What Is Blood Pressure?

Blood is carried from the heart to all parts of your body in vessels called arteries. Blood pressure is the force of the blood pushing against the walls of the arteries. Each time the heart beats (about 60–70 times a minute at rest), it pumps out blood into the arteries. Your blood pressure is at its highest when the heart beats, pumping the blood. This is called systolic (sis-TOL-ik) pressure. When the heart is at rest, between beats, your blood pressure falls. This is the diastolic (di-a-STOL-ik) pressure.

Blood pressure is always given as these two numbers, the systolic and diastolic pressures. Both are important. Usually they are written one above or before the other, such as 120/80 mmHg (measured in millimeters of mercury, a unit for measuring pressure). When the two measurements are written down, the systolic pressure is the first or top number, and the diastolic pressure is the second or bottom number (for example, 120/80). If your blood pressure is 120/80, you say that it is "120 over 80."

Blood pressure changes during the day. It is lowest as you sleep and rises when you get up. It also can rise when you are excited, nervous, or active.

Still, for most of your waking hours, your blood pressure stays pretty much the same when you are sitting or standing still. That level should be lower than 120/80 mmHg. When the level stays high, 140/90 mmHg or higher, you have high blood pressure. With high blood pressure, the heart works harder, your arteries take a beating, and your chances of a stroke, heart attack, and kidney problems are greater.

What Is Normal Blood Pressure?

A blood pressure reading below 120/80 mmHg is considered normal. In general, lower is better. However, very low blood pressure can sometimes be a cause for concern and should be checked out by a doctor.

Doctors classify blood pressures under 140/90 mmHg as either normal or prehypertension.

• Normal blood pressure is lower than 120/80 mmHg.
• Prehypertension is blood pressure between 120 and 139 for the top number, or between 80 and 89 for the bottom number. For example, blood pressure readings of 138/82, 128/89, or 130/86 are all in the prehypertension range. If your blood pressure is in the prehypertension range, it is more likely that you will end up with high blood pressure unless you take action to prevent it.

What Is High Blood Pressure?

A blood pressure of 140/90 mmHg or higher is considered high blood pressure. Both numbers are important. If one or both numbers are usually high, you have high blood pressure.

If you are being treated for high blood pressure and have repeated readings in the normal range, you still have high blood pressure.

There are two levels of high blood pressure: stage 1 and stage 2 (see the chart below).

Categories for Blood Pressure Levels in Adults (in mmHg, millimeters of
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Category	Systolic (top number)	Diastolic (bottom number)
Normal	Less than 120	Less than 80
Prehypertension	120–139	80–89
High blood pressure
Stage 1	140–159	90–99
Stage 2	160 or higher	100 or higher

a	For adults 18 and older who are not on medicine for high blood pressure; are not having a short-term serious illness; and do not have other conditions, such as diabetes and kidney disease.
Note: 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 mmHg would be stage 2 high blood pressure.

There is an exception to the above definition of high blood pressure. A blood pressure of 130/80 mmHg or higher is considered high blood pressure in people with diabetes and chronic kidney disease.