![]() ![]() ![]() In the aorta, these concentric elastic layers are arranged in as many as fifty distinct layers ( Figure 4). Elastic fibers are gathered together in sheets arranged in concentric layers throughout the thickness of the media. In these sizable vessels, there is a thick, highly developed media (tunica media) of which elastic fibers are the dominant component. The aorta, pulmonary trunk and brachiocephalic trunk are examples of the large, elastic arteries of the arterial system. Such impedance “mismatches” occur when the aorta branches for example (such as the bronchial or esophageal arteries branching from the thoracic aorta, or the celiac or superior mesenteric arteries of the abdominal aorta), or further in the periphery where the relatively elastic nature of the aortic wall gives way to the thicker and more muscular vessels. This wave is propagated in the vessel lumen and as it travels distal to the heart, it encounters differences in the physical impedance offered by the vasculature ( Figure 3). ![]() In a simplified and idealized model of the arterial system, a forward propagated pressure wave (P f) is created in the aorta following systole ( Figure 2). This returning “reflected” wave, by virtue of its timing in the cardiac cycle, offers a favorable diastolic pressure at the root of the aorta (near the openings of the coronary vasculature) that enhance coronary flow ( Figure 1). Reflected waves from many points of impedance mismatch in the system summate to form an aggregate, backward (toward the heart) propagated wave that normally returns to the central aorta in late systole and early diastole. These mismatches tend to amplify the forward propagated wave and also produce a partial wave reflection. As this wave propagates distally, it encounters regions of varying impedance mismatch arising from varying properties of the vascular wall and vascular diameter. When the heart contracts, the resulting forward propagated wave interacts with the inherent impedance of the aorta that creates a relatively low velocity (pulse wave velocity PWV) pressure wave 11. This modulation is also critical to normal function because of the concept of the “reflected wave”. (b) Muscular arteries, especially those of the lower limb ( eg femoral, popliteal, posterior tibial), which are capable of altering tone (the slightly contracted basal state of vascular smooth muscle) allowing them to modulate the velocity of the pressure wave that is conducted to them from the larger vessels upstream 10. The arterial system can be functionally (as well as structurally) divided into two sub-systems: (a) the large elastic arteries ( eg the aorta, the carotid vessels, the iliac arteries), which store blood ejected from the heart during systole, and expel it to the periphery during diastole, thereby ensuring that the peripheral circulation receives a steady flow of blood during both cardiac cycles which are significantly different in terms of pressure. Therefore predictably, there is a consistent increase in the incidence and prevalence of the surrogate markers of vascular stiffening in these conditions and these are typically pulse pressure and isolated systolic hypertension 6, 7, 8, 9.Īrteries deliver blood at high pressure to peripheral vascular beds. Central arterial stiffening is now fully recognized as an important consequence of aging that has been shown to provoke deleterious vascular phenotypes in diseases such as diabetes, atherosclerosis and renal disease among others 5. Stiffening in the larger central arterial system, such as the aortic tree, significantly contributes to cardiovascular diseases in older individuals and is positively associated with systolic hypertension 1, coronary artery disease 2, stroke 2, heart failure 3 and atrial fibrillation 4. While generalized stiffening of the vasculature as a hallmark of normal aging has been recognized even in ancient medical texts, systematic scientific evaluation of arterial stiffening and particularly the type that affects the central arterial axis ( ie the aortic system and its central branches) has only matured as a clinical and research discipline in recent decades. ![]()
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