The thoracic aorta, a segment of the body’s largest artery, resides within the chest cavity. It serves as a conduit for oxygenated blood flow from the heart to various regions of the body.
Anatomical Overview
The thoracic aorta is located within the chest, specifically in the posterior mediastinum, the space behind the heart and in front of the spine. It begins near the third intercostal space, obliquely to the left of the midline, and descends to the twelfth thoracic vertebra (T12). At T12, it passes through the diaphragm to become the abdominal aorta. This large vessel is generally curved, facing forward as it descends, and has an approximate average radius of 1.16 cm.
It lies in front of the vertebral column, gradually shifting from the left side towards the midline as it descends. To its right are the azygos vein and thoracic duct, while the left pleura and lung are to its left. In front of the thoracic aorta are the root of the left lung, the pericardium, the esophagus, and the diaphragm.
Segments and Branches
The thoracic aorta is divided into three main anatomical segments: the ascending aorta, the aortic arch, and the descending aorta. Each segment gives rise to specific branches that supply blood to various regions of the body.
The ascending aorta is the initial part, originating from the left ventricle of the heart and extending upwards behind the sternum. Its only branches are the left and right coronary arteries, which arise from the aortic root, just above the aortic valve cusps. These arteries supply oxygenated blood to the heart muscle.
The aortic arch continues from the ascending aorta, forming a curve positioned to the left of the trachea. Three major arteries branch off the aortic arch: the brachiocephalic trunk, the left common carotid artery, and the left subclavian artery. The brachiocephalic trunk, the first and largest branch, divides into the right subclavian artery, supplying the right arm, and the right common carotid artery, which delivers blood to the right side of the head and neck, including the brain. The left common carotid artery supplies the brain and the left side of the head and neck, while the left subclavian artery provides blood to the left arm and the back of the brain.
The descending thoracic aorta is a continuation of the aortic arch, beginning around the fourth thoracic vertebra and extending downwards through the chest cavity. This segment gives off numerous branches that supply blood to structures within the thorax.
Visceral Branches
Pericardial arteries, supplying the pericardium.
Bronchial arteries, providing blood to the lungs.
Esophageal arteries, nourishing the esophagus.
Mediastinal arteries, which supply lymph nodes and other tissues in the posterior mediastinum.
Parietal Branches
Posterior intercostal arteries, nine pairs that supply the intercostal spaces.
Subcostal arteries, located below the twelfth ribs and supplying the abdominal wall muscles.
Superior phrenic arteries, which supply the diaphragm.
Physiological Role
The primary function of the thoracic aorta is to transport oxygenated blood from the heart’s left ventricle to the rest of the body. This process involves the aorta’s ability to handle the pulsatile flow of blood ejected by the heart.
The aorta’s elasticity and strength maintain consistent blood pressure and flow throughout the circulatory system. During systole, when the heart contracts, the elastic walls of the aorta expand to store about half of the blood volume ejected by the left ventricle. This expansion dampens the high pulsatile pressure from the heart. During diastole, when the heart relaxes, the aorta recoils, releasing the stored blood volume into the peripheral circulation. This recoil ensures a more continuous and less pulsatile blood flow to the body’s tissues, a mechanism often referred to as the Windkessel effect.
Understanding Aortic Structural Issues
The thoracic aorta, despite its strength, can develop structural problems that affect its integrity. Two common issues are aneurysms and dissections, both involving changes to the vessel’s wall.
An aortic aneurysm is a localized bulging or widening of the aortic wall, defined as an increase in diameter of at least 50% compared to its normal size. This occurs when the aortic wall weakens, causing it to expand outward under the constant pressure of blood flow.
An aortic dissection involves a tear in the innermost layer of the aortic wall, the intima. Once this tear occurs, blood can leak into the middle layer, the media, forcing the layers apart and creating a new, abnormal channel known as a false lumen alongside the original true lumen. This separation can compromise blood flow to arteries branching off the aorta and, if the blood in the false lumen ruptures through the outer layers, can lead to complete aortic rupture.
Several factors can contribute to the weakening and structural changes in the aortic wall that lead to aneurysms and dissections. High blood pressure, or hypertension, can weaken the aortic wall over time due to constant elevated pressure. Atherosclerosis, characterized by plaque buildup, can also weaken the aortic walls. Genetic connective tissue disorders, such as Marfan syndrome, can result in a congenitally weaker aortic wall, increasing susceptibility to these issues. Aging also contributes to the degradation of elastic fibers and an increase in collagen within the aortic wall, leading to increased stiffness and a greater risk of structural issues.