Blood pressure is one of the vital signs that health care professionals measure to assess general health. It quantifies the force of blood against the walls of the arteries as it travels through them.
Doctors and nurses measure blood pressure using a sphygmomanometer, which reports pressure in millimeters of mercury (mm Hg). This device measures the pressure of blood from one heartbeat to the next, a sequence known as the "cardiac cycle."
Blood pressure is expressed as two numbers. The first number measures the pressure of blood at the peak (systole) of a heartbeat. This is when the heart is contracting and blood is pumped from the upper chambers of the heart (atrium) to the lower chambers (ventricles). A systolic value below 120 mm Hg is considered healthy.
The second number in a blood pressure reading — the diastolic value — measures pressure when the heart is relaxed and refilling with blood after a heartbeat. Diastolic values below 80 mm Hg are considered healthy. Blood pressure is typically described as systolic pressure over diastolic pressure, such as 120/80 mm Hg.
When a person's systolic or diastolic measurements are higher than these optimal values, they are said to have high blood pressure, or hypertension. Hypertension poses a serious risk to cardiovascular health and can result in heart attack and stroke.
People with hypotension, on the other hand, have below-normal blood pressure, which may mean that their hearts are not able to circulate blood adequately. The National Institutes of Health considers blood pressure that's lower than 90/60 mm Hg a sign of hypotension. The condition is most commonly a side effect of medications, a complication of heart attack or the result of significant blood loss.