Steady-state pleural fluid flow and pressure and the effects of lung buoyancy

Both theoretical and experimental studies of pleural fluid dynamics and lun g buoyancy during steady-state, apneic conditions are presented. The theory , shows that steady-state, top-to-bottom pleural-liquid flow, creates a pre ssure distribution that opposes lung buoyancy. These two forces may, balanc e, permitting dynamic lung floating, but when they, do not, pleural-pleural contact is required. The animal experiments examine pleural-liquid pressur e distributions in response to simulated reduced gravity, achieved by, lung inflation with perfluorocarbon liquid as compared to air The resulting dec rease in lung buoyancy modifies the force balance in the pleural fluid, whi ch is reflected in its vertical pressure gradient. The data and model show that the decrease in buoyancy with perfluorocarbon inflation causes the ver tical pressure gradient to approach hydrostatic. In the microgravity analog ue, the pleural pressures would be toward a more uniform distribution, cons istent with ventilation studies during spaceflight. The pleural liquid turn over predicted by the model is computed and found to be comparable to exper imental values from the literature. The model provides the flow field, whic h can be used to develop a full transport theory for molecular and cellular constituents that are found in pleural fluid.