Abnormality on pulmonary function testing Abnormal functional residual capacity 2021-04-21T15:47:00Z A deviation from normal values for the functional residual capacity, which is defined as the volume remaining in the lungs after a normal, passive exhalation. Functional residual capacity (FRC), is about 3L in a normal adult. FRC is the total amount of air in a person's lungs at the lowest point of their tidal volume (TV; the volume of air a person normally inspires and expires). The FRC is a lung capacity, consisting of the sum of two or more volumes. It also cannot be measured directly using spirometry and has to be calculated. This because FRC is a combination of the expiratory reserve volume (ERV) and the residual volume (RV). The residual volume is the amount of air remaining the lung after expelling as much air from the lungs as possible. The residual volume can never be exhaled; thus, it cannot be measured using spirometry and is the air causing the alveoli to remain open. The ERV is the reserve amount of air that can be exhaled forcefully, after passive exhalation. Therefore, the FRC can be represented as the equation: FRC=RV+ERV.