Vital capacity (VC)

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:The sum uses inspiratory reserve volume, tidal volume and expiratory reserve volume.^[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.^[ref][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.