The Act of Breathing

In the modem era, our frrst real understanding of the act of breathing came when Rahn et al. (l) published their classic paper of the pressure-volume curves of the respiratory system, lungs, and chest wall. From the relaxation curve of the respiratory system, it became possible to calculate the pressures required by the respiratory muscles to change lung volume from its equilibrium position-functional residual capacity (FRC)-at which the pressure difference across the system [mouth pressure (Pm) relative to body surface pressure] during relaxation with the glottis open is zero. The pressures at volumes greater than FRC must be produced by inspiratory muscles, whereas those below FRC must be developed by expiratory muscles.