Alveolar surface tension, lung inflation, and hydration affect interstitial pressure [Px(f)].

Peribronchoarterial interstitial fluid pressure [Px(f)] was measured by wicks inserted between bronchus and artery of dog lobes filled with air, saline, 6% dextran in saline, or mineral oil. Five inflations were made to total lung capacity, with one min stops at eight selected volume levels in each cycle. Deflation recoil (measured as transpulmonary pressure, Ptp) was largest for air and least for saline and dextran, and it fell between these extremes for mineral oil. Correspondingly, Px(f) was most negative for air, slightly less negative for mineral oil, and least for saline and dextran. On the first cycle, the Px(f) for saline and dextran were nearly equal, but in later cycles Px(f) with saline drifted fairly rapidly toward alveolar pressure. By plotting Px(f) vs. Ptp, all first-cycle curves were brought toward a single line. During later cycles, Ptp and Px(f) always changed together along this line, except for saline. We conclude that 1) at fixed vascular pressure, Px(f) depends mainly on Ptp and less on lung volume; 2) large changes in Px(f) with saline suggest that at least some fluid can enter this interstitial space quite rapidly; and 3) peripheral tissue swelling with saline causes some reduction in Ptp, and both swelling and lower recoil contribute to increased trapping of saline.