Capillary Exchange Modeling
Barrier-Limited and Flow-Limited Distribution
In the well-perfused visceral organs, active flow occurs in most capillaries, and they are packed closely. In this situation, lateral diffusion equilibration is relatively rapid and the distribution of exchanging materials is governed chiefly by the permeability of the capillary walls. We modeled extravascular distribution of exchanging substances from this kind of capillary and illustrated the changes expected in the outflow profile with increasing permeability, the evolution from the barrier-limited to the flow-limited case. We then examined the two extremes of the assemblies of such capillaries in an organ. In one, the large-vessel transit times are constant and the capillary transit times account for the outflow distribution of the vascular reference substance; in the other, the capillary transit times are constant but the large-vessel transit times vary. The barrier-limited and flow-limited cases corresponding to these are very different. In the case intermediate between these two extremes, the transit times in both the large vessels and the capillaries in the organs vary. If the organ is functionally homogeneous, the distribution of capillaries supplied by each large vessel is the same, and the situation may be described by a product distribution. The formulation for this intermediate case may then be used both to quantify capillary permeability and to describe the distributions of large-vessel and capillary transit times.
- well-perfused visceral organs
- multiple indicator dilution method
- spatially distributed models
- transcapillary exchange
- assemblies of large-vessel transit times
- diffusible indicators
- assemblies of capillary transit times
- capillary permeability
- Received December 30, 1969.
- Accepted August 10, 1970.
- © 1970 American Heart Association, Inc.