Functional residual capacity decreases in which condition?

• A. Obstructive disease
• B. Restrictive disease
• C. Normal
• D. Mixed disease

Answer: (B)

Functional residual capacity (FRC) is the volume remaining in the lungs after a normal exhalation and is set by the balance between the lungs’ elastic recoil and the chest wall’s outward recoil. In restrictive disease, lung and/or chest wall compliance is reduced, so the lungs can’t expand and hold as much air. This lowers the total lung volume, including the expiratory reserve, so the end-expiratory level shifts downward. The result is a decreased FRC.

In contrast, obstructive diseases tend to increase FRC because air gets trapped and the lungs become hyperinflated, raising the end-expiratory volume. Mixed disease varies with which pattern dominates, but a restrictive component typically lowers FRC.