This model describes the flow characteristics of a single vessel with resistance to flow, R, nonlinear compliance, C, and input flow of Fin.

Description

  The model simulates pressure and division of fluid flow through a compliant 
  vessel with resistance to flow, R, and vessel compliance, C, given an input 
  flow of Fin.  The compliant vessel follows a nonlinear pressure volume 
  curve which takes into account the resistance to vessel collapse that 
  occurs at low or negative pressures and the concave form of the curve at 
  high pressures. The pressure-volume function used is that proposed by 
  Drzewiecki et al. (Am J Physiol, Heart Circ Physiol 273:H2030-H2043, 1997).
  
  The flow out of the vessel is related to the resistance by the fluid 
  equivalent of Ohm's Law.  

                        Fout = (Pin - Pout) / R

  where Pin is the pressure at the vessel entrance, Pout is the pressure at
  the end of the vessel and R is determined from Poiseuille's Law as:
                         R = 128*mu*L / pi*D^4

  where mu is the fluid viscosity, L is the vessel length, and D is the 
  vessel diameter.

  Pin is given from the formulation by Drzewiecki et al. and is given by
  the following expression:

                           _     _            _      _
                          |     |      V/L-Ab  |      |
               Pin = a * <  exp<  b * --------  > - 1  > 
                          |_    |_       Ab   _|     _|
                                     _             _
                                    |         n     |
                         - EIhat * <  (Ab*L/V)  - 1  >  + Pb
                                    |_             _|

  where a and b are constants which determine the nonlinear shape of the
  vessel elastance, V is the vessel volume, Ab is the vessel cross-
  sectional area at buckling, EIhat is the normalized flexural rigidity 
  of the vessel wall, n determines the curvature of the pressure volume
  curve in compression and Pb is the intraluminal pressure at buckling
  which is given by:
                            _     _           _      _
                           |     |      Ab-A0  |      |
                 Pb = a * <  exp<  b * -------  > - 1  > 
                           |_    |_      A0   _|     _|

  where A0 is the vessel cross-sectional area at an intraluminal 
  pressure of zero. For more details about this formulation and the
  parameter values given for different vessels see Drzewiecki et al.

  The flow into the vessel and the flow out of the vessel are different
  because of the change in volume which adds or subtracts flow from that
  leaving the vessel depending on whether the pressure is increasing or
  decreasing in the vessel.  So we have for vessel compliance, Fcomp:

                          Fcomp = Fin - Fout

  The change in vessel volume as a function of time is determined from
  the flow attributed to the vessel diameter change and is given by:

                            dV/dt = Fcomp  

  It should be noted in the code that the compliance, C, is calculated by 
  taking the inverse of the derivative of the Pin with respect to V. 

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.