Aorta
An elastic artery and the largest blood vessel in the vertebrate body, the aorta distributes blood from the heart to the circulatory system. Oxygenated blood is pumped into the aortic arch from the left ventricle. Several small arteries branch from the top of this arch to deliver blood to the head and upper torso. The aorta then curves sharply back to become the dorsal aorta, which runs parallel to the spinal column before it branches into many smaller arteries that supply blood to the rest of the body.
Like all other large blood vessels, the aorta is composed of three layers called tunicas. The interior layer, or tunica intima, is composed of a lining of very thin, flat endothelial cells that sit on a thin connective tissue layer called the basal lamina. The middle layer, or tunica media, is the thickest and most unique part of the aorta. Composed of 40 to 70 thin sheets of elastin (a springy, flexible protein) interspersed with layers of spindle-shaped smooth muscle cells, the media makes the aorta limber and yet extremely strong. The elastic nature of the aorta is important in maintaining an even blood pressure and in keeping blood flowing smoothly through the vascular system. During contraction of the left ventricle (systole), the aorta stretches and stores blood. When the ventricle relaxes (diastole), the elastic wall of the aorta contracts and prevents blood pressure from falling to zero.
The outermost layer of the aorta, the tunica adventitia, is composed of a loose layer of collagen (protein) fibers that strengthen and help protect this important blood vessel. Nerves and small blood vessels (called vasa vasorum or vessels of vessels) run through this layer. These small blood vessels supply oxygen and nutrients to the muscle cells of the outer layers of the tunica media (which are too far from the lumen of the aorta to be reached by simple diffusion). The nerves regulate the contractions of the muscle cells and thus help control blood pressure. The adventitia also contains fibroblasts, which lay down the collagen and elastic fibers of the aorta wall as well as macrophages, lymphocytes, and other cells of the immune system, which locate and destroy damaged cells and infectious agents such as bacteria and viruses.
Because the aorta, especially the aortic arch, is subjected to very high blood pressure when the heart beats, its elastic walls often weaken and fail as a person ages. A weakened section of the aorta can balloon out into an aneurysm, which can suddenly burst. Or, in the case of a sharp blow, such as an automobile accident, the aorta can rip away from the heart. In either case the victim bleeds to death in a matter of minutes.
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