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The functioning of the sphygmomanometer can be analized from a physical point of view only if we accept some simplifications: blood must be considered an ideal liquid; and some factors that could interfere with the measurement (such as hormones, blood-fluidifying substances etc.) must be ignored. Despite these simplifications, that will certainly damage the validity of the measurements obtained, the sphygmomanometer still remains a basic, fundamental instrument for medicine.
In the symbolism used in this explanation, p0 is the atmosferic pressure in mmHg, pmax is the systolic pressure and pmin is the diastolic pressure as they are usually measured (in mmHg over atmosferic pressure). Therefore, pmax o min+p0 are respectively the maximum or minimum pressure measured in mmHg with atmosferic pressure included.
In the beginning of the measurement procedure, in the cuff there is air with pressure p0 equal to atmosferic pressure; the manometer linked to the cuff is made so that it takes this value as its zero, therefore it must show 0 mmHg. At the same time, in the brachial artery blood flows with a pressure varying between the diastolic minimum and the systolic maximum.
When the cuff gets inflated with the pump, air pressure inside it increseas. The artery, subject to a stronger external pressure, tends then to get narrower. Because of Leonardo's Principle, blood velocity increseas while the artery's diameter decreases. When the pressure in the cuff becomes stronger than the diastolic pressure, the blood has enough propulsion to pass in the artery almost only during the systole. This becomes more and more true as the pressure in the cuff gets similar to the systolic. These two factors make it so that blood changes the way it moves. Its flow, which was laminar, becomese turbulent. The irregular passage of blood in the artery causes the so-called Korotkoff sounds, a series of dull and rhytmic sounds that can be auscultated with a phonendoscope. This property of blood is very important for the measurement of its pressure.
As soon as the pressure in the cuff is stronger than the maximum pressure of blood (more than pmax+p0 the bloodstream in the artery stops completely because of its obstruction. With the phonendoscope no sound can be heard. At this point it is possible to begin the real measurement.
Opening the valve, the cuff deflates slowly balancing its internal pressure with the atmosferic pressure. In the instant in which the pressure in the cuff gets equal to pmax+p0, blood returns to flow in the artery, again in turbulent flow because of the little diameter of the artery: Korotkoff sounds can be heard again with a phonendoscope. The manometer shows in this instant the systolic pressure pmax measured in mmHg over atmosferic pressure.
Korotkoff sounds cease as soon as external pressure (the pressure of the cuff) is not so strong to stop blood from flowing in the artery, and blood returns to move in laminar flow. It is assumed that this happens in the instant in which blood pressure is equal to diastolic pressure pmin+p0; the manometer shows therefore the diastolic pressure pmax, measured in mmHg over atmosferic pressure. Nowadays, not all physicists agree if the true diastolic pressure is during the last Korotkoff sound or in the first moment of silence.
