Functional residual capacity of lung is defined as
**Core Concept**
Functional residual capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. It represents the balance between lung inflation and lung recoil forces. FRC is a critical parameter in understanding lung mechanics, particularly in respiratory diseases.
**Why the Correct Answer is Right**
FRC is the sum of the residual volume (RV) and the expiratory reserve volume (ERV). RV is the volume of air remaining in the lungs after a maximal expiration, while ERV is the additional volume of air that can be exhaled after a normal expiration. The lung's tendency to collapse and the chest wall's recoil forces determine the balance between RV and ERV, ultimately defining FRC. The FRC is approximately 2.2-2.5 liters in adults.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe FRC. FRC is not the volume of air in the lungs after a maximal inhalation.
**Option B:** This option is incorrect because it is not the correct formula for calculating FRC. FRC is the sum of RV and ERV, not the difference between VC and RV.
**Option C:** This option is incorrect because it is not the standard definition of FRC. FRC is not the volume of air in the lungs at the end of a forced expiration.
**Clinical Pearl / High-Yield Fact**
FRC is an important parameter in assessing respiratory function, particularly in patients with chronic obstructive pulmonary disease (COPD) or other lung diseases. A decreased FRC may indicate lung hyperinflation or air trapping, which can lead to respiratory failure.
**Correct Answer:** D. The volume of air present in the lungs at the end of passive expiration.