Concurrent flow model for extraction during transcapilary passage. J.B. Bassingthwaighte: Circ Res 35:483-503, 1974.

Description

Figure 3: Increasing Permeability increases maximum Extraction.

 Model consists of a BTEX20 for the permeant tracer, and a BTEX10 for
the non-permeant tracer. Code from the Vacuscular operator model was
included to model the venous transport function in Figure 8. 

ABSTRACT
A model for capillary-tissue exchange in a uniformly perfused organ with 
uniform capillary transit times and no diffusional capillary interactions 
was designed to permit the exploration of the influences of various 
parameters on the interpretation of indicator-dilution curves obtained at 
the venous outflow following the simultaneous injection of tracers into 
the arterial inflow. These parameters include tissue geometric factors, 
longitudinal diffusion and volumes of distribution of tracers in blood 
and tissue, hematocrit, volumes of nonexchanging vessels and the sampling 
system, capillary permeability, P. capillary surface area, S, and flow of 
blood- or solute-containing fluid, F's. An assumption of instantaneous 
radial diffusion in the extravascular region is appropriate when 
intercapillary distances are small, as they are in the heart, or 
permeabilities are low, as they are for lipophobic solutes. Numerical 
solutions were obtained for dispersed input functions similar to normal 
intravascular dye-dilution curves. Axial extravascular diffusion showed 
a negligible influence at low permeabilities. The "instantaneous extraction" 
of a permeating solute can provide an estimate of PS/F's, the ratio of the 
capillary permeability-surface area product to the flow, when PS/F's lies 
between approximately 0.05 and 3.0; the limits of the range depend on the 
extravascular volume of distribution and the influences of intravascular 
dispersion. The most accurate estimates were obtained when experiments were 
designed so that PS/F's was between 0.2 and 1.0 or peak extractions were 
between 0.1 and 0.6.

Equations

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