Which calculation can be used to determine functional residual capacity (FRC)?

Functional residual capacity (FRC) is defined as the volume of air remaining in the lungs after a normal expiration. It can be calculated by summing the residual volume (RV) and the expiratory reserve volume (ERV). Therefore, the correct approach is to subtract the volumes that are not part of the FRC from the total lung capacity (TLC).

The correct calculation for FRC is obtaining it from the total lung capacity (TLC) by subtracting the inspiratory reserve volume (IRV) and the tidal volume (VT). This method ensures that you are accounting for the volumes that are actively involved in inspiration and normal breathing, leaving you with the air that exists in the lungs after expiration, which constitutes the FRC.

This approach reflects the relationship between the various lung volumes and capacities, allowing for a clear understanding of how much air remains available for gas exchange after typical breathing efforts. The structural relationship of these lung volumes helps in clinical assessments of pulmonary function.