Simple Model of an axially distributed two-region capillary Blood-Tissue EXchange unit with consumption in interstitium

Description

   This capillary-tissue model consists of two "concentric" cylinders,
 a central capillary of volume Vcap with flow Fcap along the capilary
 of length L in which there is axial dispersion in the x-dimension. 
 Radial diffusion is assumed instantaneous in both regions. The convective 
 mean transit time is Vcap/Fcap = 3 seconds with the parameters above.
 The capillary length L is set at 1 mm, an approximation for cardiac 
 capillaries. In the partial differential equation the boundary 
 conditions at the capillary entrance, where x = x.min, are set so that
 diffusion in the upstream direction is balanced by the convection and
 no material is lost there from the capillary-tissue unit. Material is
 lost by convection into the outflow with concentration Cout. 
   The conductance for exchange between the capillary and the ISF is
 purely passive, equivalent to diffusional exchange through the clefts
 between endothelial cells. Visf is the interstitial water space,
 in which the solubility is assumed to be the same as in the capillary
 plasma. Consumption, occuring only in the ISF, is by a first order reaction
 with rate constant Gisf. (Note that Fcap, PS, and Gisf all have the same
 units, those of flux per unit mass of tissue, ml/(g*min).
   The dispersion terms represent a combination of molecular diffusion
 and dispersion due to intravascular velocity profile, eddies, disturbed
 flow, branching, all causing some mixing. This model can be made into a
 stirred tank (compartmental) model by raising the dispersion coefficients
 to high values, e.g. 10^(-1) cm^2/sec, oblitering axial gradients to result
 in uniform concnetrations from end to end. Dcap = 1e-4 cm^2/sec is a good
 approximation for the observed physiological dispersion in capillaries.

Equations

The equations for this model may be viewed by running the JSim model applet and clicking on the Source tab at the bottom left of JSim's Run Time graphical user interface. The equations are written in JSim's Mathematical Modeling Language (MML). See the Introduction to MML and the MML Reference Manual. Additional documentation for MML can be found by using the search option at the Physiome home page.