Correct Answer: A. Expiratory reserve volume + residual volume
Functional residual capacity (FRC) is the volume of air remaining in the lungs after a normal expiration at rest. It is a composite volume comprising two subdivisions: expiratory reserve volume (ERV) and residual volume (RV). The ERV is the maximum volume that can be expired after a normal expiration (typically 1000–1200 mL), and RV is the volume that cannot be expelled even with forced expiration (typically 1200–1500 mL). Thus, FRC = ERV + RV, typically 2200–2400 mL in healthy adults. FRC is clinically critical because it maintains alveolar patency, prevents airway collapse during expiration, and ensures continuous gas exchange. In Indian clinical practice, FRC measurement via helium dilution or nitrogen washout is used to assess obstructive airway disease (COPD, asthma) and restrictive lung disease. A reduced FRC suggests loss of elastic recoil or airway collapse; an elevated FRC indicates air trapping. This is a fundamental definition question that tests understanding of lung volume subdivisions as taught in Guyton's Physiology and Harrison's Principles.
Why the other options are wrong
B. Volume that remains after forced expiration — This describes residual volume (RV) alone, not FRC. While RV is a component of FRC, FRC includes both ERV and RV. This is a classic NBE trap that tests whether students confuse a single subdivision with the composite volume. Forced expiration removes all ERV, leaving only RV behind. C. Tidal volume + inspiratory reserve volume — This describes inspiratory capacity (IC), not FRC. IC = TV + IRV and represents the maximum volume that can be inspired from functional residual capacity. This option deliberately pairs inspiratory volumes to confuse students who haven't memorized the four primary subdivisions and their combinations. D. Volume expired after normal inspiration — This is vague and misleading—it could describe tidal volume (TV), which is the volume expired after normal inspiration (~500 mL). This trap exploits imprecise language and tests whether students understand that FRC is a resting state volume, not a volume measured after inspiration.
High-Yield Facts
- FRC = ERV + RV and is typically 2200–2400 mL in healthy adults; it is the volume at functional residual capacity (resting state after normal expiration).
- FRC maintains alveolar patency and prevents airway collapse during expiration; loss of FRC leads to atelectasis and hypoxemia in conditions like ARDS and acute respiratory failure.
- Reduced FRC is seen in restrictive lung disease (pulmonary fibrosis, pneumonia), supine position, and obesity; elevated FRC indicates air trapping in obstructive disease (COPD, asthma).
- FRC cannot be measured by simple spirometry; it requires helium dilution, nitrogen washout, or body plethysmography—essential in Indian pulmonary function testing labs.
- Closing capacity (CC) > FRC in small airways disease leads to airway closure during expiration, causing ventilation–perfusion mismatch and hypoxemia.
Mnemonics
FRC = ERV + RV Functional Residual Capacity = Expiratory Reserve Volume + Residual Volume. Both ERV and RV are 'leftover' volumes after expiration; FRC is their sum. Lung Volumes: TIRE Tidal (500 mL) + Inspiratory Reserve (3000 mL) + Residual (1500 mL) + Expiratory Reserve (1200 mL). FRC = E + R. Use when recalling all four subdivisions.
NBE Trap
NBE pairs "volume that remains after forced expiration" (option B) with FRC to exploit the common student error of confusing residual volume (a single subdivision) with functional residual capacity (a composite volume). Students who know RV is "unremovable air" may select B without recognizing that FRC includes ERV as well.
Clinical Pearl
In Indian ICUs, FRC collapse is a major cause of hypoxemia in mechanically ventilated patients and ARDS cases. PEEP (positive end-expiratory pressure) is applied to restore FRC and prevent atelectasis—a bedside principle that directly stems from understanding FRC = ERV + RV and its role in maintaining alveolar recruitment.
_Reference: Guyton & Hall Textbook of Medical Physiology, Ch. 37 (Pulmonary Ventilation); Harrison's Principles of Internal Medicine, Ch. 246 (Pulmonary Function Testing)_