Evaluation of the Early Extraction Method off Determining Capillary Permeability by Theoretical Capillary and Organ Models

The early extraction method of measuring capillary permeability was evaluated with theoretical models of capillaries and organs. The organs consisted of feeder lines and a capillary bed that was either homogeneous or heterogeneous with respect to capillary length or flow. The capillary permeability of the model organ determined by the early extraction method was compared with the actual permeability of the capillaries from which the organ was constructed. The main results of this analysis are: (1) If all the capillaries are identical and if the extravascular volume of distribution of the diffusible indicator is at least three times as large as the capillary volume, then the early extraction method provides a satisfactory measure of the capillary permeability. (2) If the capillaries are identical but the extravascular volume is about the same size as the capillary volume, tJien the early extraction method can lead to a qualitative underestimate of the permeability, namely assigning a significant diffusion limitation to a substance which is actually flow limited. (3) If the capillaries are not identical, either through differences of flow or length, then the early extraction method can again lead to the assignment of a significant diffusion limitation to a substance which is actually flow limited. ADDITIONAL