What is calculated by subtracting Expiratory Reserve Volume (ER) from Functional Residual Capacity (FRC)?

Functional Residual Capacity (FRC) is defined as the volume of air remaining in the lungs after a normal tidal expiration. It consists of both the Expiratory Reserve Volume (ER) and the Residual Volume (RV). When you subtract the Expiratory Reserve Volume from the Functional Residual Capacity, you are isolating the Residual Volume itself.

By this calculation, you are essentially determining how much air remains in the lungs after all the air that can be forcefully exhaled (which is the Expiratory Reserve Volume) has been expelled. Therefore, the Residual Volume represents the air that cannot be expelled and remains in the lungs, which plays a critical role in maintaining gas exchange and preventing lung collapse.

The other choices represent different lung volume metrics that do not directly result from this subtraction: Total Lung Capacity (TLC) includes all lung volumes, Forced Vital Capacity (FVC) refers to the maximum amount of air that can be forced out after taking a deep breath, and Vital Capacity measures the air volume that can be exhaled following a maximum inhalation. Each of these concepts involves different principles and calculations, effectively distinguishing them from the straightforward relationship between FRC and RV.