Vertebral arteries branch off from the subclavian arteries and then pass through the costotransverse foramina c6 to c2 cervical vertebrae. They enter our skulls via the foramen magnum and merge to form the basilar artery at the pontomedullary junction. At the base of the brain the carotid and basilar systems join to form the circle of Willis (named of Thomas Willis 1621-1675). This tends to allow adequate brain perfusion despite occlusion of a main vessel.
In a recent case that was missed my the clients GP he also had a flat back which was the result of a sway back posture. Once this was corrected it removed a great deal of pressure off the cervical spine.