Which capacity is defined as the total volume of air that remains in the lungs after a normal expiration?

The correct option is Functional Residual Capacity.

Explanation

Respiratory capacities are calculated by summing specific respiratory volumes to describe the functional status of the lungs. These measurements are essential for assessing pulmonary function.

Analysis of Options:

• Functional Residual Capacity (FRC): This is defined as the volume of air that remains in the lungs after a normal expiration. It is the sum of the Expiratory Reserve Volume (ERV) and the Residual Volume (RV). Mathematically, FRC = ERV + RV.
• Vital Capacity (VC): This is the maximum volume of air a person can breathe in after a forced expiration. It includes the Expiratory Reserve Volume, Tidal Volume, and Inspiratory Reserve Volume (VC = ERV + TV + IRV).
• Expiratory Capacity (EC): This is the total volume of air a person can expire after a normal inspiration. It consists of the Tidal Volume and the Expiratory Reserve Volume (EC = TV + ERV).
• Total Lung Capacity (TLC): This refers to the total volume of air accommodated in the lungs at the end of a forced inspiration. It includes the Residual Volume, Expiratory Reserve Volume, Tidal Volume, and Inspiratory Reserve Volume (TLC = RV + ERV + TV + IRV).

Key Takeaway: Functional Residual Capacity (FRC) specifically refers to the air remaining in the lungs during the pause between breaths in a normal respiratory cycle.