The major effect of PEEP on the lungs is to increase FRC. In patients with decreased lung volume, both PEEP and CPAP increase FRC and tidal ventilation above closing capacity, improve lung compliance, and correct ventilation/perfusion abnormalities. The resulting decrease in intrapulmonary shunting improves arterial oxygenation. Their principal mechanism of action appears to be stabilization and expansion of partially collapsed alveoli. Recruitment (reexpansion) of collapsed alveoli occurs at peep or CPAP levels above the inflection point, defined as the pressure level on a pressure–volume curve at which collapsed alveoli are recruited (open); with small changes in pressure there are large changes in volume. Although neither peep nor CPAP decreases total extravascular lung water, studies suggest that they do redistribute extravascular lung water from the interstitial space between alveoli and endothelial cells toward peribronchial and perihilar areas. Both effects can potentially improve arterial oxygenation.