Blood pressure basics

Blood pressure is measured in millimeters of mercury (mm Hg) at two points: when the heart beats (systolic) and between beats (diastolic). The systolic reading is usually stated in front of (or over) the diastolic one, for example, 110/75 mm Hg. Both numbers matter; after age 50, systolic becomes more key as it indicates how flexible your blood vessels are.

If your systolic blood pressure is 140 or higher, or your diastolic is 90 or higher, you have high blood pressure, or hypertension. If your systolic is between 120 and 139 or your diastolic is between 80 and 89, you have prehypertension -- you’re at risk of developing high blood pressure, and your health risks are already elevated. If you have diabetes or kidney disease or have had previous cardiovascular problems, such as a heart attack or stroke, 130/80 mm Hg is the threshold for high blood pressure. About one-third of American adults have high blood pressure. Among those age 55, more than 90% either have it or will develop it.

Blood pressure fluctuates all day -- it goes down when you nap at your desk and comes back up (and then some) when your boss yells at you to wake up. “High blood pressure” refers to pressure that is chronically high. One bad reading isn’t conclusive; doctors generally recommend several for a diagnosis.

High blood pressure is not usually curable. Even if you can get your pressure back below 120/80 mm Hg, you still have the condition and still need to treat it.