Estimation of fluid flow velocities in cortical brain tissue driven by the microvasculature

Abstract

We present a modelling framework for describing bulk fluid flow in brain tissue. Within this framework, using computational simulation, we estimate bulk flow velocities in the gray matter parenchyma due to static or slowly varying water potential gradients--hydrostatic pressure gradients and osmotic pressure gradients. Working with the situation that experimental evidence and some model parameter estimates, as we point out, are presently insufficient to estimate velocities precisely, we explore feasible parameter ranges resulting in a range of estimates. We consider the effect of realistic microvascular architecture (extracted from mouse cortical gray matter). Although the estimated velocities are small in magnitude (e.g. in comparison to blood flow velocities), the passive transport of solutes with the bulk fluid can be a relevant process when considering larger molecules transported over larger distances. We compare velocity magnitudes resulting from filtration and pulsations. Filtration can lead to continuous directed fluid flow in the parenchyma, while pulsation-driven flow is (at least partly) reversible. For the first time, we consider the effect of the vascular architecture on the velocity distribution in a tissue sample of ca. 1 mm3 cortical gray matter tissue. We conclude that both filtration and pulsations are potentially potent drivers for fluid flow. Summary (lay abstract)Fluid transport through brain tissue is dynamic, but the basic properties of this flow and its variability are poorly characterised. Disturbed fluid transport was linked to Alzheimers disease. Moreover, fluid flow may be exploited to administer drugs to the brain. Using computer simulations, we estimate bulk flow velocities in the gray matter functional brain tissue due to changes in blood pressure and solute concentrations in unseen detail. We show that microvessel pulsations as filtration across the blood capillaries are potentially potent drivers of fluid flow in brain tissue.