost layer of an arterial wall and is composed of connective tissue containing elastic fibers and another layer of protein called the external basement membrane. Smooth muscle cells comprise the tunica media, or middle coat. The muscle cells contract or relax to constrict or dilate the artery.

See also Coronary Angiogram, Angiography, Angioplasty, and Cardiac Catheterization.