 Residual lung volume

Residual lung volume, RV, is the gas remaining in the lungs after maximal expiration. It is smaller than the quiet end-expiratory functional residual capacity by the expiratory reserve volume:Thus residual gas is not the tidal volume or a direct measurement of gas exchanged in one breath.^[ref]

For stipulated FRC 2.5 L and expiratory reserve volume 1.1 L, RV is 1.4 L. Since spirometry records changes rather than an absolute lung-gas baseline, exhaling into a spirometer cannot by itself identify the gas still inside.

The 1956 experiment by Arthur B. DuBois and colleagues made that hidden volume accessible through body plethysmography. Their apparatus combined an airtight chamber, a mouthpiece shutter, capacitance pressure transducers and an oscillograph. Respiratory efforts against the closed shutter related mouth-pressure changes to changes in chamber volume.^[ref]

The physical idea follows Boyle's law. For a fixed gas amount at approximately fixed temperature, writeTo first order in the small changes,An original ideal calculation uses effective gas pressure 100 kPa, pressure increase 0.2 kPa and volume decrease 4 mL, giving $V\simeq 2000$ mL. This estimates the gas volume at the instant of airway closure. If closure occurs at quiet end-expiration, the result corresponds to an FRC-level measurement; expiratory reserve volume must still be subtracted to obtain residual lung volume. Pressure equilibration, thermal assumptions and correct volume conventions are part of the physical measurement, not consequences of the algebra alone.