Place a finger just below the bone that protrudes on the inside of your wrist, just below your thumb. That rhythmic pulsing you feel is, quite literally, the surge of blood propelled from your heart. Your heart beats roughly 100,000 times a day. Each time it does, it pushes blood into your blood vessels and, through them, to all the cells of your body.

Blood pressure is a measure of the force of blood moving with each heartbeat and of the force those elastic vessel walls exert on the blood flowing through them. The first person ever to measure these forces was Stephen Hale, an eighteenth-century English clergyman who was also a leading botanist, chemist, physiologist, and inventor.

Having done pioneering work on the physiology of plants, Hale moved on to animals. In 1733, he used a nine-foot-long vertical glass tube to measure the pressure exerted by blood in blood vessels. His “patient” was a horse. Blood pressure keeps nutrients and oxygen moving to the body’s cells in the vast recycling system that is your circulation.

It’s called “circulatory” because the entire system is a constantly repeating cycle. You may have noticed over the years when you’ve cut yourself that sometimes the blood that escapes is bright red and sometimes it’s dark. The bright red blood is full of freshly oxygenated cells and loaded with nutrients.

The dark red blood is, in effect, the “usedup” blood that’s circling its way back to exchange carbon dioxide for oxygen. The squeezing of the heart moves the blood around and creates the pressure. The pressure must be sufficient to overcome the force of gravity and reach your head when you stand. Quite simply, blood pressure is what keeps you alive. But if it’s too high, it can also harm you.

Silent Killer

When blood pressure is too high for too long it can scar, stiffen, and narrow the insides of your blood vessels, forcing your heart to work harder than it should. Like any other muscle, working harder can cause the heart to thicken. Unlike other muscles, however, thickening can make the heart weaker, not stronger.

A bulging bicep may be a good thing; a heart that is too muscular is not. At the same time, narrower and less elastic blood vessels create conditions in which blood clots can become stuck and block blood flow. In time, faced with these challenges, the heart can begin to wear out, sending less blood to your body with each beat than your body really needs, a condition known as “heart failure.”

But you may never be aware that any of this is happening. High blood pressure—also called hypertension—is known as “the silent killer” because it generally has no outward symptoms. It can sometimes cause headaches or dizziness or fatigue, but these symptoms are vague and often attributed to other, less serious causes.

All too often, the first “symptom” is a heart attack or stroke. The higher your blood pressure, the higher your risk. More than 65 million Americans have high blood pressure— that’s one out of every three adults. For every ten people who have high blood pressure, three have no idea they have it.

Of those who know they have hypertension, some 15 percent (about 1 in 6) are not being treated. Of those who are being treated, almost one half aren’t being treated regularly enough or well enough to bring their blood pressure down to safe levels. Your chances of having high blood pressure increase as you age.

Researchers from the Framingham Heart Study recently found that Americans who are fifty-five years old and older have a 90 percent chance of developing high blood pressure during their lifetime.

Blood Pressure Tests

The only way high blood pressure can be detected is by a blood pressure measurement. For this reason, and because health professionals now understand the direct link between high blood pressure and other diseases, one of the first things a nurse or doctor will do when you have an appointment—for almost any reason—is check your blood pressure.

Blood pressure is measured in much the same way as atmospheric pressure is assessed with a weather barometer: in units of millimeters of mercury (or, mmHg). The pressurized cuff placed around your arm effectively determines the point at which your existing blood pressure is overcome by the pressure in the cuff.

Unlike other measures, like your temperature, however, blood pressure gives you two numbers, not one. The person taking your blood pressure will document the results as the first number “over” the second number. The first number in a blood pressure reading is your systolic blood pressure. That’s the pressure produced in your blood vessels during each contraction, or “beat” of your heart.

The second number is your diastolic blood pressure. It’s the pressure inside your arteries when your heart relaxes between beats. Unlike temperature, there is no one number that is considered normal for blood pressure. Higher blood pressure is generally associated with higher risk, but there is no number that is considered just right.

In general, lower blood pressure is better as long as you do not have any side effects, such as light-headedness. The measurement of blood pressure is not always consistent. Blood pressure varies from individual to individual and, for that matter, at different times of the day, depending upon the demands and stresses placed upon your heart.

If you’re caught in traffic and rush in late for your doctor’s appointment, for example, the chances are you’ll have a higher blood pressure reading than if you had a leisurely walk. Because of this variation, national experts have made some suggestions about what you should do before your blood pressure reading.

National guidelines suggest that patients should follow these steps:

• Avoid smoking or drinking caffeine 30 minutes before having your blood pressure measured.

• Rest for 5 minutes before the measurement is taken.

• When your blood pressure is being measured, be seated with your feet flat on the floor, back and arm supported, arm at heart level; after the first measurement, rest for 2 minutes before the second measurement; a second measurement is recommended to confirm the first. If the first two readings differ by more than 5 mmHg, then a third measurement is recommended.