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Functional Residual Capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. At FRC, the opposing elastic recoil forces of the lungs and chest wall are in equilibrium and there is no exertion by the diaphragm or other respiratory muscles.

FRC is the sum of Expiratory Reserve Volume (ERV) and Residual Volume (RV) and measures approximately 2100 mL in a 70 kg, average-sized male (or approximately 30ml/kg) .It cannot be estimated through spirometry, since it includes the residual volume.