By Sarah Samaan, Rosanne Rust, Cynthia Kleckner

Blood pressure is a measure of the force that blood pulses through the arteries in the body. It’s usually measured with a blood pressure cuff that consists of an inflatable arm cuff and a pressure gauge called a sphygmomanometer.

The cuff inflates to a point that the brachial artery (the main artery in the arm) is briefly closed off. Then air is gradually released until a pulse is either heard with a stethoscope placed in the crease of the elbow or detected by an electronic machine. When medical staff folks take your blood pressure, they’re listening for two things:

  • The point at which they hear sound is the systolic pressure, and it occurs when the heart contracts and blood is forced through the arteries.

  • The point at which the pulse is no longer audible is the pressure when the heart relaxes; it’s called the diastolic pressure.

Blood pressure is measured in millimeters of mercury (mm Hg), a term that derives from the time of the original mercury-based blood pressure gauges.

Figuring out what those numbers indicate

Normal blood pressure is conventionally considered to be below 120/80. Some studies peg the ideal blood pressure at 115/75, although many people who are perfectly healthy naturally run even lower than that.

Some people with lower blood pressure experience dizziness and lightheadedness as a result. This situation is called hypotension. In those cases, seek medical attention to sort out the cause of the problem.

Pressures at or above 120/80 but lower than 140/90 fall into the pre-hypertension range. About 30 percent of American adults fall into this category. Although doctors don’t generally prescribe medication to treat pre-hypertension, a pressure in this range is a red flag because the condition is associated with a higher risk for heart disease and stroke.

The good news? Simple lifestyle changes — including the DASH diet, exercise, and weight loss of as little as 10 pounds (when needed) — can often bring the blood pressure back down to the normal range.

Blood pressure varies from individual to individual, but here are the ranges for low, normal, and high blood pressure:

  • Hypotension: 90/60 or lower

  • Normal: 120/80 or lower

  • Pre-hypertension: 121/81 to 140/90

  • Hypertension: 141/91 or higher

Understanding high blood pressure readings

Blood pressure isn’t a single set of numbers. On a typical day, your blood pressure is likely to fluctuate up or down by about 20 percent. That’s why your doctor will probably want to obtain at least three different readings on three different days before diagnosing you with hypertension. (Of course, if you have dangerously high blood pressure, you usually don’t need to wait to start treatment.)

Sometimes high blood pressure isn’t really high blood pressure but rather “white coat hypertension” — also known as the spike in blood pressure that occurs when you visit the doctor. In fact, a study from the VA hospital system found an average 10-point difference between home blood pressure readings and office measurements. That’s why having your own blood pressure machine at home is so useful.

Although white coat hypertension is usually just a nuisance, nearly half of all individuals with the condition eventually develop persistently high blood pressure.

On the flip side, some people may experience lower readings in a doctor’s office and higher readings at home. This situation is referred to as “masked hypertension,” and it’s yet another reason why keeping tabs on your own blood pressure is a good idea.

Although you can have both systolic and diastolic hypertension, elevations in just one or the other of the two numbers are common. People under the age of 50 are more apt to have high diastolic pressures, whereas those over 50 tend to have higher systolic numbers. That’s because the systolic pressure often reflects hardening of the arteries, which is more common as people age.

Both numbers are important, so you can’t ignore your readings just because only one of the two is high.

Just to make matters a little bit trickier, guidelines published in The Journal of the American Medical Association in 2013 recommended that for folks over 60 without diabetes or kidney disease, blood pressure medication shouldn’t be started until the systolic pressure is consistently over 150.

That doesn’t mean that 150 is an ideal systolic blood pressure; it simply recognizes that medications have benefits and risks, and that in many cases, you should hold off on drug treatment a little longer and work even harder on creating and maintaining a healthy lifestyle.