The effect of the gradient of vascular permeability on fluid and plasma protein exchange in the mesenteric microcirculation.

The present work is a theoretical analysis of the intramural and transmural flows of fluid and plasma proteins. The transvascular fluxes are treated on the basis of a phenomenological approach resulting from the irreversible thermodynamics. The concept leads to a generalization of the well known Starling equation for the fluid exchange and to an additional equation which accounts for the exchange of plasma proteins. The key element in the theory is the incorporation of axial variations of the vascular permeability in the model. Numerical results are obtained for the cat mesentery taking into account the particular angioarchitecture of that tissue. The strength of the present model lies in the precise localization of the transvascular fluxes which results from the differential treatment of the fluid dynamic and mass transfer problem. The results confirm vital microscopic findings of various authors that fluid filtration and the plasma exchange occur predominantly on the arteriolar and venular side of the microcirculatory bed. In contrast, the exchange in the capillaries is relatively small regardless of the actual value of the gradient of permeability. In the organ specific situation studied, the transvascular fluxes are directed outward everywhere to the tissue. The general effect of the vascular gradient of permeability is to enhance the local transvascular fluxes on the venous side.