Left Subclavian Artery

The left subclavian artery is the third and final major branch of the aortic arch. It is a large elastic artery responsible for supplying oxygenated blood to the left upper limb, as well as parts of the thorax, neck, spinal cord, and brain. Due to its direct origin from the aortic arch, the left subclavian artery has a longer intrathoracic course compared to the right subclavian artery, which arises from the brachiocephalic trunk.

Origin and Course

The left subclavian artery arises directly from the aortic arch, posterior to the origin of the left common carotid artery. From its origin, it ascends vertically through the superior mediastinum, arches laterally toward the root of the neck, and passes posterior to the left sternoclavicular joint to enter the thoracic outlet.

After passing over the first rib, the artery continues laterally and changes name to the axillary artery at the lateral border of the first rib. Along its course, it remains largely posterior to the clavicle and anterior scalene muscle, protected within the deep structures of the lower neck and upper thorax.

Parts of the Left Subclavian Artery

Anatomically, the left subclavian artery is divided into three parts based on its relationship to the anterior scalene muscle:

1. First Part

Extends from the origin of the artery at the aortic arch to the medial border of the anterior scalene muscle. This portion lies in the superior mediastinum and lower neck, deep to the sternoclavicular joint and medial clavicle.

2. Second Part

Located posterior to the anterior scalene muscle. This part lies deeper and is shorter in length. It is closely related to the brachial plexus and subclavian vein, which lies anterior to the muscle.

3. Third Part

Extends from the lateral border of the anterior scalene to the lateral border of the first rib, where it continues as the axillary artery. This segment lies superior and posterior to the first rib and is the most superficial part, often used as a surgical landmark.

Branches

The subclavian artery gives rise to several major branches before becoming the axillary artery. These branches typically arise from the first part of the artery, although some variability exists.

1. Vertebral Artery

Ascends through the transverse foramina of the cervical vertebrae (C6 to C1) to enter the cranial cavity via the foramen magnum. It supplies the brainstem, cerebellum, and posterior part of the cerebral hemispheres.

2. Internal Thoracic Artery

Descends posterior to the costal cartilages near the sternum and gives off anterior intercostal arteries, pericardiacophrenic branches, and superior epigastric and musculophrenic arteries. It supplies the anterior thoracic wall and diaphragm.

3. Thyrocervical Trunk

Arises near the medial border of the anterior scalene muscle and divides into several branches, including:

• Inferior thyroid artery – supplies the thyroid gland and larynx
• Transverse cervical artery – supplies the trapezius and scapular region
• Suprascapular artery – supplies the supraspinatus and infraspinatus muscles

4. Costocervical Trunk

Usually arises from the second part of the subclavian artery and divides into:

• Superior intercostal artery – supplies the first and second posterior intercostal spaces
• Deep cervical artery – supplies deep neck musculature

5. Dorsal Scapular Artery (Variable)

May arise directly from the subclavian artery or from the transverse cervical artery. It runs along the medial border of the scapula and supplies the rhomboids and levator scapulae muscles.

Anatomical Relationships

Thoracic Part

• Anterior: Left vagus nerve, left phrenic nerve, left brachiocephalic vein, and thymus (in children)
• Posterior: Apex of the left lung and pleura, esophagus, and thoracic duct
• Medial: Trachea and esophagus
• Lateral: Left lung apex and scalene muscles

Neck Part

• Anterior: Subclavian vein (separated by the anterior scalene), clavicle
• Posterior: First rib, lower part of the brachial plexus
• Superior: Brachial plexus trunks pass superior and posterior to it

Surface Anatomy

Although the left subclavian artery itself is not directly palpable, it can be approximated by tracing a curved line from the sternoclavicular joint to the midpoint of the clavicle. Compression against the first rib or clavicle in cases of trauma or bleeding may sometimes be necessary to control subclavian bleeding, though such procedures are complex and high risk.

Function

The left subclavian artery is a critical supplier of oxygenated blood to the upper limb, posterior brain, cervical spinal cord, anterior thoracic wall, diaphragm, and parts of the scapular region. Through the vertebral artery, it also contributes to the posterior cerebral circulation via the basilar artery, making it important not just for limb perfusion but also for cerebrovascular integrity.

Its complex branching pattern supports a wide distribution network, ensuring collateral flow through anastomoses with intercostal, cervical, and thoracic arterial systems.

Embryological Origin

The left subclavian artery develops from the left seventh intersegmental artery, which arises from the dorsal aorta. Unlike the right subclavian artery, which develops from multiple embryonic segments (including the right fourth aortic arch and right dorsal aorta), the left subclavian forms more directly and maintains its origin from the aortic arch.