Where Is the Aorta?
The aorta is the great artery rising from the left side of the heart, through which the blood passes on its way to all parts of the body except the lungs, which are supplied by another system. After rising from the heart, the aorta forms an arch, descending down the left side of the body. It passes through the diaphragm into the abdomen, where it divides into two lesser arteries.
One of these terminates under the end of the backbone, where man has the vestige of a tail. In animals it continues into the tail as the caudal artery. The aorta is one of the elastic or conducting—arteries, which take the blood to the muscular—or distributing—ones, which connect with the veins and smaller blood vessels.
Three valves at the exit of the aorta protect the heart from any back pressure that might develop in the artery and force the blood in the wrong direction. Inflammation caused by an illness such as rheumatic fever may sometimes cause these valves to leak. This is a serious condition requiring considerable medical care and attention. But in normal cases your aorta will continue to function reliably throughout your life.
The aorta is an elastic artery, and as such is quite distensible. The aorta consists of a heterogeneous mixture of smooth muscle, nerves, intimal cells, endothelial cells, fibroblast-like cells, and a complex extracellular matrix. The vascular wall consists of several layers known as the tunica externa, tunica media, and tunica intima.
The thickness of the aorta requires an extensive network of tiny blood vessels called vasa vasorum, which feed the tunica externa and tunica media outer layers of the aorta. The aortic arch contains baroreceptors and chemoreceptors that relay information concerning blood pressure and blood pH and carbon dioxide levels to the medulla oblongata of the brain. This information is processed by the brain and the autonomic nervous system mediates the homeostatic responses.
Within the tunica media, smooth muscle and the extracellular matrix are quantitatively the largest components of the aortic vascular wall. The fundamental unit of the aorta is the elastic lamella, which consists of smooth muscle and elastic matrix. The medial layer of the aorta consist of concentric musculoelastic layers (the elastic lamella) in mammals.
The smooth muscle component does not dramatically alter the diameter of the aorta but rather serves to increase the stiffness and viscoelasticity of the aortic wall when activated. The elastic matrix dominates the biomechanical properties of the aorta. The elastic matrix forms lamellae, consisting of elastic fibers, collagens (predominately type III), proteoglycans, and glycoaminoglycans.