Fig. 2

Comparison of a blood water flow and b, c transfers of water across the blood–brain barrier and the choroid plexuses. For the choroid plexus flow and transfers in and out of the brain are shown through a ×10 magnifying glass. The very much smaller net flows of water across both barriers are shown magnified ×1000. Arrow lengths are an approximate guide to sizes of the transfers. The water flow along the blood vessels, ~85% of blood flow, is ~100-fold greater than that to the choroid plexuses. Because the transfers of water across the interfaces are blood flow limited, the transfers in and out of the brain are also ~100-fold greater at the blood–brain barrier. By contrast because net transfers reflect active secretion of fluid, the very much smaller net transfer of water is almost certainly greater across the plexuses than across the blood–brain barrier