Effects of red cell exchange on calculated sheep lung vascular permeability ot 14C-urea and 14C-thiourea

Abstract

We performed fourteen experiments on sheep to determine whether or not erythrocyte tracer transport alters the calculated of capillary permeability surface area. In each set of experiments 14C-urea or 14C-thiourea was equilibrated with: (1) a whole blood sample; (2) saline alone, and (3) packed erythrocytes alone. Aliquots of each of these samples were injected separately into the superior vena cava of shepp and multiple indicator data collected from the aorta.Lung urea or thiourea microvascular permeability surface area (PS) was calculated for each set of data using the integral extraction method. These results were compared tot he predictions of a detailed theory of microvascular transport which included red cell effects. As predicted by the theory, only small differences were found between urea PS calculation based onpre-equilibrated blood or plasma injectes. When 14C-urea in the injectate was congined to red cells, the average PS calculation was approximately 60% of the average plasma-equilibrium value (also predicted by theory). This ratio was substantially lower (25%)_when 14C-thiourea was used as an indicator, suggesting that the red cell membrane, rather than the microvascular barrier, limits thiourea exchange. We conclude that a finite red cell-plasma urea exchange rate does not significantly influence the calculation of lung vasuclar PS when th einjected blood is equilibrated with urea prior to its introduction into the vascular system.