Figure 1. Blood vessels come in two fundamental flavors—arteries and veins. Both types of vessels are composed of an inner endothelium (tunica intima) surrounded by internal elastic tissue, smooth muscle cell layer (tunica media), external elastic tissue, and fibrous connective tissue (tunica adventitia). Largercaliber arteries have thicker smooth muscle cell layers, and larger veins possess specialized structures such as valves. The two networks of tubes are completely separate at the level of the larger vessels but are linked distally through a system of fine capillaries.(Reprinted, with permission, from Cleaver and Krieg 1999 [copyright Academic Press].)