Aortic Arch

The aortic arch is a curved continuation of the ascending aorta and represents the second portion of the thoracic aorta. It plays a crucial role in distributing oxygenated blood from the heart to the upper body, including the head, neck, and upper limbs. The arch also serves as a central landmark for many anatomical relationships within the superior mediastinum.

Location and Orientation

The aortic arch is located in the superior mediastinum, posterior to the manubrium of the sternum. It begins as a continuation of the ascending aorta at the level of the second right sternocostal joint, ascends and arches posteriorly and to the left, and ends at the level of the T4 vertebra, continuing as the descending thoracic aorta. It lies anterior to the trachea initially and then arches over the left main bronchus.

Its orientation is from anterior to posterior, right to left, and slightly superior to inferior. The arch lies superior to the bifurcation of the pulmonary trunk and the left main bronchus.

Course of the Aortic Arch

The arch originates as the ascending aorta exits the left ventricle of the heart, rising upward and backward. As it curves over the pulmonary trunk and left bronchus, it forms an arch that eventually descends into the posterior mediastinum as the descending thoracic aorta. This arching path gives it a characteristic convex superior border and concave inferior surface.

The convex superior border of the arch gives rise to three large arterial branches, while the concave inferior surface gives attachment to the ligamentum arteriosum, a fibrous remnant of the fetal ductus arteriosus.

Major Branches

The aortic arch gives rise to three major branches in the following order, from right to left:

1. Brachiocephalic Trunk

This is the first and largest branch of the aortic arch. It ascends to the right side of the neck and bifurcates into the:

• Right common carotid artery – supplies the right side of the head and neck
• Right subclavian artery – supplies the right upper limb and part of the thorax

2. Left Common Carotid Artery

This is the second branch of the aortic arch. It arises just to the left of the brachiocephalic trunk and ascends into the neck to supply the left side of the head and neck.

3. Left Subclavian Artery

The third branch, it arises most laterally and supplies the left upper limb, part of the thoracic wall, and spinal cord. It arches laterally and ascends toward the root of the neck, posterior to the left scalenus anterior muscle.

Note: In some individuals, variations exist in the origin of these branches. For example, the left common carotid artery may arise from the brachiocephalic trunk, or a fourth branch called the thyroidea ima artery may be present.

Anatomical Relationships

The aortic arch is closely related to several vital thoracic structures. Understanding these relationships is essential, especially in surgical and radiological settings.

Anterior to the Arch

• Left vagus nerve
• Left phrenic nerve
• Left lung and pleura (anterior surface)
• Thymus (in children)

Posterior to the Arch

• Trachea
• Esophagus
• Thoracic duct
• Vertebral column (T4)

Inferior to the Arch

• Bifurcation of the pulmonary trunk
• Left main bronchus
• Ligamentum arteriosum
• Left recurrent laryngeal nerve (loops around the arch posterior to the ligamentum arteriosum)

Superior to the Arch

• Brachiocephalic veins
• Origins of the three arterial branches
• Left superior intercostal vein (crosses the arch)

Ligamentum Arteriosum

Attached to the inferior surface of the arch is the ligamentum arteriosum, a fibrous remnant of the fetal ductus arteriosus. It connects the aortic arch to the superior surface of the left pulmonary artery. During fetal circulation, the ductus arteriosus allows blood to bypass the non-functioning fetal lungs. After birth, it closes and forms the ligamentum arteriosum.

Relations with Nerves

• Left Vagus Nerve: Descends anterior to the aortic arch and gives off the left recurrent laryngeal nerve, which loops under the arch and ascends in the tracheoesophageal groove.
• Left Phrenic Nerve: Passes anterior to the aortic arch and continues downward along the fibrous pericardium toward the diaphragm.

Functional Importance

The aortic arch is a major arterial conduit that ensures blood is distributed efficiently from the heart to the head, neck, and upper limbs. It plays a critical role in systemic circulation and serves as a point of pressure sensing (baroreceptors and chemoreceptors) through the carotid sinus and aortic bodies located near the arch and at the origins of its branches.

The curvature of the arch helps maintain directional flow and dampens the pulsatile pressure of blood ejected from the left ventricle, protecting distal vessels from pressure surges. This arch is also a surgical and radiological landmark when evaluating mediastinal anatomy and pathology.