The lower one occurs when your heart is resting. When a doctor or other health professional takes your blood pressure, both levels are measured and recorded as numbers. These two numbers give important information about your health. The systolic pressure tells the maximum amount of pressure exerted on your arteries. The diastolic pressure tells the minimum pressure on your arteries.
Arterioles are smaller vessels that branch off from the arteries.
The arterioles regulate blood pressure. To understand how, think about how a nozzle regulates the water pressure in a hose. If the nozzle is wide open, relatively little pressure will force the water through the hose. The same principle holds true in your blood vessels. If your arterioles are narrowed for some reason, it's harder for blood to pass through them.