Transport of soluble macromolecules through the brain extracellular space (ECS), also called the interstitial space or parenchymal ECS, is facilitated by convection of cerebrospinal fluid (CSF) in the paravascular space (Ichimura et al., 1991; Weller et al., 2002; Abbott, 2004). More recently, a “glymphatic” mechanism has been proposed to mediate the clearance of solutes and macromolecules from the ECS, such as β-amyloid in neurodegenerative disease and metabolic waste during sleep, as well as to facilitate the distribution of nutrients to brain cells (Iliff et al., 2012; Jessen et al., 2015). In contrast to prior models, the glymphatic mechanism postulates that solute clearance in the ECS requires vectorial, advective flow of CSF from para-arterial to paravenous spaces though the brain parenchyma. As diagrammed in Fig. 1 A, advective flow entraps interstitial solutes in ECS fluid and enhances their clearance from brain tissue. The water permeability of astrocyte endfeet at the paravascular space/ECS interface and the ECS volume fraction in brain parenchyma have been proposed as important determinants of glymphatic flow (Iliff et al., 2012; Xie et al., 2013).