Abstract

Abstract. The Na+ recirculation theory for solute-coupled fluid absorption is an expansion of the local osmosis concept introduced by Curran and analyzed by Diamond & Bossert. Based on studies on small intestine the theory assumes that the observed recir-culation of Na+ serves regulation of the osmolarity of the absorbate. Mathematical modeling reproducing bioelectric and hydrosmotic properties of small intestine and proximal tubule, respectively, predicts a significant range of observations such as isosmotic transport, hyposmotic transport, solvent drag, anomalous solvent drag, the residual hydraulic per-meability in proximal tubule of AQP1 ()/)) mice, and the inverse relationship between hydraulic perme-ability and the concentration difference needed to reverse transepithelial water flow. The model repro-duces the volume responses of cells and lateral inter-cellular space (lis) following replacement of luminal NaCl by sucrose as well as the linear dependence of volume absorption on luminal NaCl concentration. Analysis of solvent drag on Na+ in tight junctions provides explanation for the surprisingly high meta-bolic efficiency of Na+ reabsorption. The model predicts and explains low metabolic efficiency in di-luted external baths. Hyperosmolarity of lis is gov-erned by the hydraulic permeability of the apical plasma membrane and tight junction with 6–7 mOsm in small intestine and £ 1 mOsm in proximal tubule. Truly isosmotic transport demands a Na+ recircula-tion of 50–70 % in small intestine but might be barely measurable in proximal tubule. The model fails to reproduce a certain type of observations: The reduced volume absorption at transepithelial osmotic equilib-rium in AQP1 knockout mice, and the stimulated water absorption by gallbladder in diluted external solutions. Thus, it indicates cellular regulation of apical Na+ uptake, which is not included in the mathematical treatment. Key words: Solute-coupled water transport — Hertz convection-diffusion equation — Mathematical modeling — Small intestine — Proximal tubule — Paracellular transport — Aquaporin-1 knockout — Solvent drag — Pseudo-solvent drag