Blood pressure is defined as the force in the arteries when the heart beats (systolic pressure) and when the heart is at rest (diastolic pressure). It's measured in millimeters of mercury (mm Hg). The measurement is written one above or before the other, with the systolic number on top and the diastolic number on the bottom.

High blood pressure, or hypertension, is defined in an adult as a blood pressure greater than or equal to 140 mm Hg systolic pressure or greater than or equal to 90 mm Hg diastolic pressure.

High blood pressure directly increases the risk of coronary heart disease and stroke, especially along with other risk factors.

High blood pressure can occur in children or adults, but it's more common among people over 35. It's particularly prevalent in blacks, middle-aged and elderly people, obese people, heavy drinkers and women who are taking birth control pills. It may run in families, but many people with a strong family history of high blood pressure never have it. People with diabetes mellitus, gout or kidney disease are also more likely to have hypertension.

Some people can have high blood pressure for years without knowing it. The only way to know if you have high blood pressure is to have your blood pressure checked. According to recent estimates, one in four U.S. adults has high blood pressure, but because there are no symptoms, nearly one-third of these people don't even know they have it. This is why high blood pressure is often called the "silent killer."

A single reading showing high blood pressure doesn't mean you have high blood pressure, but it's a sign that you need to watch it carefully. High blood pressure is typically only diagnosed after consistent or consecutive high readings.

If your blood pressure is normal, get it checked at least every two years. If your blood pressure is near the top of the normal range, or if you have a family history of high blood pressure, you're at higher risk. Your doctor will tell you how often to have it checked.

Categories for Blood Pressure Levels in Adults*

When systolic and diastolic blood pressures fall into different categories, the higher category should be used to classify blood pressure level. For example, 160/80 mm Hg would be stage 2 hypertension (high blood pressure).

BLOOD PRESSURE LEVEL (mm Hg)
Category	Systolic	Diastolic
Optimal**	less than 120	less than 80
Normal	less than 130	less than 85
High Normal	130-139	85-89
High Blood Pressure
Stage 1	140-159	90-99
Stage 2	160-179	100-109
Stage 3	greater than or equal to 180	greater than or equal to 110
* For those not taking medicine for high blood pressure and not having a short-term serious illness. These categories are from the National High Blood Pressure Education Program. ** Optimal blood pressure with respect to heart disease risk is below 120/80 mm Hg. However, unusually low readings should be evaluated for clinical significance.