Physiology, Functional Residual Capacity

Functional residual capacity (FRC), is the volume remaining in the lungs after a normal, passive exhalation. In a normal individual, this is about 3L. The FRC also represents the point of the breathing cycle where the lung tissue elastic recoil and chest wall outward expansion are balanced and equal. Thus, the FRC is unique in that it is both a volume and related directly to two respiratory structures.FRC is the total amount of air in a person’s lungs at the lowest point of their tidal volume (TV), where the tidal volume is 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 expiratory reserve volume (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+ERVFRC is also the point at which two forces are at equilibrium; the inner recoil forces of the lung due to the elastic tissue of the alveoli, and the chest wall which wants to expand outwards.