A 60-year-old smoker has calf pain when walking that eases with rest. Plaque (atherosclerosis) has narrowed the leg arteries. Walking muscles need more oxygen, but the narrowed artery can't deliver — pain (intermittent claudication). Rest lowers demand, pain fades. The same plaque process in coronary arteries causes heart attacks and in brain arteries causes strokes — one disease, three territories.