Airway closure and fluid filtration in the lung

We have investigated the effect of airway closure on fluid balance in the isolated, perfused rabbit lung in which the alveolar, pleural and vascular pressures could be regulated independently. The rate of filtration into the interstitial space was monitored by continuous weighing. A reduction in pleural pressure of 0.5 kPa with open airways increased fluid filtration by 0.11 g min-1. Reduction in pleural pressure of 0.5 kPa with the main airways closed reduced the alveolar pressure by 0.4 kPa and increased fluid filtration by 0.86 g minp1. We concluded that the pleural pressure is transmitted poorly to the alveolar interstitial space in the normal lung with open airways. However, airway closure allows the pleural pressure to be transmitted almost completely to this interstitium, with interstitial oedema as a possible consequence.