Aortic arch crosses the left mainstem bronchus. The first branch is the right carotid artery, the second branch is the left carotid artery, the third branch is the subclavian artery, the fourth branch is the right subclavian artery arising from the posteromedial aspect of the distal aortic arch and continuing posterior to the esophagus to the right hand side of the body. [from
HPO]