Normal functional residual capacity is:
**Core Concept**
Normal functional residual capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. It represents the balance between the inward recoil of the lungs and the outward recoil of the chest wall. This balance is crucial for maintaining lung compliance and preventing atelectasis.
**Why the Correct Answer is Right**
The correct answer is based on the concept of lung volumes and capacities. 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 exhalation. ERV is the additional volume of air that can be exhaled after a normal exhalation. The normal FRC value is approximately 2.2-2.5 liters in adults. This value is essential for maintaining adequate oxygenation and carbon dioxide elimination.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is below the normal range for FRC. A value of 1.5 liters would indicate a restrictive lung disease or atelectasis.
**Option B:** This option is incorrect because it is above the normal range for FRC. A value of 3.5 liters would indicate a obstructive lung disease or hyperinflation.
**Option C:** This option is incorrect because it is not a specific value for FRC. A range of 1-5 liters is too broad and does not provide a clear answer.
**Clinical Pearl / High-Yield Fact**
FRC is an important parameter in assessing lung function, especially in patients with respiratory diseases. A decrease in FRC can indicate restrictive lung disease, while an increase can indicate obstructive lung disease. It is essential to measure FRC accurately in patients with respiratory symptoms to guide treatment and management.
**Correct Answer:** 2.2-2.5 liters.