A model of fluid, erythrocyte, and solute transport in the lung.

A mathematical model of fluid, solute, and red cell transport in the lung has been developed that includes the effects of simultaneous changes in lung vascular and interstitial volumes. The model provides separate arterial, microvascular, and venous pulmonary regions and a systemic vascular region in addition to a pulmonary interstitial compartment. Pressure, volume, hematocrit, flow, and concentration of up to 12 solutes and tracers can be computed in each compartment. Computer code is written in the C programming language, with Microsoft Excel serving as a user interface. Implementation is currently on PC-486 microcomputer systems, but the core program can easily be moved to other computer systems. The user can select different models for the blood-interstitial barrier (e.g., multiple pore, nonlinear Patlak equation), osmotic pressure-concentration relationships (e.g., Nitta, Navar-Navar), solute reflection coefficients interstitial macromolecule exclusion, or lymph barrier characteristics. Each model parameter or a combination of parameters can be altered with time in a predetermined fashion. The model is particularly useful in interpreting lung experimental data where simultaneous changes occur in vascular and extravascular compartments. Several applications are presented and discussed, including interpretation of optical filtration experiments, venous occlusion experiments, external detection of macromolecular exchange, and blood-lymph studies that use exogenous tracers. A number of limitations of the model are identified and improvements are proposed. A major strength of the model is that it is specifically designed to incorporate newly discovered relationships as the field of lung physiology expands.