Tracer flow, permeability, and partial conductance.

It is often not possible to evaluate a permeability coefficient for net flow P from the small flows produced by physiological gradients of concentration or electrical potential. The common use of a tracer permeability coefficient P-x for this purpose, under the assumption that P-x = P, requires that the species be transported passively, and that there be no significant coupling between its flow and that of other chemical species, and between the flows of its tracer and abundant isotopes (isotope interaction). These conditions are often not satisfied. However, for passive transport in the absence of coupling of flows of different chemical species the measurement of tracer flow at two values of electrical potential difference evaluates (P-x/P) and thus P. In the presence of coupling of flows of different chemical species, although these measurements no longer evaluate P, they evaluate the partial conductance G. A graphical method of evaluating (p-x/P), P, and G is presented.