state the principle in use or law which is involved in function of sphygmomanometre?

Question

and how it works

Answer 1

A sphygmomanometer is little more than an inflatable cuff and an accurate pressure gauge. The person taking the blood pressure reading places the cuff around the patient's arm at the horizontal level of the heart. (The level is important to avoid static pressure differences as a result of elevation differences biasing the measurement.)

Attached to the cuff is a bulb with a release valve. The operator inflates the cuff to a pressure greater than that of the blood coming from the heart, temporarily cutting off the blood supply to the arm.

To take the reading, the operator then listens for blood flow to the arm with a stethoscope, gradually releasing the pressure on the cuff and watching the pressure meter. The operator notes the pressures when the blood first starts to flow and when full flow is restored, then releases the pressure and removes the cuff.

There is nothing special about the pressure gauges used with these devices. The name "sphygmomanometer" is probably better reserved for the old-fashioned version with a U-shaped tube with the bottom of the U filled with mercury. One leg of this U is open to the atmosphere and is marked off in distance units, usually millimeters, perhaps with a correction to be described later. The other end of this U is connected to the blood pressure cuff.

When pressure is applied to the cuff, this pushes the mercury up into the tube open to the atmosphere, against the weight of the mercury in the column. The pressure measurement is the difference in the heights of the two columns. To improve the size of the scale, the mercury in the column connected to the bulb is considerably wider than that connected to the air. This limits the amount of elevation difference of the mercury in this part of the manometer and puts most of the height difference in the column to be read. There is a slight correction in the scale for the small height changes that occur in the side connected to the cuff. Because these devices must be level to read accurately and would lose their mercury out the top if tilted too far, they are usually wall-mounted.

Because of the large amount of mercury in these devices, you don't see them much anymore. The cuff pressure is more usually measured these days with electronic strain gauges measuring the deflection of a diaphragm or by a semiconductor device that serves as both diaphragm and strain gauge or by a mechanical gauge much like that used to measure barometric air pressure or altitude in an airplane.

Answer 2

This surprisingly simple looking device to determine sys- and diastolic blood pressure actually employs a number of physical characteristics. [See source.]

Sound is what the medic listens to with her stethescope. The sound is the presence or lack of the pulse in an artery in your arm. The sound is picked up by the receiving side of the stethescope placed over an arm artery, is transmitted by the tubes, and is fed into each ear by the listening ends of the scope. All this working correctly depends on setting up standing waves of sound within the tubes. And that is the physics of sound generation and channeling.

The medic tightens an arm band around the arm with the artery that is being listened to. That cuts off the flow of blood through the artery, much like a tournequet would do. The medic hears nothing in the scope at this point. Then the medic gradually releases the grip of the band on the arm.

The amount of pressure put on the arm band is read in the manometer tube filled with a known, high density fluid...like mercury. Manometer is just a device for reading pressure, the tire pressure gauge at your local petrol/gasoline station is a manometer for example. But it doesn't use the tube filled with fluid to measure the pressure.

As the pressure on the arm is released, there is a point where the blood begins to flow again. It does this because, the blood's pressure overcomes the arm band pressure, which is read on the manometer. So at the time the blood begins to flow, the BP equals the pressure showing on the manometer. And, voila, that's your systolic blood pressure.

The medic knows the blood is flowing again because she can hear the pumping pulse through the stethescope. After noting the systolic pressure, which is the high side pressure from the heart contracting, the medic continues to release arm band pressure on the arm. The pumping sound (pulse) continues as the arm band pressure is released. But then, it stops...there is no more pumping sound.

That's the diastolic (low BP) blood pressure. The diastolic BP is the base line blood pressure that exists in your arteries even when the heart is relaxed and not contracted. The pumping sound disappears just the arm band pressure equals the diastolic BP because the artery is now fully distended and does not transmit the pulse sounds even though they are there in the artery.

So there you have it: the physics of sound and Pascal's Law are invoked in the blood pressure measruing device.