Arteries

Arteries are regarded as the resistance vessels of the cardiovascular system because their walls are strong, resilient, and capable of withstanding the high pressure generated during each heartbeat. Every cardiac contraction produces a pressure surge as blood is ejected into the arterial system. Thanks to their thick muscular walls, arteries maintain a round shape even when empty, which explains their circular appearance in histological sections.

Arteries are classified by size into three principal types: conducting (elastic or large) arteries, distributing (muscular or medium) arteries, and resistance (small) arteries.

The conducting arteries are the largest vessels and serve primarily to transport blood rapidly to major branches that supply the organs—much like interstate highways feeding secondary roads. Representative examples include the aorta, common carotid, subclavian, pulmonary trunk, and common iliac arteries. Structurally, these arteries contain an internal elastic lamina at the junction between the tunica interna and tunica media, though it may appear incomplete or blend with the elastic fibers of the media. The tunica media is composed of 40–70 concentric layers of elastic sheets interspersed with smooth muscle, collagen, and additional elastic fibers. Perforations within these elastic sheets allow the passage of nerves and the vasa vasorum, as well as intercellular communication among smooth muscle cells through gap junctions. An external elastic lamina separates the tunica media from the tunica externa, although it can be difficult to distinguish microscopically. The tunica externa is relatively thin (less than half the thickness of the media) and contains sparse connective tissue and vasa vasorum. Functionally, conducting arteries expand during ventricular systole to accommodate the surge of blood, thereby reducing pressure on smaller downstream vessels. During diastole, their elastic recoil helps maintain arterial pressure and smooth out fluctuations in blood flow. In contrast, arteries stiffened by atherosclerosis lose this elastic property, leading to increased stress on downstream vessels and a heightened risk of aneurysm formation.

The distributing arteries (also known as muscular or medium arteries) are smaller branches that deliver blood to specific organs, analogous to state highways branching from major routes. Most named arteries, such as the brachial, femoral, renal, and splenic arteries, belong to this group. Their tunica media contains up to 40 layers of smooth muscle, accounting for roughly three-quarters of the total wall thickness. In histological sections, smooth muscle is more prominent than elastic tissue, although both the internal and external elastic laminae are usually thick and clearly visible.

The resistance arteries (or small arteries) represent the terminal branches of the arterial system and are the primary regulators of blood flow into tissues—comparable to city streets that control local traffic. These vessels are typically less than 0.1 mm in diameter and contain one to five layers of smooth muscle. The smallest of these, the arterioles, possess only one to three layers of smooth muscle and minimal tunica externa. Arterioles are chiefly responsible for modulating perfusion to individual organs and tissues by altering vascular resistance.

Metarterioles are short transitional vessels that play two key roles in microcirculation. First, they connect arterioles to capillaries, serving as channels that distribute blood into capillary beds. Second, they provide bypass pathways, permitting blood to flow directly from arterioles to venules when capillary perfusion is reduced or temporarily halted. Unlike continuous smooth muscle layers found in other arteries, metarterioles possess individual smooth muscle cells spaced intermittently along their walls. Each of these cells forms a precapillary sphincter that encircles the entrance to a single capillary. When these sphincters constrict, blood flow through the corresponding capillary is reduced or stopped, redirecting circulation to other vascular regions where demand is greater.