A four phase model of capillary tracer exchange.

Abstract

In previously proposed models of capillary tracer exchange, red cell membranes have usually been assumed tobe either infinitely permeable or completely impermeable to tracer molecules. Permeability of the extravascular cellular compartment has been treated previously, but never in conjunction with finite capillary and red cell permeability effects. Our objective was to examine the situation encountered with multiple indicator experiments in which tracer exchange may be limited by red cell, tissue and capillary membranes. A four phase model is presented which accounts for plasma, red cell, interstitial, and extravascular cellular regions. Results from this model indicate that transcapillary tracer exchange is affected by a minimum of seven dimensionless parameters. The influence of relatively low red cell permeability is most pronounced when the dimensionless capillary permeability is high (i.e., αcap ⩾ 1). Deviations in transcapillary extraction values from those corresponding to infinitely permeable erythrocytes can be kept below 5% when capillary permeability is low (i.e., αcap ⩽ .15) by pre-equilibrating the injectate with tracer prior to injection. The additional barrier in the extravascular region necessarily decreases overall transvascular tracer exchange but does not affect extraction values in the vicinity of the appearance time.