Large water fluxes continuously take place between the different compartments of the brain as well as between the brain parenchyma and the blood or cerebrospinal fluid.

Disturbances in this well-regulated water homeostasis may have deleterious effects on brain function and may be fatal in cases where water accumulates in the brain following pathologies such as ischemia, haemorrhage, or brain trauma.

The molecular pathways by which water molecules cross the blood brain barrier are not well-understood, although the discovery of Aquaporin 4 (AQP4) in the brain improved the understanding of some of these transport processes, particularly under pathological conditions.