Subclavian artery and its branches

Introduction

The left and right subclavian arteries are two major arteries in the thorax that lie beneath the clavicles. They receive blood flowing from the aortic arch, and once they pass the lateral border of the first rib, they become known as the axillary arteries. The axillary arteries supply the upper limbs. This article will describe the parts and branches of the subclavian arteries, together with its course and development.

Development

During prenatal development, the right subclavian artery arises from the fourth aortic arch of the right dorsal aorta (between the 4th and 7th intersegmental arteries) and the right 7th intersegmental artery. The left subclavian artery develops from the left 7th intersegmental artery.

As embryonic development progresses, the subclavian arteries course in the neck, after running posterior to the sternoclavicular joints and ascending through the superior thoracic aperture to gain access into the root of the neck.

Apart from supplying blood to the upper limbs (through their terminal branch, the axillary arteries), they also send branches to the neck and brain. These branches includes the:

• Vertebral artery
• Internal thoracic artery
• Thyrocervical trunk
• Dorsal scapular artery​

Parts

Each of the subclavian arteries is made up of three parts, defined in relation to the anterior scalene muscle of the neck. Tributaries to the neck and brain arise from the three parts of the subclavian artery as follows:

First part

This part of the subclavian artery is medial to the anterior scalene muscle. Lying posterior to this part are the cervical pleura, apex of the lung, and sympathetic trunk. Branches from this part of the subclavian artery are the:

• Vertebral artery
• Internal thoracic artery
• Thyrocervical trunk

Second Part

This part of the subclavian artery is posterior to the anterior scalene muscle. It gives rise to the costocervical trunk as it courses upwards.

Third Part

The third part of the subclavian artery lies lateral to the anterior scalene muscle and gives off the dorsal scapular artery. This part lies on the first rib and it is the longest, most superficial part of the subclavian artery. Its pulsations can be felt by applying deep pressure in the omoclavicular triangle. The inferior trunk of the brachial plexus lies directly posterior to this part of the subclavian artery. The suprascapular artery may also arise from the third part, however in most individuals, this branch arise from the first part of the subclavian artery as a branch of the thyrocervical trunk.

Course

Right Subclavian Artery

The right subclavian artery originates from the brachiocephalic trunk, behind the upper border of the right sternoclavicular joint. It courses upwards above the clavicle, superomedially, and then posterior to the scalenus anterior (anterior scalene muscle), to the lateral border of the first rib, when it becomes the axillary artery.

Left Subclavian Artery

The left subclavian, in the majority of individuals, originates from the aortic arch independently after the brachiocephalic trunk and the left common carotid artery have branched off. It arises below the left common carotid artery and ascends into the neck lateral to the medial border of scalenus anterior, crosses behind this muscle and then descends towards the lateral border of the first rib, where it becomes the axillary artery .

Branches

Vertebral Artery

This branch of the subclavian artery originates from the first part, and it is composed of four parts – cervical, vertebral, suboccipital and cranial.

• Cervical: It arises from the first part of the subclavian artery and ascends in the pyramidal space formed between the scalene and longus muscles. 
• Vertebral: It passes deeply to course through the foramina of the transverse processes of cranial vertebrae C1-C6. Occasionally, the vertebral artery may enter a transverse foramen superior to the 6th cervical vertebra.
• Suboccipital: It courses in a groove on the posterior arch of the atlas before it enters the cranial cavity through the foramen magnum.
• Cranial: It supplies branches to the medulla, spinal cord, parts of the cerebellum, and the dura of the posterior cranial fossa. At the caudal border of the pons of the brainstem, the vertebral arteries anastomose to form the basilar artery, which participates in the formation of the the Circle of Willis, a loop of cerebral vessels.

Internal Thoracic Artery

This branch arises from the anteroinferior aspect of the first part of the subclavian artery, and passes inferomedially into the thorax. It has a thoracic and cervical part.

Thyrocervical trunk

The thyrocervical trunk arises from the anterosuperior aspect of the first part of the subclavian artery, near the medial border of the anterior scalene muscle. It has three branches:

• Inferior thyroid artery: It is the largest and most important one because it is the primary visceral artery of the neck.
• Suprascapular artery: It supplies muscles on the posterior surface of the scapula 
• Transverse cervical artery: It sends branches to the muscles in the lateral cervical region, the trapezius and medial scapular muscles.
• Inferior thyroid and ascending cervical arteries:  These are the terminal branches of the thyrocervical trunk. The latter artery supplies the lateral muscles of the upper neck.

Costocervical Trunk

This branch of the subclavian artery arises from the posterior aspect of the second part, posterior to the anterior scalene on the right side, and usually just medial to this muscle on the left side. The costocervical trunk passes posterosuperiorly and divides into the superior intercostal and deep cervical arteries, which supply the first two intercostal spaces and the posterior deep cervical muscles respectively.

Dorsal Scapular Artery

This branch arises as the deep descending branch of the transverse cervical artery, but it may be an independent branch arising from the third part of the subclavian artery. When it is a branch of the subclavian, the dorsal scapular artery passes laterally through the trunks of the brachial plexus, anterior to the middle scalene. It then runs deep to the levator scapulae and the rhomboid muscles, supplying both and participating in the arterial anastomoses around the scapula.