Influence of astrocyte endfoot water permeability, Pf, on solute movement in brain ECS. (A, top) Schematic showing hydrostatic and osmotic water transport across the astrocyte endfoot barrier and hydrostatic (advective) fluid movement in the gaps between endfeet. (bottom) Pseudocolored images showing tracer solute accumulation in ECS, as in Fig. 3, for Pf = 0, 0.004, and 0.04. For all computations in this figure, D = 10−10 m2/s and α = 0.2. (B, top) Kinetics of tracer solute accumulation in ECS for the indicated Pf. (bottom) Half-filling time for the indicated parameter sets. (C) Influence of active ion pumping to create an osmotic imbalance between the para-arterial space and ECS. (top) Schematic of possible effects of ion pumping into and out of the ECS across astrocyte endfeet, in which osmotically driven water transport across endfeet changes pressure in the ECS and hence the driving force for advective fluid movement from the para-arterial space into the ECS. (bottom) Pseudocolored images at time = 600 s for Pf = 0.04 cm/s showing accumulation of solutes A and B in ECS for the indicated active ion pumping flux, JApump = 1.5 × 10−3 mol/m3 (active pumping into the ECS) and JApump = −1.5 × 10−3 mol/m3 (pumping from the ECS). (D) Kinetics of tracer solute accumulation in ECS for ΔP = 1 and 5 mmHg.
