What Is a Plethysmograph?

A plethysmograph is an instrument that measures volume changes in an organ or the entire human body.

Plethysmograph

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A plethysmograph is an instrument that measures volume changes in an organ or the entire human body.

Features

Lung plethysmographs are often used to measure functional residual capacity. Traditional plethysmographs are usually a closed box the size of a telephone booth with an air inlet.

After the routine exhalation, the mouthpiece is closed. Then let the patient take a deep breath. At this time, the human lungs swell, the air pressure inside them decreases, and the lung capacity increases. Because it is a closed system, the air pressure inside the box will increase.

Boyle's law is used to measure those unknown lung volumes. First, the change in chest volume was calculated. The initial air pressure inside the box is set equal to the known atmospheric pressure. When calculating the volume of air in the new box, the change in the volume of the thorax can be obtained.

Once this result is obtained, the volume of the original gas can be determined using the Boel's law.

There are also plethysmographs that measure the legs and feet to determine blood circulation.