Lung volumes and elasticity.

The respiratory system is an elastic structure changing volume when pressures are generated by inspiratory or expiratory muscles. If the muscles are relaxed the respiratory system returns to its relaxation volume (Vr) (referred to in the pediatric literature as the elastic equilibrium volume [EEV]), which in normal subjects is the end-expired volume or functional residual capacity, FRC, where alveolar pressure is equal to atmospheric pressure.