Functional residual capacity

Functional residual capacity (FRC) is the volume of air present in the lungs at the end of passive expiration.

[1] At FRC, the opposing elastic recoil forces of the lungs and chest wall are in equilibrium and there is no exertion by the diaphragm or other respiratory muscles.

In order to measure RV precisely, one would need to perform a test such as nitrogen washout, helium dilution or body plethysmography.

In restrictive diseases, the decreased total lung capacity leads to a lower FRC.

[5] Obese and pregnant patients will have a lower FRC in the supine position due to the added tissue weight opposing the outward recoil of the chest wall thus reducing chest wall compliance.