aorta

the main artery of the greater circulatory system, which supplies blood to all organs of the body.

The wall of the aorta consists of three membranes, that is, the interior (a layer of endothelial cells), intermediate (numerous rows of elastic fibers), and exterior (bundles of connective tissue fibers). Because of the elasticity of the aorta’s walls an uninterrupted flow of blood in the arteries is assured. In man, mammals, and birds, the aorta emerges from the left ventricle of the heart, forming an enlargement at its very beginning, called the aortic spindle, goes up (ascending aorta), turns back and to the left in man and mammals (arch of the aorta) and back and to the right in birds, and goes down (descending or dorsal aorta). In reptiles there are two aortic arches, that is, the right, or arterial, arch, emerging from the left ventricle of the heart; and the left, or venous, arch from the right ventricle. When they unite, they form a common aorta with mixed blood. In amphibians an arterial cone emerges from the only ventricle, and from it one pair (in ecaudates) or two pairs (in caudates) of aortic arches branch out, which form the dorsal aorta when they unite. In fishes and cyclostomes the ventricle passes into the abdominal aorta, which carries venous blood through the system of arterial arches into the gills (in fishes) and gill pouches (in cyclostomes). The blood which is oxygenated there is collected in the dorsal aorta. Of the invertebrates, mollusks and arthropods have aortas.