Models of lung transvascular fluid and protein transport.

Transport theory has been applied to lymph flow (QL), protein lymph to plasma concentration ratios (L/P), and permeability surface area for urea (PSu) in unanesthetized sheep. Three models of the plasma-interstitial barrier have been used: a single pathway fiber matrix model, a continuous cylindrical-pore model with log normal distribution of filtration coefficients, and a cylindrical two-pore model. The fiber matrix model was unable to match measured PSu, QL, and L/P. The continuous-pore model was capable of describing the data, but the fitted median pore size was inconsistent with a continuum theory. The two-pore model described steady-state data and was used in additional model applications. We explored the 90% confidence limits for the fitted structural parameters of the two-pore theory. We found that many sets of model parameters were capable of fitting the available experimental data. We therefore sought combinations of parameters that might characterize the microvascular barrier under baseline and altered permeability situations. One combination that looks promising is the ratio of large-pore to small-pore radius raised to the sixth power and multiplied by the large-pore frequency. This value remains relatively constant following elevations in microvascular pressure, saline infusions, and plasma infusions but increases dramatically after endotoxin infusion.