Anatomy, Physiology, and Pharmacology of the Vascular Wall

Arterial Anatomy

The arterial tree can be divided into three separate categories: large elastic arteries, medium muscular arteries, and small arteries. The aorta and its major branches are classified as large elastic arteries, while the distributing arteries to major organs comprise the muscular arteries, and the arteries within organs comprise the small arteries. From small arteries, blood flow travels through arterioles to capillary beds, then to postcapillary venules and small veins, and blood flow then returns to the heart via larger veins. Collateral arteries are a special class of muscular arteries that traverse from one artery to another, rather than feeding into arterioles. Normally there is little flow through collateral arteries and they have low shear stress. However, when the main conduit artery is obstructed, collateral artery flow and shear stress increase substantially, as a compensatory mechanism, which after adaption eventually can restore up to one-third of the normal conduit artery blood flow ( Fig. 3.1 ).

The aortic media is composed of well-defined lamellar units; each unit consists of a concentric plate of elastin and a circumferentially oriented layer of smooth muscle cells, surrounded by a network of type III collagen fibrils embedded in a matrix of basal lamina. Finer elastin fibers compose a network between lamellae, as do bundles of interstitial type I collagen. As the aorta becomes more distant from the heart, the percentage of collagen increases and the elastin decreases, so that the thoracic aorta and its major branches have more elastin than collagen and the abdominal aorta has more collagen than elastin.

The tangential tension on an artery wall can be approximately estimated with Laplace's law (tension is proportional to the product of the radius and the pressure). Because blood pressure is similar in animals of all sizes, the average wall tangential tension per lamellar unit is remarkably constant across different species. The upper two-thirds of the thoracic aorta, which is thicker than 28 lamellar units, also contains a medial vasa vasorum. The dependence of the abdominal aortic wall on luminal nutrition, because of the lack of a medial vasa vasorum, may explain its increased propensity to aneurysm formation.

In contrast to elastic arteries, where collagen and elastin comprise approximately 60% of the dry weight of the media, muscular arteries contain proportionally more smooth muscle cells and less collagen and elastin, allowing them to alter their diameter rapidly through vasodilation or vasoconstriction. In addition, their ratio of media to lumen is higher, contributing to their function as resistive arteries ( Fig. 3.2 ). Elastin is diminished as arteries become smaller, and the internal and external elastic lamellae become discontinuous and fragmented. The smallest arteries (arterioles) consist of only an endothelium, a layer of smooth muscle cells, and a filamentous collagenous adventitia. At the capillary level, only the endothelium remains, supported by an occasional contractile connective tissue cell known as a pericyte .

The differentiation of these three types of arteries has significance because each class of vessel is subject to different pathology and diseases. Atherosclerosis affects the elastic and muscular arteries, whereas medial calcific sclerosis is limited to muscular arteries, and small arteries are subject to diffuse fibromuscular thickening and hyalinization.

Venous Anatomy

Veins, as befitting their role as capacitance vessels under low-pressure conditions, are larger and thinner walled than arteries. Despite this, the vasa vasorum in veins is denser than in arteries and has increased medial wall flow, suggesting that luminal blood flow is less capable of providing nutritional support to the vein wall. Disruption of the vasa vasorum in veins, during vein graft harvest, may lead to wall damage and worse patency rates. The subendothelial layer of the intima is missing entirely in veins, and an internal elastic lamina is present only in larger veins. The medial layer contains few smooth muscle cells, collagen, and elastin. Thin bicuspid valves, consisting of two layers of endothelium sandwiched around a layer of connective tissue, support unidirectional flow to the heart. They are present in larger numbers in peripheral extremity veins and are rarely present in central veins. The contractile state of both venules and veins is largely controlled by sympathetic adrenergic activity.

Veins can be divided into three systems: deep (located below the muscular fascia, paralleling arteries and providing approximately 90% to 95% of the extremity drainage), superficial (providing the remaining 5% to 10% of drainage), and perforating (connecting the deep and superficial systems). The usual direction of flow is from the superficial to the deep system. There is substantially more variation in venous anatomy than arterial anatomy. The function of venous valves is to decrease the hydrostatic pressure from the weight of blood within each segment of the venous system when standing and to prevent retrograde flow. Entrapment of venous valves from fibrotic scarring resulting from incomplete resolution of deep venous thrombosis can cause venous obstruction, estimated to contribute to approximately 80% of chronic venous insufficiency. The other 20% results from primary valvular incompetence. Evidence suggests that the increased collagen with decreased smooth muscle and elastin found in varicose veins may occur prior to the development of incompetent valves, leading to reflux.