How does blood pressure measuring apparatus (sphygmomanometer) work?

As a dynamic pump, the heart forces the 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. This is also called systole hence systolic pressure. The lowest pressure occurs when this valve is relaxed, called diastole, hence diastolic pressure. The 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. A physician would state such average readings 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 a sphygmomanometer). 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 press down on the artery, so the mercury in the tube rises, due to the pressure. In other words, the pressure of the blood in the brachial artery is transmitted through the air contained in the cuff of the mercury. Slowly, the air is released for loosening a valve attached to the rubber ball. Moments later, as air is released from the cuff utilizing a valve on the bulb, normal blood flow returns and the mercury starts to fall. As the mercury falls, the physician listens for sounds through the stethoscope.

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 (lower) pressure.

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 the column containing liquid mercury, which is considered an environmental hazard.

The pressure is transmitted to a diaphragm at the back of such a device, and pointer needle moves across a round dial to the appropriate value. These handy instruments can be used at homes simply to measure a person’s blood pressure, which needs regular monitoring, with no need to visit the physician often. Anyone can learn how to operate the apparatus and check the blood pressure.

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