Below is an abridged Wikipedia description. We've all used one during our Pulmo function test. Very interesting discussion for those that are just a little curious.
I got curious about this because of my new oximeter {described elsewhere} has one for blood flow. But what I learned from this piece, is I may be able to answer a question since my very early days. I have felt my lungs were larger than normal. The key word here is "normal" because all of those %'s we get are based on "normal" for a person of your weight and size. According to the article my "actual" lung size may be hidden in this data. I'll keep you posted.
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Pulmonary plethysmographs are commonly used to measure the functional residual capacity (FRC) of the lungs—the volume in the lungs when the muscles of respiration are relaxed—and total lung capacity.
In a traditional plethysmograph, the test subject is placed inside a sealed chamber the size of a small telephone booth with a single mouthpiece. At the end of normal expiration, the mouthpiece is closed. The patient is then asked to make an inspiratory effort. As the patient tries to inhale (a maneuver which looks and feels like panting), the lungs expand, decreasing pressure within the lungs and increasing lung volume. This, in turn, increases the pressure within the box since it is a closed system and the volume of the box compartment has decreased to accommodate the new volume of the subject.
The difference between full and empty lungs can be used to assess diseases and airway passage restrictions. An obstructive disease will show increased FRC because some airways do not empty normally, while a restrictive disease will show decreased FRC. Body plethysmography is particularly appropriate for patients who have air spaces which do not communicate with the
bronchial tree; in such patients
helium dilution would give an incorrectly low reading.
Another important parameter, which can be calculated with a body plethysmograph is the airway resistance. During inhalation the chest expands, which increases the pressure within the box. While observing the so-called resistance loop (cabin pressure and flow), diseases can easily be recognized. If the resistance loop becomes planar, this shows a bad compliance of the lung. A COPD, for instance, can easily be discovered because of the unique shape of the corresponding resistance loop.[1]