Functional residual capacity increases in which condition?

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

Answer: (D)

Functional residual capacity is the volume in the lungs after a normal exhale, and it increases when air is trapped in the lungs at the end of expiration. In obstructive diseases, the airways are narrowed and expiratory flow is limited, so during expiration not all the air can be pushed out. This leads to air trapping and a higher end-expiratory lung volume, which raises the functional residual capacity. In contrast, restrictive diseases make the lungs stiffer and smaller, so the end-expiratory volume is reduced, not increased. Normal lungs sit at a baseline FRC, and mixed disease can vary but the characteristic rise due to air trapping points to obstructive pathology.