= Expiratory reserve volume
{title2=ERV}
\b[Expiratory reserve volume], ERV, is the additional gas that can be exhaled after the end of a quiet expiration, down to maximal expiration. It lies between <functional residual capacity> and <residual lung volume>:
$$
\mathrm{ERV}=\mathrm{FRC}-\mathrm{RV}.
$$
It is distinct from the ordinary <tidal volume> and does not include the gas left after the maneuver.<Standardisation of the measurement of lung volumes, 2023 update by Nirav R. Bhakta and others>[Definitions and subdivisions of lung volume; Figure 1]{ref}
For stipulated FRC 2.5 L and <residual lung volume> 1.4 L, ERV is 1.1 L. Beginning instead at the end of a 0.5 L tidal inspiration permits a 1.6 L expiration down to that same residual endpoint. That larger excursion includes <tidal volume> as well as <expiratory reserve volume>, so its starting point changes its name and interpretation.
A shift in the ordinary end-expiratory level also changes ERV without necessarily changing <residual lung volume>. If that lower endpoint stays at 1.4 L while FRC changes from 2.5 to 2.0 L, ERV changes from 1.1 to 0.6 L. This is an original comparison of reference levels, not a claim about a particular disease or posture.
The identity is useful for combining measurements. A measured FRC supplies an absolute gas-volume reference; a linked expiratory maneuver supplies the increment needed to calculate <residual lung volume>. The technical standard requires consistent linked maneuvers for this calculation.<Standardisation of the measurement of lung volumes, 2023 update by Nirav R. Bhakta and others>[Measurement of FRC and calculation of RV]{ref} An incomplete expiration changes the observed excursion, so a reported ERV should remain attached to its maneuver and endpoints.
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