We have attempted to determine the membrane parameters, permeability-surface area product (PS) and reflection coefficient (σ), necessary to describe solute exchange in the pulmonary circulation. Experimental lymph-to-plasma ratios were obtained for total protein (Rtot) and albumin (Ralb) using lung lymph collected from goats with chronic lung lymph fistulae. Lymph flow (QL) and solute flow (QS) were varied by increasing microvascular pressure in a stepwise manner by inflating a balloon in the left atrium. The increased QL and decreased Ralb from multiple experiments in four goats were used to plot PS vs. σ using the equation of Patlak et al. (J. Theor. Biol. 5: 426-442, 1963). The region of intersection of two or more such curves should identify the two membrane constants. However, these curves rarely intersected suggesting that perhaps the lung barrier was not adequately representable by a single homoporous membrane. To evaluate heteroporosity, the three-pore model of Blake and Staub (Microvasc. Res. 12: 197-220, 1976) was investigated. By use of their pore sizes and relative pore distributions, values for the fluid filtration coefficient, PS, and σ of each pore type were calculated, and finally QL and RS were predicted at various microvascular pressures. When these calculated values of QL and R were plotted using the equation of Patlak et al., an equivalent homoporous PS and σ could still be found. Therefore, a heteroporous membrane does not appear to explain the failure of our data plots to intersect. Possible alternative models are suggested.
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