The correct answer is D. Functional residual capacity.
Scientific Explanation
A standard spirometer measures the volume of air inspired and expired by the lungs as a function of time. It can directly measure all lung volumes and capacities that are exchangeable with the environment.
Functional Residual Capacity (FRC) is defined as the volume of air remaining in the lungs at the end of a normal tidal expiration. It is calculated as:
Why it cannot be measured by spirometry:
The Constraint of Residual Volume: A spirometer can only measure air that moves in and out of the lungs. The Residual Volume (RV) is the volume of air remaining in the lungs even after a maximal exhalation. Because RV cannot be exhaled, it cannot be recorded by a spirometer.
Dependency: Since FRC includes the Residual Volume component, FRC cannot be determined via simple spirometry.
How FRC is measured:
To measure FRC (and consequently Total Lung Capacity and Residual Volume), indirect methods are required that do not rely on airflow alone. These include:
Helium Dilution Technique: Uses a closed-circuit system with a known concentration of inert helium gas.
Nitrogen Washout Technique: Uses an open-circuit system where nitrogen is washed out of the lungs by breathing pure oxygen.
Body Plethysmography: Uses Boyle's Law (P1V1 = P2V2) inside a sealed box to measure thoracic gas volume.
Analysis of Distractors
A. Vital Capacity (VC): This is the maximum amount of air a person can expel from the lungs after a maximum inhalation (VC = TV + IRV + ERV). Since all these components are exchangeable volumes, VC is easily measured by spirometry.
B. Tidal Volume (TV): This is the volume of air inhaled or exhaled during a normal breathing cycle. It is the most basic measurement performed by a spirometer.
C. Expiratory Reserve Volume (ERV): This is the additional amount of air that can be expired from the lungs by determined effort after normal expiration. This is directly measurable by spirometry.
