Narrowness reduces blood flow directly. Stiffness causes inadequate blood flow by preventing arteries from dilating when a region of the brain becomes more active and needs more oxygen or when blood pressure drops. Weakness in arteries can lead to reduced blood flow if the weak area bulges outward, forming an aneurysm . Though the region with the aneurysm is initially wider than normal, blood swirling in aneurysm can clot. As the clot enlarges, it can partially or completely block blood flow through the artery.
Emboli formed by atherosclerotic plaque can also block arteries.
As the heart becomes weaker, blood flow to the brain decreases. In either case, the clot from the heart may embolize into arteries leading to or through the brain. Ischemia then follows when the embolus blocks an artery. Sometimes many emboli develop, causing multiple strokes. When atherosclerosis weakens an artery in the brain, the artery can burst, causing hemorrhaging within the brain.