Functional Residual Capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. At FRC, the elastic recoil forces of the lungs and chest wall are equal but opposite and there is no exertion by the diaphragm or other respiratory muscles.
A lowered or elevated FRC is often an indication of some form of respiratory disease. For instance, in emphysema, the lungs are more compliant and therefore are more susceptible to the outward recoil forces of the chest wall. Emphysema patients often have noticeably broader chests because they are breathing at larger volumes. In healthy humans, FRC changes with body posture. Obese patients will have a lower FRC in the supine position.