The aorta has a complex intrinsic biology and sophisticated mechanical properties involving intrinsic relaxation and contraction that interact with left ventricular ejection to enhance hemodynamic function. 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. 106–1).1,2 The innermost lining of the tunica intima is the endothelium, resting on a thin basal lamina. The subendothelial tissue comprises 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, comprising elastin, smooth muscle cells, collagen, and ground substance. The predominance of elastic fibers in the aortic wall and their arrangement as circumferential lamellae distinguish this elastic artery from the smaller muscular arteries. A lamellar unit is made up of two concentric elastic lamellae and the smooth muscle cells, collagen, and ground substance contained within.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 is known to surgeons as the "strength" layer of the aorta and is essential for secure suturing of aortic 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.
Transverse section of the wall of a large elastic artery demonstrating the well-developed tunica media containing elastic lamellae. Pararosaniline–toluidine blue stain; medium magnification. Reproduced with permission from The circulatory system.1.
The ascending aorta is approximately 3 cm in diameter, depending on age, gender, and body surface area. The diameter of the aortic arch is similar. Descending in the posterior mediastinum, the thoracic aorta tapers slightly to about 2 to 2.3 cm. The abdominal aorta narrows to 1.7 to 1.9 cm in its distal portion. The aortas of males are larger than those of females, and aortic root dimension increases with age, height, and weight. The gender difference in aortic root dimension is not entirely explained by body surface area.5
Hemodynamic Function of the Aorta
The force of left ventricular (LV) ejection creates a pressure wave that traverses the aorta, producing radial expansion and contraction of the arterial walls.6 Potential energy derived from myocardial contraction ...