A 22-year-old man undergoes spirometry as part of a routine pre-participation physical. He has no medical history and takes no medications. The technician records tidal volume (TV) 500 mL, inspiratory reserve volume (IRV) 3,000 mL, expiratory reserve volume (ERV) 1,200 mL, vital capacity (VC) 4,700 mL, and inspiratory capacity (IC) 3,500 mL. The report lists functional residual capacity (FRC) as 'not obtainable by spirometry.' A medical student observing the encounter asks the supervising physician which component of FRC makes it impossible to measure by this method.
Which of the following correctly identifies the unmeasurable component of FRC and explains why spirometry cannot detect it?