Aortic arch

The aortic arch is the curved portion of the aorta, the main artery that carries oxygenated blood from the heart to the rest of the body. It connects the ascending aorta to the descending aorta and gives rise to several major arteries that supply the head, neck, and upper limbs.

Location

The aortic arch is located in the superior mediastinum, a central compartment of the thoracic cavity. It starts at the level of the second right sternocostal joint, just above the heart, and arches to the left and backward over the left main bronchus. The aortic arch lies behind the manubrium of the sternum and passes in front of the trachea and the esophagus. It gives rise to three main branches: the brachiocephalic trunk, left common carotid artery, and left subclavian artery, which supply blood to the upper body.

Structure and Anatomy

Origin

The aortic arch is a continuation of the ascending aorta, which arises from the left ventricle of the heart. It begins at the level of the second right sternocostal joint (approximately at the level of the T4 vertebra in the thoracic spine). The aortic arch originates where the ascending aorta makes a sharp bend, turning to the left and posteriorly.

Course

The aortic arch follows a curved, C-shaped path as it moves from its origin in the ascending aorta to its transition into the descending aorta. Its course is as follows:

• It arches upward from the ascending aorta, curving posteriorly and to the left.
• The highest point of the aortic arch is located at about the level of the T3 or T4 vertebra.
• It descends slightly as it passes behind the left main bronchus and in front of the trachea and esophagus, ending at the beginning of the descending thoracic aorta, just to the left of the vertebral column at the level of T4.

Branches

The aortic arch gives rise to three major branches that supply the head, neck, and upper limbs. From right to left, these are:

• Brachiocephalic Trunk: This is the first and largest branch of the aortic arch. It divides into the right common carotid artery and the right subclavian artery, which supply blood to the right side of the head, neck, and right upper limb.
• Left Common Carotid Artery: The second branch arises directly from the aortic arch and ascends through the neck to supply the left side of the head and neck.
• Left Subclavian Artery: The third branch arises from the aortic arch and travels laterally to supply blood to the left upper limb and parts of the neck and chest.

Anatomical Relations

The aortic arch is surrounded by several important anatomical structures within the superior mediastinum:

• Anteriorly: The aortic arch is positioned behind the manubrium of the sternum and in front of the trachea.
• Posteriorly: It lies anterior to the vertebral column and the esophagus, making its position vital in both respiratory and digestive functions.
• Right: To the right of the aortic arch is the superior vena cava and the right pulmonary artery.
• Left: To the left of the aortic arch are the left pulmonary artery and the left lung.

Ligamentum Arteriosum

The ligamentum arteriosum is a small fibrous remnant of the ductus arteriosus, a fetal blood vessel that connected the pulmonary artery to the aorta. It is located between the aortic arch and the pulmonary trunk and attaches to the lower border of the arch. This structure is a key anatomical landmark and is closely related to the left recurrent laryngeal nerve, a branch of the vagus nerve that loops under the arch.

Transition to the Descending Aorta

After curving leftward and posteriorly, the aortic arch transitions into the descending thoracic aorta. This marks the end of the arch at the level of the T4 vertebra. The descending aorta continues down through the posterior mediastinum, supplying the thoracic organs and tissues before passing through the diaphragm into the abdominal cavity.

Variations

Anatomical variations in the aortic arch are relatively rare but can include differences in the number or position of its branches. Common variations include:

• Left Common Carotid Originating from the Brachiocephalic Trunk: In some individuals, the left common carotid artery arises from the brachiocephalic trunk instead of directly from the aortic arch.
• Aortic Arch Anomalies: Rare anomalies, such as a double aortic arch or a right aortic arch, can occur. These variations are often associated with congenital heart conditions and can affect the course of nearby structures such as the trachea and esophagus.

Size and Dimensions

The aortic arch has a relatively large diameter, typically ranging from 2.5 to 3.5 cm, depending on the individual’s body size and age. The diameter can vary with health conditions and can become dilated in certain vascular disorders, such as aneurysms.

Surrounding Vascular Structures

The aortic arch is closely related to several major vascular structures:

• Superior Vena Cava: This large vein, which returns deoxygenated blood from the upper body to the heart, is positioned to the right of the aortic arch.
• Pulmonary Arteries: The left pulmonary artery arches over the left main bronchus and lies adjacent to the aortic arch. The right pulmonary artery lies below the brachiocephalic trunk.
• Pulmonary Veins: These veins drain oxygenated blood from the lungs into the left atrium of the heart and lie inferior to the aortic arch.

Nerves Related to the Aortic Arch

Several important nerves pass close to or under the aortic arch, including:

• Left Recurrent Laryngeal Nerve: A branch of the vagus nerve, it loops under the aortic arch near the ligamentum arteriosum before ascending to the larynx. Damage to this nerve can result in hoarseness or vocal cord paralysis.
• Vagus Nerve (Left and Right): The vagus nerve descends through the neck and thorax, providing parasympathetic innervation to various organs. The left vagus nerve runs anterior to the aortic arch.
• Phrenic Nerve: Both the left and right phrenic nerves pass near the aortic arch as they descend toward the diaphragm, where they provide motor innervation.

Developmental Considerations

The aortic arch is a structure that develops during the early stages of fetal life from the aortic arches (pharyngeal arch arteries). The transformation of these embryonic vessels into the mature aortic arch, along with its branches, is essential for the proper formation of the cardiovascular system. Developmental abnormalities can result in vascular anomalies such as coarctation of the aorta or vascular rings, which may compress nearby structures like the trachea or esophagus.

Aortic Isthmus

The aortic isthmus is a region located just distal to the left subclavian artery, where the aorta narrows slightly. It is the point where the ductus arteriosus (or its remnant, the ligamentum arteriosum) connects to the arch. The aortic isthmus is a key area in the anatomy of the aorta, as it is a common site for coarctation (narrowing) of the aorta, which can affect blood flow to the lower parts of the body.

Role in Fetal Circulation

During fetal life, the aortic arch plays a vital role in the circulation of oxygenated blood from the placenta to the developing fetus. Blood flows through the ductus arteriosus, bypassing the lungs, and enters the aorta, allowing oxygen-rich blood to reach the fetal body. After birth, the ductus arteriosus closes, and the aortic arch assumes its postnatal function of distributing oxygenated blood from the left ventricle to the systemic circulation.

Function

Distribution of Oxygenated Blood to the Systemic Circulation

The primary function of the aortic arch is to distribute oxygenated blood from the left ventricle of the heart to the systemic circulation, supplying essential organs and tissues in the head, neck, upper limbs, and thorax. The aortic arch connects the ascending aorta to the descending aorta, ensuring that blood flows efficiently to different parts of the body. This distribution is vital for maintaining overall body function, as the oxygen and nutrients carried by the blood are essential for cellular metabolism, organ function, and tissue repair.

Supply to the Head and Neck

The aortic arch plays a key role in supplying blood to the head and neck through two of its major branches:

• Right Common Carotid Artery (via the brachiocephalic trunk): This artery supplies the right side of the head and neck. It ascends through the neck and divides into the internal carotid artery, which supplies the brain, and the external carotid artery, which supplies the face and scalp.
• Left Common Carotid Artery: This artery arises directly from the aortic arch and supplies the left side of the head and neck. Like the right common carotid, it branches into the internal and external carotid arteries.

Through these branches, the aortic arch ensures that critical structures such as the brain, eyes, facial muscles, and skin receive a consistent and reliable supply of oxygenated blood. The brain, in particular, requires a steady flow of blood for proper function, and any disruption in this supply can lead to serious neurological consequences.

Blood Supply to the Upper Limbs

The aortic arch also provides blood to the upper limbs through the following arteries:

• Right Subclavian Artery (via the brachiocephalic trunk): This artery supplies the right upper limb, as well as parts of the right shoulder and thorax. It continues as the axillary artery in the arm, providing blood to the arm, forearm, and hand.
• Left Subclavian Artery: This artery arises directly from the aortic arch and supplies the left upper limb. Like the right subclavian artery, it transitions into the axillary artery as it travels into the arm.

By providing blood to the arms and hands, the aortic arch supports motor activity, sensory function, and muscle performance in the upper limbs, enabling essential tasks such as gripping, lifting, writing, and other fine motor activities.

Formation of Collateral Circulation

The aortic arch plays a role in collateral circulation through its branches, which form connections with other major arteries in the upper body. For example, the vertebral arteries, which are branches of the subclavian arteries, contribute to the Circle of Willis, a vital network of arteries at the base of the brain that ensures continuous blood flow to the brain even if one major artery becomes blocked or narrowed. The anastomoses between the subclavian, common carotid, and vertebral arteries help maintain stable circulation to the brain, head, neck, and upper limbs.

Nutrient Delivery and Waste Removal

Through its branches, the aortic arch ensures the delivery of oxygen, glucose, and essential nutrients to the organs and tissues in the head, neck, upper limbs, and thorax. This nutrient delivery supports the metabolic needs of tissues, allowing them to carry out vital processes such as energy production, growth, repair, and immune defense. The blood supplied by the aortic arch also facilitates the removal of metabolic waste products, such as carbon dioxide and lactic acid, from tissues. This waste removal is critical for maintaining the overall health and function of the body’s cells and preventing the buildup of toxic byproducts.

Maintenance of Blood Pressure and Perfusion

The aortic arch plays an important role in maintaining blood pressure and tissue perfusion throughout the upper body. As blood is pumped out of the left ventricle into the aorta, the aortic arch helps regulate the pressure with which blood is distributed to the head, neck, and upper limbs. The curvature of the aortic arch helps moderate the force of blood ejected from the heart, ensuring that blood reaches its target organs without causing damage to smaller blood vessels. Additionally, the major branches of the aortic arch ensure that blood reaches the vital organs and tissues at appropriate pressures, facilitating adequate perfusion and oxygenation.

Hemodynamic Regulation through Baroreceptors

The aortic arch contains baroreceptors, which are specialized sensory receptors that monitor changes in blood pressure. These baroreceptors detect the stretch in the walls of the aorta and relay information to the brainstem, particularly to the vasomotor center in the medulla oblongata. When blood pressure rises, the baroreceptors trigger reflexive adjustments in heart rate and blood vessel diameter, helping to reduce blood pressure to a normal range. Similarly, when blood pressure falls, the baroreceptors stimulate increases in heart rate and vasoconstriction to raise blood pressure. This feedback loop helps maintain hemodynamic stability, ensuring that the body’s tissues receive a steady supply of blood under varying conditions.

Connection between Pulmonary and Systemic Circulation

During fetal development, the aortic arch plays a role in connecting the pulmonary circulation to the systemic circulation through the ductus arteriosus, which is a fetal blood vessel that allows blood to bypass the lungs. After birth, the ductus arteriosus closes and forms the ligamentum arteriosum, a fibrous remnant. While this function is no longer active after birth, it highlights the role of the aortic arch in the development of the cardiovascular system and its transition from fetal to postnatal circulation.

Support for the Upper Thorax

The aortic arch provides some of the blood supply to the thoracic wall and upper chest through branches of the subclavian arteries, which give rise to the internal thoracic arteries. These arteries supply blood to the chest wall, ribs, and associated muscles, ensuring the thoracic region has the necessary oxygen and nutrients to support respiratory movements, structural integrity, and protection of vital organs like the heart and lungs.

Role in Respiratory Support

By supplying blood to the intercostal muscles, diaphragm, and muscles of the thorax, the aortic arch indirectly supports respiratory function. Oxygenated blood reaching these muscles enables them to contract and relax effectively during respiration, facilitating normal breathing patterns. This ensures that the lungs are able to expand and contract, exchanging oxygen and carbon dioxide efficiently with the bloodstream. The aortic arch, through its branches, helps maintain this vital respiratory support system.

Regulation of Oxygen Supply to the Brain

The aortic arch, via the right and left common carotid arteries, is essential for maintaining oxygen supply to the brain. The internal carotid arteries, which branch off from the common carotid arteries, are critical for supplying the brain with oxygen-rich blood. Any disruption in the function of the aortic arch, such as an aortic dissection or stenosis, can impair blood flow to the brain, leading to neurological symptoms such as dizziness, fainting, or stroke. The aortic arch’s ability to consistently deliver blood to the carotid arteries is vital for preserving brain function.

Maintenance of Vascular Integrity

The aortic arch also contributes to the vascular integrity of the body’s major arteries. The elasticity and structural strength of the aortic arch allow it to absorb the pulsatile pressure from the heart’s ejections, protecting smaller blood vessels in the systemic circulation from the high pressure generated by each heartbeat. This ability to buffer pressure fluctuations helps prevent damage to more delicate vascular structures and maintains the smooth, continuous flow of blood throughout the circulatory system.

Clinical Significance

The aortic arch is clinically significant due to its role as a major conduit for blood flow to the head, neck, upper limbs, and thorax. Its anatomy and proximity to critical structures make it a vital area of focus in both vascular and cardiothoracic medicine.

Aortic Arch Aneurysms

A thoracic aortic aneurysm can occur in the aortic arch, leading to the weakening and ballooning of the arterial wall. If not treated, an aneurysm can rupture, causing life-threatening bleeding. Aneurysms in this region may require surgical repair, such as aortic arch replacement or stent-grafting.

Aortic Dissections

Aortic dissection is a serious condition where a tear occurs in the inner layer of the aortic wall. When it affects the aortic arch, it can disrupt blood flow to the brain, neck, and upper limbs, leading to symptoms like stroke, pain, or organ failure. Emergency surgery is often required to repair the dissection.

Surgical Access and Procedures

The aortic arch is often involved in cardiothoracic surgeries, such as coronary artery bypass grafting (CABG), aortic arch aneurysm repair, and procedures for congenital heart defects. Its branches are important landmarks during these surgeries, and the proximity to nerves (e.g., the left recurrent laryngeal nerve) can result in complications such as hoarseness or vocal cord paralysis if injured.

Coarctation of the Aorta

Coarctation of the aorta, a congenital narrowing of the aorta near the aortic arch, can cause increased blood pressure in the upper body and reduced blood flow to the lower body. Treatment often involves surgical correction or balloon angioplasty.