residualvolume

Residual volume (RV) is the volume of air remaining in the lungs after a maximal exhalation. This air cannot be expelled by voluntary effort and remains in the airways and alveoli due to elastic recoil and airway closure. RV, together with expiratory reserve volume, makes up the functional residual capacity (FRC), the lung volume at the end of a normal tidal exhalation; FRC equals RV plus ERV.

RV cannot be measured by standard spirometry, which only records air moved in and out during breathing maneuvers. Instead, RV is estimated using body plethysmography or gas-dilution methods such as nitrogen washout or helium dilution. These techniques assess trapped or residual air to determine lung volumes beyond what spirometry can provide.

Normal RV values vary with sex, height, and age. In healthy adults, RV is typically around 1.2–1.5 liters in men and 0.9–1.1 liters in women, with larger RV values associated with greater body size and increasing age. Variability among individuals means reference ranges are population-specific.

Clinical significance: an increased RV is a hallmark of obstructive lung diseases characterized by air trapping and hyperinflation, such as chronic obstructive pulmonary disease, emphysema, and severe asthma. A reduced RV can occur in restrictive lung diseases that lower overall lung volumes or in conditions that diminish lung compliance. Measuring RV helps assess hyperinflation, disease severity, and response to therapy, complementing other lung function tests.