Water influx and efflux across the human blood–brain barrier each amount to roughly 40,000 mol day−1. The difference between the influx and efflux is very much less. Not even the normal direction of the net flux of water across the blood–brain barrier is known with any certainty, partly because it is so small. The available evidence suggests that scaled for a human there is a net movement from blood to brain amounting perhaps to ~ 10 mol day−1 (see [4]). For comparison metabolic production of water within the brain is ~ 3.3 mol day−1 and the amount of water in the CSF produced by the choroid plexuses is ~ 28 mol day−1.