= Inspiratory capacity
{title2=IC}
\b[Inspiratory capacity], IC, is the maximum volume that can be inhaled from the end of a quiet expiration to maximal inspiration. Its lower reference is <functional residual capacity>, and its upper reference is <total lung capacity>:
$$
\mathrm{IC}=\mathrm{TLC}-\mathrm{FRC}=V_T+\mathrm{IRV}.
$$
The last expression combines <tidal volume> with <inspiratory reserve volume>.<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 <total lung capacity> 6.0 L and <functional residual capacity> 2.5 L, IC is 3.5 L. If <tidal volume> is 0.5 L, the corresponding <inspiratory reserve volume> is 3.0 L. The two descriptions use the same maximum inspiratory endpoint but different starting points.
The starting point is essential when interpreting a change. In an original comparison, keep <total lung capacity> at 6.0 L while moving the quiet end-expiratory level from 2.5 to 3.2 L. IC falls from 3.5 to 2.8 L, although the maximum inspiratory volume has not changed. Thus a lower <inspiratory capacity> need not imply a lower <total lung capacity>; an independently specified baseline is required to distinguish the possibilities.
<Inhalation> through a volume-measuring apparatus can determine an inspiratory increment without directly measuring the gas already present in the <lungs>. A stable starting level and complete endpoint connect that increment to the defined capacity. The technical standard therefore links capacity maneuvers with an identified FRC-level measurement.<Standardisation of the measurement of lung volumes, 2023 update by Nirav R. Bhakta and others>[Measurement of FRC and linked capacity maneuvers]{ref} The values in the example illustrate endpoint bookkeeping rather than reference limits for a particular person.
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