The homeostatic cellular water efflux rate constant, kio, has a significant contribution from cell membrane sodium pump activity previously unmeasurable. With high extracellular contrast agent concentration or ultra-low magnetic field, kio can be precisely determined by two-site-exchange analysis of in vivo 1H2O longitudinal relaxation data. With the low field case, there is an inversion of the apparent tissue compartmental contributions from the true values. The NMR shutter-speed organizing principle informs an analysis spanning the entire range of conditions.
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