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To understand the functioning of a sphygmomanometer, it is necessary to make it clear what is blood pressure.
Blood pressure measured with a sphygmomanometer is more properly called arterial blood pressure (since it is relative to blood flowing in arteries), opposing to venous blood pressure (the one of blood flowing in veins). Since these two blood pressures are very different, it is important that the distinction between them is clear.
If blood was a static fluid, it would have a pressure determined only by its height and density (because of Stevin's Principle). But blood is continuosly pumped in bloodstream, cycling between systole and diastole; its pressure varies therefore between a maximum (during the systole) and a minimum (during the diastole).
Blood pressure is measured in a standard location, the brachial artery, the main artery in the arm. The measurement provides values measured in mmHg over atmospheric pressure (see Physical Analysis of the Functioning). The average value of systolic pressure at rest is about 120 mmHg; the one of the dyastolic pressure is 80 mmHg. In different locations, values are different: for example, in capillary vessels, average maximum pressure is about 40 mmHg; minimum is about 30 mmHg.
Arterial blood pressure is constantly controlled by the body with the emission of hormones in very complex processes - it will do to know that physical and mental fatigue and illnesses can vary the values of arterial blood bressure.
