between systolic and diastolic values. Elastic arteries have agreater amount of elastic tissue and a smaller amount of smoothmuscle in their walls, compared with other arteries. The elasticfibers are responsible for the elastic characteristics of the bloodvessel wall, but collagenous connective tissue determines thedegree to which the arterial wall can stretch.The tunica intima of elastic arteries is relatively thick. Theelastic fibers of the internal and external elastic membranes mergeand are not recognizable as distinct layers. The tunica media con-sists of a meshwork of elastic fibers with interspersed, circularsmooth muscle cells and some collagen fibers. The tunica adven-titia is relatively thin.

Muscular Arteries

Tunica intima

Tunica media

Tunica adventitia

FIGURE 21.5 Comparison of an Artery and a Vein

The typical structure of a medium-sized artery (A) and a vein (V). Notethat the artery has a thicker wall than the vein. The predominant layerin the wall of the artery is the tunica media, with its circular layersof smooth muscle. The predominant layer in the wall of the vein is thetunica adventitia, and the tunica media is thinner than in the artery.

Muscular arteries include medium-sized and small arteries. Theuse of muscular in the name of these vessels refers to their thicktunica media. The walls of some muscular arteries are relativelythick, compared with their diameter, mainly because the tunicamedia contains 25–40 layers of smooth muscle (figure 21.6 b ). Thetunica intima of the muscular arteries has a well-developed inter-nal elastic membrane. The tunica adventitia is composed of a rela-tively thick layer of collagenous connective tissue that blends withthe surrounding connective tissue. Muscular arteries are frequentlycalled distributing arteries because the smooth muscle cells allowthem to partially regulate blood supply to different body regionsby either constricting or dilating.Smaller muscular arteries range from 40 μm to 300 μm indiameter. Those that are 40 μm in diameter have approximatelythree or four layers of smooth muscle in their tunica media,whereas arteries that are 300 μm across have essentially the samestructure as the larger muscular arteries. The small muscular arter-ies are adapted for vasodilation and vasoconstriction.

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