Role of pulmonary lymphatics and interstitium in visceral pleural fluid exchange

If the pulmonary lymphatics and/or interstitium of the lung participate in visceral pleural (VP) fluid exchange, then increasing the water content of the interstitial compartment by lymphatic ligation (L) should increase the filtration and/or decrease the absorption of fluid across the VP membrane. To test this hypothesis, the hydraulic conductivity (Lp) and diffusional permeability for protein (Pd) of the visceral pleura were determined in eight spontaneously breathing anesthetized chronic L dogs and seven sham-operated (S) controls. The left lung was enclosed in a water-impermeable membrane, creating a VP space; fluid flux was determined as the filtration or reabsorption of water and protein in the VP space. Intravascular and pleural pressures and the protein concentration of plasma and pleural fluid were monitored during two or more experimental periods in which different mixtures of plasma and saline were introduced in the VP space. At similar pulmonary capillary pressures, the extravascular lung water of the L animals [5.36 (+/- 0.32) g/g dry blood-free lung] was significantly (P less than 0.001) greater than that of the S dogs [3.79 (+/- 0.13)]. The Lp of the L dogs [6.8 (+/- 1.6) X 10(-7) ml X s-1 X cm-2 X Torr-1] was virtually identical (P greater than 0.8) to the 6.7 (+/- 1.4) ml X s-1 X cm-2 X Torr-1 of the S controls. Similarly, Pd in the L group [4.3 (+/- 0.5) X 10(-6) cm/s] was similar (P greater than 0.5) to that of the S animals [5.1 (+/- 1.1) X 10(-6) cm/s]. We conclude that neither the pulmonary lymphatics nor the interstitium of the lung contribute to VP fluid exchange.