Can both High blood pressure and Low blood pressure cause stroke?

It sounds like both can but this isn't exactly explained in my text book so I'm not sure,plus I cant find much online that explains WHY. I know low blood pressure(hypotension) can cause many types of strokes due to having low blood flow (ischemia)...BUT what about high blood pressure(hypertension)..if extreme? And HOW or WHY does that happen? I'd really apprieciate sites. thanks.