Blood pressure (strictly speaking: vascular pressure) refers to the force exerted by circulating blood on the walls of blood vessels, and constitutes one of the principal vital signs. The pressure of the circulating blood decreases as blood moves through
arteries, arterioles, capillaries, and veins; the term
blood pressure generally refers to
arterial pressure, i.e., the pressure in the larger arteries, arteries being the blood vessels which take blood away from the heart.
Arterial pressure is most commonly measured via a sphygmomanometer, which uses the height of a column of mercury to reflect the circulating pressure (see Non-invasive measurement). Although many modern vascular pressure devices no longer use mercury, vascular pressure values are still universally reported in millimetres of mercury (mmHg).
The systolic
arterial pressure is defined as the peak pressure in the arteries, which occurs near the beginning of the cardiac cycle; the diastolic
arterial pressure is the lowest pressure (at the resting phase of the cardiac cycle). The average pressure throughout the cardiac cycle is reported as mean arterial pressure; the pulse pressure reflects the difference between the maximum and minimum pressures measured.
Typical values for a resting, healthy adult human are approximately 120 mmHg (16 kPa) systolic and 80 mmHg (11 kPa) diastolic (written as 120/80 mmHg, and spoken as "one twenty over eighty") with large individual variations. These measures of
arterial pressure are not static, but undergo natural variations from one heartbeat to another and throughout the day (in a circadian rhythm); they also change in response to stress, nutritional factors, drugs, or disease.
Hypertension refers to
arterial pressure being abnormally high, as opposed to hypotension, when it is abnormally low. Along with body temperature,
blood pressure measurements are the most commonly measured physiological parameters.
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