What is the relationship defined by the equation Functional Residual Capacity (FRC)?

Functional Residual Capacity (FRC) is defined as the volume of air remaining in the lungs after a normal expiration. The definition of FRC can be broken down into its components, which include Residual Volume (RV) and Expiratory Reserve Volume (ERV).

Residual Volume is the amount of air that remains in the lungs even after a forceful expiration, preventing lung collapse. Expiratory Reserve Volume refers to the additional air that can be forcibly exhaled after the expiration of a normal tidal breath.

Therefore, FRC is the sum of these two volumes: RV and ERV. This is why the equation FRC = RV + ERV is correct, as it accurately reflects the relationship between these lung capacities and volumes. Understanding this relationship is essential for grasping concepts in respiratory physiology, particularly in assessing lung function and pathology.

The other relationships presented in the choices do not correctly define FRC. For instance, the relationships involving Tidal Volume (TV) and Inspiratory Reserve Volume (IRV) do not incorporate both components necessary to establish FRC.