Functional residual volume is?
**Core Concept**
Functional residual capacity (FRC) is the volume of air present in the lungs at the end of a passive expiration. It represents the balance between the inward recoil of the lungs and the outward recoil of the chest wall, and is a critical factor in maintaining adequate gas exchange.
**Why the Correct Answer is Right**
FRC is the sum of the residual volume (RV) and the expiratory reserve volume (ERV). The residual volume is the volume of air that remains in the lungs after a maximal exhalation, while the expiratory reserve volume is the additional volume that can be exhaled beyond the normal expiration. The FRC is approximately 2.2-2.5 L in a normal adult, and is influenced by the elastic properties of the lungs and chest wall, as well as the tone of the respiratory muscles.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a recognized lung volume. The lung volumes are: tidal volume (TV), inspiratory reserve volume (IRV), expiratory reserve volume (ERV), residual volume (RV), and dead space volume (Vd).
**Option B:** This option is incorrect because it is not the correct formula for calculating FRC. The correct formula is FRC = RV + ERV.
**Option C:** This option is incorrect because it is not a recognized lung volume. The lung volumes are: tidal volume (TV), inspiratory reserve volume (IRV), expiratory reserve volume (ERV), residual volume (RV), and dead space volume (Vd).
**Option D:** This option is incorrect because it is not the correct unit of measurement for FRC. The unit of measurement for FRC is liters (L).
**Clinical Pearl / High-Yield Fact**
The FRC is an important concept in understanding the pathophysiology of respiratory diseases, such as chronic obstructive pulmonary disease (COPD) and cystic fibrosis, where the lung volumes are often altered.
**Correct Answer: D. 2.2-2.5 L**