A simple network thermodynamic method for series-parallel coupled flows. III. Application to coupled solute and volume flows through epithelial membranes.

Abstract Using the network thermodynamic methods presented in the first two papers in this series (Mikulecky, Wiegand & Shiner, 1977; Mikulecky, 1977), an application to the coupled flows of salt and volume through epithelial membranes is developed. In another paper, the kidney proximal tubule is treated as a specific example (Thomas & Mikulecky,1978; Thomas, 1977). Two different experimental situations are examined in detail. The first is the case of transport across an epithelium with no external driving forces and a pump; the second is the case with transepithelial driving forces for both volume flow and solute flow but with no pump. For the linear network, the result for the most general case with a pump and transepithelial driving forces is obtained by superposition. The results obtained for the linear model lead to a number of interesting conclusions. The composition of transported fluid is independent of pump rate but does depend on the characteristics of the various membranes in the epithelium. The link between the genetic determination of epithelial membranes and the tonicity of blood is discussed. The absence of a need for a “standing gradient” in the interstitial space is clearly demonstrated, although the actual tonicity of that space can play a significant role, as is shown in detail in the work on the kidney proximal tubule. The topological aspects of the experimental design affect the manifestation of Onsager reciprocity in different parts of the system. Also, much of the epithelial function can be seen in terms of simple network concepts, such as the current divider properties of parallel pathways. The changes in topology brought about by different experimental design can be seen to alter these current-division patterns.