Figure 1.

Illustration of fluid and interstitial waste solutes movement in the brain between the parenchyma and perivascular spaces surrounding the penetrating vasculature. The periarterial influx in the figure shows the inward movement of CSF along the periarterial space, which mixes with the ISF and interstitial waste solutes in the parenchyma with the help of AQP-4 and gaps between astrocytic end-feet, and then exit via perivenous efflux pathway (along the perivenous space). White arrows indicate the direction of blood flow in the vasculature. Green arrows show the CSF and CSF-ISF fluid transport (whether by diffusion, bulk flow, or both remains elusive). Purple dots indicate the interstitial waste solutes that exit the parenchyma through gaps between the astrocytic end-feet into the perivenous space, which are then drained to the CSF. Reproduced with permission from [15].