Pressurevolume relationship

This is determined by the compliance of the lungs and chest wall. The inspiratory pressure-volume relationship contains three components: an initial increase in pressure with no significant volume change; a linear increase in volume as pressure increases (the slope of which represents respiratory system compliance); and a further period of pressure increase with no volume increase. These three phases are separated by two inflexion zones, the lower representing the opening pressure of the system after flow resistance has been overcome in smaller airways and the upper approximating to total lung capacity. The expiratory pressure-volume relationship should normally approximate the inspiratory curve, returning to functional residual capacity. In patients with small airway collapse, separation of the inspiratory and expiratory curves occurs (hysteresis) as gas is trapped in smaller airways at the end of expiration.

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