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 Compliance (ΔV/ΔP) is the change in lung volume (ΔV) per unit change in transpulmonary pressure (ΔP) and it has been estimated as 22.7 l/kPa (3.0 l/mmHg).

 Lung compliance can be measured in an awake spontaneously breathing horse in a laboratory setting by measuring the mouth to intrathoracic pressure and lung volume change using a pneumotachograph (device that measures airflow over time) attached to a facemask. In anesthetized horses, pulmonary compliance and pressure volume curves are determined using positive pressure to achieve a change in lung volumes from FRC to total lung capacity. (see Figure 4.3)

 Compliance is the slope of the pressure‐volume curve and, due to the non‐linear shape of the curve, it fluctuates with lung volume.

 In the spontaneously breathing animal with normal lungs, tidal volume breathing occurs on the steep portion of the curve. As a result, for a given change in intrapleural pressure, there is a greater change in lung volume than would occur if tidal volume breathing occurred at the extremes of the curve.