A Theory for the Quantification of Transcapillary Exchange in the Presence of Shunt Flow

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Abstract
A theory is proposed to treat the transcapillary exchange of diffusion-limited substances in the presence of shunt circulation. With the simultaneous use of a nondiffusible indicator to correct for the distribution of intravascular transit and two diffusible indicators with different diffusion coefficients, the triple-indicator method can be used to assess the behavior of the diffusion-limited indicators in the exchange circuit. Application of this theory to data on perfused skeletal muscle allows the calculation of capillary blood flow, indicator extraction, and capillary diffusion capacity in the exchange circuit. The initial extraction of Na and Rb in the exchange circuit before back diffusion occurs shows a biphasic relation to flow rate. As the overall blood flow is increased to 10-15 ml min-1 100 g-1, decrease of indicator extraction is associated with an increase of capillary blood flow and a decrease of diffusion capacity in the exchange circuit. The increase of indicator extraction at higher flows is attributable to an increase of diffusion capacity in the exchange circuit. The biphasic changes in capillary diffusion capacity with blood flow has been attributed to the influences of metabolic factors and distending pressure.