= Inspiratory reserve volume
{title2=IRV}
\b[Inspiratory reserve volume], IRV, is the additional gas that can be inhaled after the end of an ordinary tidal inspiration, up to maximal inspiration. It is the portion of <inspiratory capacity> beyond <tidal volume>:
$$
\mathrm{IRV}=\mathrm{IC}-V_T=\mathrm{TLC}-(\mathrm{FRC}+V_T).
$$
The starting point is end-inspiration, rather than the quiet end-expiratory level used for <inspiratory capacity>.<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 a stipulated <total lung capacity> of 6.0 L, <functional residual capacity> 2.5 L and <tidal volume> 0.5 L, end-tidal inspiration reaches 3.0 L and leaves IRV 3.0 L. The full 3.5 L <inspiratory capacity> includes both the ordinary 0.5 L breath and this additional excursion.
A deeper ordinary breath changes the subdivision even if the absolute endpoints remain fixed. Holding <functional residual capacity> at 2.5 L and <total lung capacity> at 6.0 L while increasing <tidal volume> to 1.0 L reduces IRV to 2.5 L. Thus a change in <inspiratory reserve volume> need not imply that maximum inspiratory volume changed.
The name describes an available volume increment, not a separate anatomical compartment sealed off from tidal gas. In the ideal volume ledger, boundaries between subdivisions are reference states on the same gas-volume coordinate. A useful measurement consequently states the ordinary breathing level and the maximum endpoint, rather than interpreting IRV independently of the preceding breath.
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