 Large arteries or elastic arteries such as the aorta and pulmonary arteries convey blood from the heart to the systemic and pulmonary circulations, respectively (see Fig. 13.2). Their main branches—the brachiocephalic trunk, common carotid, subclavian, and common iliac arteries—are also classified as elastic arteries.
 Medium arteries or muscular arteries (most of the “named” arteries of the body) cannot be sharply distinguished from elastic arteries. Some of these arteries are difficult to classify because they have features that are intermediate between the two types.
 Small arteries and arterioles are distinguished from one another by the number of smooth muscle layers in the tunica media. By definition, arterioles have only one or two
FIGURE 13.13 • Molecular mechanism of blood vessel vasoconstriction. Contraction of vascular smooth muscle in a blood vessel (vasoconstriction) decreases its diameter and increases vascular resistance, leading to increased systemic blood pressure. Binding of angiotensin II and thrombin to vascular endothelial cells stimulate synthesis of endothelium-derived factors that regulate smooth muscle contraction. These include endothelins (the most potent family of vasoconstrictors), prostaglandin H2 (PGH2), and its derivative thromboxane A2. They bind to their own receptors on the smooth muscle cell membrane, causing an influx of Ca2 and an increase in the release of intracellular-stored Ca2 from the sarcoplasmic reticulum. The reduced rate of nitric oxide (NO) production, which is a potent vasodilatator, or inactivation of NO by the superoxide anion (O2–) has a stimulating effect on smooth muscle contraction. (Based on Noble A, Johnson R, Thomas A, Bass P. The Cardiovascular System. London, New York: Churchill Livingstone, 2005.) layers, and small arteries may have as many as eight layers of smooth muscle in their tunica media.
Elastic arteries have multiple sheets of elastic lamellae in their walls.
From a functional standpoint, elastic arteries serve primarily as conduction tubes; however, they also facilitate the continuous and uniform movement of blood along the tube. Blood flow occurs as follows: The ventricles of the heart pump blood into the elastic arteries during systole (the contraction phase of the cardiac cycle). The pressure generated by contraction of the ventricles moves the blood through the elastic arteries and along the arterial tree. Simultaneously, it also causes the wall of the large elastic arteries to distend. The distension is limited by the network of collagenous fibers in the tunica media and tunica adventitia (Fig. 13.14). During diastole (the relaxation phase of the cardiac cycle), when no pressure is generated by the heart, the recoil of the distended elastic arteries serves to maintain arterial blood pressure and the flow of blood within the vessels. Initial elastic recoil forces blood both away from and back toward the heart. The flow of blood toward the heart causes the aortic and pulmonary valves to close. Continued elastic recoil then maintains continuous flow of blood away from the heart.
The tunica intima of the elastic artery consists of endothelium, subendothelial connective tissue, and an inconspicuous internal elastic membrane.
The tunica intima of elastic arteries is relatively thick and consists of the following.