How does blood pressure measuring apparatus (sphygmomanometer) work?

As a dynamic pump, the heart forces blood around an impressive network of arteries and veins which, if joined end to end, would circle the Earth two-and-a-half times. The pressure that is exerted during this process can most conveniently be measured in the brachial artery that passes through the upper arm. The greatest pressure occurs when the heart valve that pumps the blood (the ventricle) contracts and the lowest pressure occurs when this valve is relaxed. Determination of blood pressure therefore consists of two measurements, that of the greatest value and the lowest.

Blood pressures are recorded in millimeters of mercury (mmHg), about 120 millimeters being the normal high or systolic value, and around 80 millimeters the low diastolic value. Such average readings would be stated by a physician as ‘120’ over ‘80’. This means that the pressure exerted by the pumping action of the heart would physically suffice to raise a column of liquid mercury to these heights. Standard atmospheric pressure at sea level, by way of comparison, is 760 millimeters at 0° Celsius.

The apparatus used by physicians for measuring blood pressure is called sphygmomanometer (picture, above). It consists of (1) a pressure gauge housing a vertical glass column containing a reservoir of mercury; (2) a tube connected to a cuff, or wrapping sleeve that can be filled with air; and (3) a hollow rubber ball which pumps air into the cuff. To take blood pressure, the physician wraps the cuff around the patient’s arm. Air is first pumped into the cuff by squeezing the rubber ball. A stethoscope is placed over the artery of the arm just below the cuff. Forcing air into the cuff causes it to pass down on the artery, so the mercury in the tube rises. In other words, the pressure of the blood in the brachial artery is transmitted through the air contained in the cuff of the mercury. Moments later, as air is released from the cuff by means of a valve on the bulb, normal blood flow returns and the mercury starts to fall.

The first sound heard through the stethoscope is that of the systolic (upper) pressure, and the level of the mercury column measures that value. The next sound is the diastolic pressure, its valve being similarly observed from the calibrations on the pressure gauge.

Some physicians use another kind of apparatus which consists of a simple pressure gauge, which is calibrated in terms of millimeters of mercury. This handy instrument has eliminated the need for column containing liquid mercury which is considered an environmental hazard. Pressure is transmitted to a diaphragm at the back of such device, and pointer needle moves across a round dial to the appropriate value.

A person’s blood pressure never stays at a particular level, but varies according to his own level of physical activity and also his state of mind.

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