The inside lining of your blood vessels makes a chemical that keeps them loose; if it stops, the vessels get stiff and blood pressure goes up.

“Vascular resistance is controlled by the endothelium, the thin layer of cells lining every artery in your body, which produces nitric oxide to keep vessels relaxed and compliant. When nitric oxide production falls, vessels stiffen, resistance climbs, and pressure rises, even if your heart rate and blood volume haven't changed at all.”