Arterial pressure is most commonly measured via a sphygmomanometer, which historically used the height of a column of mercury to reflect the circulating pressure.[2] Today BP values are still reported in millimetres of mercury (mmHg), though aneroid and electronic devices do not use mercury.
For each heartbeat, BP varies between systolic and diastolic pressures. Systolic pressure is peak pressure in the arteries, which occurs near the end of the cardiac cycle when the ventricles are contracting. Diastolic pressure is minimum pressure in the arteries, which occurs near the beginning of the cardiac cycle when the ventricles are filled with blood. An example of normal measured values for a resting, healthy adult human is 115 mmHg systolic and 75 mmHg diastolic (written as 115/75 mmHg, and spoken [in the US] as "one fifteen over seventy-five"). Pulse pressure is the difference between systolic and diastolic pressures.
Systolic and diastolic arterial BPs 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, disease, exercise, and momentarily from standing up. Sometimes the variations are large. Hypertension refers to arterial pressure being abnormally high, as opposed to hypotension, when it is abnormally low. Along with body temperature, respiratory rate, and pulse rate, BP measurements are the most commonly measured physiological parameters.
Arterial pressures are usually measured non-invasively, without penetrating skin or artery. Measuring pressure invasively, by penetrating the arterial wall to take the measurement, is much less common, and usually restricted to a hospital setting.
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