How can functional residual capacity (FRC) be calculated?

Functional Residual Capacity (FRC) represents the volume of air remaining in the lungs at the end of a normal expiration. It can be calculated effectively using the relationship between total lung capacity (TLC) and inspiratory capacity (IC). In this context, TLC is the total volume of the lungs when fully inflated, which consists of the sum of all the lung volumes, while inspiratory capacity includes the tidal volume (VT) and the inspiratory reserve volume (IRV).

By subtracting inspiratory capacity from total lung capacity, you isolate the volume of air that remains in the lungs after a normal expiration. The formula can be summarized as:

FRC = TLC - IC

This calculation is essential in clinical practice for assessing lung function and diagnosing various respiratory disorders. Understanding FRC helps to identify conditions such as obstructive or restrictive lung disease, where the function of the lungs may be compromised.

The other choices do not yield FRC accurately because they involve erroneous combinations of lung volume definitions. For instance, calculating using vital capacity (VC) minus expiratory reserve volume (ERV), or attempting combinations that confuse reserve volumes, does not correctly represent the residual air left in the lungs and thus does not provide the correct measure of functional residual capacity.