Vasomotion and transvascular exchange of fluid and plasma proteins.

The effect of spontaneous arteriolar vasomotion on the transvascular exchange of fluid and plasma proteins has been studied theoretically. The model combines fluid dynamic principles with a phenomenological approach to transvascular exchange on the basis of irreversible thermodynamics. The analytical treatment of the intravascular flow together with consideration of local changes of the morphology and vascular membrane characteristics makes it possible to determine the spatial and temporal variation of transvascular fluid and plasma protein fluxes. Since the undulations of the arteriolar diameter have low frequencies (3 to 10 per cycle), the transvascular fluxes are in phase with the vasomotion. The model combines hemodynamics, microhemorheology and mass transfer at the vessel wall and provides pressures, velocities, plasma protein concentration, hematocrit and apparent blood viscosity at any position in the microvascular bed at any time. In addition, global parameters as the total filtration rate and the total transvascular mass flow rate of plasma proteins are determined as functions of time. Numerical results are obtained for the cat mesentery with the terminal arteriole exhibiting vasomotion. The vascular arrangement studied comprised a terminal arteriole, capillaries and a venule together with appropriate side branches. The results show dramatic temporal changes in the hemodynamic parameters. Fluid filtration occurs along the entire vascular length at all times except during vasoconstriction when small absorption rates occur on the venous side. The transvascular fluxes achieve maximum values either on the arteriolar or on the venular side depending on the moment of the vasomotion cycle. The transvascular exchange rates in the mid-capillaries are generally low.