Transvascular exchange of fluid and plasma proteins

A theoretical model of transvascular exchange of fluid and plasma proteins in the microcirculation is developed based on fundamental laws of the fluid mechanics and on phenomenological transport equations of the irreversible thermodynamics. Intravascular axial changes of the pressure, flow and plasma protein concentration are taken into account as well as axial gradients of vascular permeability. Proper nondimensionalization of the resulting equations leads to the identification of dimensionless parameters which combine the transport characteristics of the endothelial wall and the intravascular flow resistance. In the theory, the dependence of the reflection coefficient on the transport coefficients of the vascular wall and on the plasma protein concentration is established. The model is applied to the cat mesentery and the rat intestinal muscle. The numerical simulations indicate that taking into account vascular protein permeability yields considerable differences in the axial distribution of the plasma protein concentration and transvascular fluxes in comparison with the case of protein impermeability of the endothelial wall. The results show that the maximum of the transvascular fluid and plasma protein movement resides at the site of the small venules while a minimum of the exchange occurs at the site of the midcapillaries.