CHAPTER 93: DISEASES OF THE AORTA

THE NORMAL AORTA

The aorta has complex biological and mechanical properties involving intrinsic relaxation and contraction that interact with left ventricular ejection to enhance hemodynamics. The major conductance vessel of the body, the aorta is an elastic artery with a trilaminar wall: the tunica intima, tunica media, and tunica adventitia (Fig. 93–1).^1,2 The innermost lining of the tunica intima is the endothelium, resting on a thin basal lamina. The subendothelial tissue is comprised of fibroblasts, collagen fibers, elastic fibers, and mucoid ground substance. An internal elastic membrane forms the outer lining of the tunica intima. The tunica media is approximately 1 mm thick, comprised of elastin, smooth muscle cells, collagen, and ground substance. The predominance of elastic fibers in the aortic wall and their arrangement as circumferential lamellae distinguish it from the smaller muscular arteries. A lamellar unit is made up of two concentric elastic lamellae containing smooth muscle cells, collagen, and ground substance.^3,4 The thoracic aorta incorporates 35 to 56 lamellar units and the abdominal aorta about 28 units.⁵ Surrounding the tunica media is the tunica adventitia, which is composed of loose connective tissue, including fibroblasts, relatively small amounts of collagen fibers, elastin, and ground substance. The adventitia strengthens the aorta and is essential to aortic surgeons for secure suturing of tissues. Within the tunica adventitia lie the nervi vasorum and vasa vasorum. The arteries arising along the course of the aorta give rise to the vasa vasorum, which develop into a capillary network supplying the adventitia and media of the thoracic aorta. The vasa vasorum do not supply the media of the abdominal aorta. Unlike the elastic fibers of the arterial wall, which are highly distensible, collagen is inelastic and provides the tensile strength required to prevent deformation and rupture of the aortic wall. Sophisticated biaxial strength testing of the human thoracic aorta has permitted new, detailed understanding of the tensile properties of the aorta (contributed by elastin and collagen) and permitted mechanical modeling of dissection and rupture events.⁶

The ascending aorta is approximately 3 cm in diameter, depending on age, sex, and body surface area. Among approximately 3500 individuals in the Multi-Ethnic Study of Atherosclerosis, the mean diameter of the ascending aorta was 3.2 ± 0.4 cm,^7,8 and in none of these normal individuals exceeded 5 cm. The diameter of the aortic arch is similar. Descending in the posterior mediastinum, the aorta tapers slightly to about 2 to 2.3 cm and the abdominal aorta narrows further to 1.7 to 1.9 cm ...