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

We performed fourteen experiments on sheep to determine whether or not erythrocytes tracer transport alters the calculated value of capillary tracer 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 sheep 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 to the 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 calculations based on pre-equilibrated blood or plasma injectates. When 14C-urea in the injectate was confined to red cells, the average PS calculation was approximately 60% of the average plasma-equilibrated 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 cel-plasma urea exchange rate does not significantly influence the calculation of lung vascular PS when the injected blood is equilibrated with urea prior to its introduction into the vascular system.