The brain requires unobstructed blood flow to nourish its many functions. Very high, sustained blood pressure will eventually cause blood vessels to weaken. Over time these weaken vessels can break, and blood will leak into the brain. The area of the brain that is being fed by these broken vessels start to die, and this will cause a stroke. Additionally, if a blot clot blocks a narrowed artery, blood ceases to flow and a stroke will occur.
Imagine a garden hose hooked up to a spigot. When the hose is flexible and there are no kinks in it, you can turn on the water full blast and it will flow easily through the hose. But if there’s a kink in the hose, the water doesn’t flow as well beyond the kink. And the pressure inside the hose builds up behind the kink. Or imagine there is gunk inside the hose blocking the path of the water. Your arteries are a lot like that garden hose.