Fig. 1

Schematic representation of the ChP and derivation of the computational domain. a Macrostructure of the ChP containing capillaries in the stroma and epithelial cells on the outer surface of it. Protected pools of CSF are within a distance of \({l}_{\text{prot}}\) from the epithelial surface, as shown in the main panel. The first inset shows the two main epithelial cell borders (basolateral and luminal membranes), separated by tight junctions. The second inset shows epithelial microvilli, in between which standing osmotic gradients can be present. b Simplified representation of the microvillar zone with homogeneously distributed cylindrical microvilli (radius \({r}_{\text{mv}}\) and length \({l}_{\text{mv}}\)), spaced \(p\) apart from each other. A FU (blue shaded region) with the net water flow direction along the z-axis is shown. c Isolated FU with the 3D computational domain in blue and defined boundaries. Solutes are injected with a flux of \(\phi\) into the FU, normal to the \(\partial {\Gamma }_{\text{I}}\) boundary, which corresponds to the luminal membrane. \(\partial {\Gamma }_{\text{II}}\) and \(\partial {\Gamma }_{\text{III}}\) show the opening to the protected pool and adjacent FUs, respectively. d Simplified 1D geometry of the FU (a cylinder with hydraulic diameter of \(d\)) with its boundaries. \(\partial {\Omega }_{\text{I}}\) and \(\partial {\Omega }_{\text{II}}\) are the FU base and tip, respectively. In contrast to the 3D model, solute injection is applied through the governing equations