7th February 2024

Lung volumes and capacities

As clinicians, we need to be aware of several lung volumes and capacities. 

Lung volumes and their definitions:

• Tidal volume (TV): volume of air that is inspired or expired during normal resting breathing.
• Inspiratory reserve volume (IRV): additional volume of air that can be maximally inspired on top of normal tidal volume inspiration.
• Expiratory reserve volume (ERV): additional air volume that can be forcefully expired after normal tidal volume expiration.
• Residual volume (RV): volume of air remaining in the lung after maximal expiration.

Lung capacities are a combination of two or more lung volumes. Lung capacities and their definitions:

• Vital capacity (VC): air volume that can be maximally expired after maximum inspiration.
  • VC = IRV + TV + ERV
• Functional residual capacity (FRC): volume of air remaining in the lung at the end of resting expiration.
  • FRC = ERV + RV
• Total lung capacity (TLC): volume of air in the lung after maximal inspiration
  • TLC = IRV + TV + ERV + RV

Image Source: Wikimedia Commons (2012). Lung Volumes Updated, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/1/16/Lungvolumes_Updated.png  (Accessed: 4 January 2024).

Other important measurements that can be obtained by the spirometry measurements include:

• Forced vital capacity: Volume of air that can be forcibly expired after maximal inhalation.
• FEV1: Air volume that can be forcibly expired during the first second of expiration after maximal inhalation. 

This information can be summarised in a flow-volume loop. Changes in the contour of the loop can help support the diagnosis of restrictive or obstructive pulmonary conditions.