The estimation of the diffusion constant and solubility of O-2 in tissue using kinetics

The diffusion of a gas through a substance in which it is soluble is analog ous to the passage of electric current through a circuit with both capacita nce and resistance. We model steady-state diffusion employing this analogy, and extend the model to include a description of the kinetics of systems u nder circumstances of changing partial pressure, applying two physical cons tants from electrical circuitry to gas diffusion: capacitance (zeta) and re sistance (R). We represent the substrate of the diffusion as a capacitor be ing charged through a resistor after the rapid imposition of a voltage chan ge. Using the insight derived from this model we have devised an experiment al system that allows us to approximate both D, the diffusion coefficient, and a, solubility, directly from the kinetic data. We do this by recording the exponential change in P-O2 on one side of a sheet of material both with and without the addition of a purely resistive barrier of known resistivit y. The method was used to estimate D and alpha for distilled water at a num ber of temperatures, olive oil, and the belly skin of Rana catesbeiana. (C) 2000 Academic Press.