I believe you are supposed to have two arteries on each side, an internal and external carotid artery.. There are many older people walking around with a blocked carotid artery, which can cause a stroke. If you were born with one missing, then your body has probably compensated with what you were born with. However, it is important that you be careful about your diet and blood pressure to prevent the build up of plaque in the arteries.. With only one you would be at a higher risk of transient ischemic attacks or worse, stroke.