Aortic Arch: Structure, Function & Diseases

The aortic arch is effectively a 180-degree elbow of the body’s aorta, transferring the nearly vertical upward ascending aorta to the nearly vertical downward descending aorta. The aortic arch lies just outside the pericardium above the origin of the ascending aorta, which originates in the left ventricle. Three arteries or arterial trunks branch from the aortic arch, supplying the head, neck, and shoulders and arms.

What is the aortic arch?

The aortic arch is the transition from the ascending aorta (aorta ascendens), which originates in the left ventricle, to the descending aorta (aorta descendens). It is a type of 180-degree bend just outside the pericardium. The transitions from the ascending aorta into the aortic arch and further into the descending aorta cannot be defined in terms of cell biology, since the structure of the vessel walls is identical in the course of the aforementioned sections of the aorta. Three arteries branch from the aortic arch, the common brachiocephalic trunk (bracheocephalic truncus), the left common carotid artery (carotid sinistra), and the left subclavian artery (subclavian sinistra). The arm-head arterial trunk branches after only a few centimeters into the right carotid artery (dextra carotid communis artery) and the right subclavian artery (dextra subclavian artery). Arteries supplying blood to the head, neck, shoulders and arms thus all arise from the aortic arch. Prenatally, there is a direct connection between the aortic arch and the pulmonary artery of the pulmonary circulation (ductus arteriosus botalli), which runs directly below the arch. This short-circuits the pulmonary circulation, which is activated only with the onset of pulmonary respiration immediately after birth. Normally, this closes the connection so that the two circuits, the pulmonary circuit and the systemic circuit, are separate.

Anatomy and structure

The aorta opens into the cranial portion of the left ventricle, to the right of the atrial septum, and forms the central arterial trunk of the systemic circulation, from which all other arterial trunks and main arteries arise. The aorta has an initial diameter of 2.5 to 3.5 cm and runs almost vertically upward. At about the point of exit from the pericardium, the aorta transitions without a discernible transition into the aortic arch, which deflects the aorta downward 180 degrees. The three-layered wall structure of the aortic arch is identical to that of the aorta and the other great arteries. The inner closure is formed by the intima (tunica intima), which is composed of a single-layer epithelium, a loose connective tissue layer, and an elastic membrane. This is followed by the middle layer, the media (tunica media). It consists of elastic fibers and one or more elastic membranes as well as smooth muscle cells. The externa (tunica externa or tunica adventitia) connects to the outside. It is characterized by elastic and collagenous connective tissue and is the carrier of the vessels that supply the arterial wall, so to speak, the vessels of the vessels (vasa vasorum), and it is the carrier of the nerve fibers that control the lumen of the aortic arch. On the lower side of the aortic arch, a small receptor corpuscle (glomus aorticum) encased in connective tissue, which contains chemoreceptors that measure the partial pressure of oxygen in the lumen of the aortic arch and transmit it to the brain via the vagus nerve. The signals are mainly used to control respiratory activity.

Function and Tasks

Primarily, the aortic arch serves to divert the ascending branch of the aorta into the descending branch. In addition, along with the other major body arteries, it performs a kind of windbox function. The arterial systolic blood pressure peak is moderated by the elastic walls of the arteries. The lumen of the large arteries, including the lumen of the aortic arch, dilates and attenuates the pressure peak. During the subsequent diastolic phase of the ventricles, the aortic valve closes, maintaining the necessary residual pressure in the arterial portion of the systemic circulation. Through the three arterial outlets in the aortic arch, it is responsible for supplying the head, neck, shoulders and arms with oxygenated blood. Indirectly, the aortic arch, as a carrier of chemoreceptors, has a function in regulating respiratory activity.The chemoreceptors bundled in the glomus aorticum react sensitively to a drop in pH towards acidic and to a drop in oxygen partial pressure. The nerve signals are processed in the brain and translated into the release of neurotransmitters that stimulate an increase in respiratory drive.

Diseases

Diseases and conditions associated with the aortic arch are usually acquired or genetic narrowing or occlusion (stenosis) in the outgoing vessels or in the aortic arch itself. Occlusion of one or more of the three outlets in the aortic arch is called aortic arch syndrome. The cause may be arteriosclerotic changes in the intima of the vessels or inflammatory processes in the vessel walls. Depending on the affected branch in the aortic arch, mild to severe symptoms occur in the deficiently supplied regions. A failure of the internal carotid artery, which also supplies the brain, results in typical neurological deficits such as visual disturbances, ringing in the ears, concentration deficits, and even consciousness and speech disorders. In about 10 percent of cases of aortic dissection, the aortic arch is affected. Tears in the intima, the inner layer of the vessel, can cause mild to severe hemorrhage between the intima and the media, the middle layer, causing serious, life-threatening aneurysms. In very rare cases, aortic isthmic stenosis, a vascular malformation that usually accompanies inherited heart defects, may be present as a genetic malformation. In many cases, aortic isthmic stenosis is also observed in the presence of monosomy X (Turner syndrome).