The aortic arch crosses the right mainstem bronchus and not the left mainstem bronchus, but does not result in the creation of a vascular ring. The first branch is the left brachiocephalic artery which divides into the left carotid artery and left subclavian artery, the second branch is the right carotid artery, the third branch is the right subclavian artery. [ DDD:dbrown HPO:sdoelken ]