MODEL FOR A PUMP THAT DRIVES CIRCULATION OF PLEURAL FLUID

Physical and mathematical models were used to study a mechanism that c ould maintain the layer of pleural fluid that covers the surface of th e lung. The pleural space was modeled as a thin layer of viscous fluid lying between a membrane carrying tension (T), representing the lung, and a rigid wall, representing the chest wall. Flow of the fluid was driven by sliding between the membrane and wall. The physical model co nsisted of a cylindrical balloon with strings stretched along its surf ace. When the balloon was inflated inside a vertical circular cylinder containing a viscous fluid, the strings formed narrow vertical channe ls between broad regions in which the balloon pressed against the oute r cylinder. The channels simulated the pleural space in the regions of lobar margins. Oscillatory rotation of the outer cylinder maintained a lubricating layer of fluid between the balloon and the cylinder. The thickness of the fluid layer (h), measured by fluorescence videomicro scopy, was larger for larger fluid viscosity (mu), larger sliding velo c ity (U), and smaller pressure difference (Delta P) between the layer and the-channel. A mathematical model of the flow in a horizontal sec tion was analyzed, and numerical solutions were obtained for parameter values of mu, U, Delta P, and T that matched those of the physical mo del. The computed results agreed reasonably well with the experimental results. Scaling laws yield the prediction that h is similar to(T/Del ta P)(mu U/T)(2/3). For physiological values of the parameters, the pr edicted value of h is similar to 10(-3) cm, in good agreement with the observed thickness of the pleural space.