The basics of blood pressure

Blood circulating through your body moves with a certain force -- that's your blood pressure. This force can be affected by how strongly the heart is pumping and by the size of the vessels the blood is moving through. Blood moves through large arteries into smaller vessels called arterioles, which can expand and contract. When they expand, blood pressure goes down. When they contract, blood pressure goes up. (The body has some very good reasons and complicated systems for doing this.)

How is blood pressure measured? The pressure is measured in millimeters of mercury (mm Hg) at two points: when the heart beats (systolic) and when the heart is between beats (diastolic). Usually blood pressure readings are stated with the systolic in front of (or over) the diastolic -- for example, 110/75 mm Hg. Systolic and diastolic pressure are both important, but after you reach 50, systolic becomes more important because it gives a measure of how flexible your blood vessels are -- or are not.

How high should your blood pressure be? Your systolic blood pressure should be less than 120, and your diastolic should be less than 80. If your systolic is 140 or higher, or your diastolic is 90 or higher, you have high blood pressure, also called hypertension. If your systolic is between 120 and 139 or your diastolic is between 80 and 89, you have prehypertension -- meaning you're at risk of developing high blood pressure, and your risk of bad outcomes is 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.

Of course, your blood pressure fluctuates all day -- it goes down when you fall asleep at your desk, and it comes back up again (and then some) when your boss yells at you to wake up. The terms "high blood pressure" and "hypertension" refer to a condition in which your blood pressure is chronically higher than it should be.

It could happen that your systolic pressure is too high but your diastolic pressure isn't. That's called isolated systolic hypertension, and it's the most common kind of high blood pressure among older people. Though it used to be considered normal for that age group, doctors now know that treating it can reduce the risk of strokes and heart attacks. But it's important to lower the systolic pressure without also lowering the diastolic pressure too much -- a too-low diastolic pressure can make the risk of strokes and heart attacks go back up.

How common is high blood pressure? Very. About one-third of all adults in America have high blood pressure. More than half of all those who are 60 or older have it. And among those age 55, more than 90% either have it already or will develop it at some point.

Is high blood pressure curable? Sometimes. For example, blood pressure may go up during pregnancy, then go down again after the baby is born, and it may go up if you take some cold medicines, then go down again if you stop. But in general, high blood pressure is not curable. Even if you treat your high blood pressure and bring it down below 120/80 mm Hg, you still have the condition and still need to treat it.

Is everyone equally likely to have high blood pressure? No. Some factors are beyond your control, including age (as your age goes up, your blood pressure tends to go up too), gender (men are more likely to have high blood pressure than women), race and ethnicity (African Americans are more likely to have high blood pressure than Caucasian or Hispanic Americans), salt-sensitivity (African Americans are also more likely to be salt-sensitive) and family history (if your parents or other close relatives have high blood pressure, you have a greater than average risk of developing it too).

But you have control over some other factors, such as how much you weigh, how much salt you consume, how much alcohol you consume, whether or not you smoke, how much exercise you get and how much stress you experience. (Of course, no one says controlling these things is easy.)

How do you know you have high blood pressure? Many people who have it -- more than 20% -- don't know. After all, blood pressure usually has no symptoms itself. But diagnosis is easy, and everyone, of every age, should be checked periodically. Because blood pressure can fluctuate depending on the situation -- for example, the stress of a doctor visit, which is so common it has a name: white coat hypertension -- one bad reading isn't necessarily conclusive. Doctors generally recommend several readings for making a diagnosis.