Tracer flux ratios: a phenomenological approach.

The kinetic behavior of tracer flows across epithelial membranes is examined and attention is called to the condition under which unidirectional tracer flows may be described by first order rate equations. It is shown that the first order nature of the tracer rate equation when combined with simple thermodynamic constraints on tracer flow yields a relation between the ratio of the unidirectional rate coefficients and thermodynamic driving forces. The form of this relation is examined for the case of simple diffusion and in the presence of coupled process.