= Vital capacity
{title2=VC}
\b[Vital capacity], VC, is the volume change between maximal inspiration and maximal expiration. It is the exchangeable difference between <total lung capacity> and <residual lung volume>:
$$
\mathrm{VC}=\mathrm{TLC}-\mathrm{RV}=\mathrm{IRV}+V_T+\mathrm{ERV}.
$$
The sum uses <inspiratory reserve volume>, <tidal volume> and <expiratory 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}
A constructed volume record with maximum 6.0 L and minimum 1.4 L gives VC 4.6 L. Subdivisions 3.0 L, 0.5 L and 1.1 L give the same result. Those increments partition the full excursion; none includes the 1.4 L that remains after maximal expiration.
<Forced vital capacity> names a specific forceful expiratory maneuver. The broader <vital capacity> can also be measured during a relaxed expiration or inspiration. The maneuver and completeness of its endpoints matter when comparing measurements.<Standardisation of the measurement of lung volumes, 2023 update by Nirav R. Bhakta and others>[Definitions and subdivisions of lung volume, inspiratory VC, expiratory VC and FVC]{ref}<Standardization of Spirometry 2019 Update by Brian L. Graham and others>[FVC Maneuver; end-of-forced-expiration considerations]{ref}
A <spirometry> trace records volume change, so the same VC can arise at different absolute lung-gas levels. A second stipulated record spanning 7.0 to 2.4 L also gives 4.6 L. Adding a common offset to every point leaves VC unchanged. This is why a precise <vital capacity> measurement alone cannot determine <residual lung volume> or <total lung capacity>.
The distinction is useful experimentally: a large recorded excursion establishes how much gas was exchanged in that maneuver, while an independent absolute-volume measurement establishes how much gas occupied the <lungs> at its endpoints.
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