Illustration of the arterial supply of the upper extremity (left side), demonstrating the relationship between the subclavian artery and the first rib.

The principal blood supply to the arm is provided by the subclavian artery. The subclavian artery arises from the upper chest and passes into the base of the neck, where it then passes up and over the first rib. It crosses the first rib behind the anterior scalene muscle and immediately in front of the brachial plexus nerve roots, and is therefore within the scalene triangle. Several branches arise from the subclavian artery just before it passes through the scalene triangle, including the vertebral artery (to the back of the brain) and the internal thoracic artery (to the inside of the anterior chest). There are also several smaller branches to the neck that arise as the subclavian artery crosses behind the anterior scalene muscle. Beyond the first rib where it passes underneath the clavicle, the subclavian artery becomes the axillary artery and passes underneath the pectoralis minor muscle. The axillary artery has a number of branches that serve the structures around the shoulder girdle, including the subscapular artery and the circumflex humeral arteries. After it passes in front of the shoulder and into the upper arm, the axillary artery becomes the brachial artery.