IT HAS been proposed1 that the cerebrospinal fluid (CSF) acts as a "sink" for the brain extracellular space (ECS). Assuming free diffusion between brain ECS and CSF, the more rapid rate of removal of the extracellular indicator from the CSF relative to its rate of entrance into the CSF and brain from blood would prevent the attainment of a concentration equilibrium between the brain ECS and plasma. The "space" calculated with reference to the plasma concentration would, therefore, be smaller than the maximal anatomical volume of distribution of the indicator.
The present experiments were designed to eliminate the effect of the CSF "sink" by experimentally maintaining an equivalent concentration of isotope in plasma and CSF. This was accomplished by an intravenous injection and simultaneous ventriculocisternal perfusion of carrier-free sodium sulfate S35 and sodium iodide 1125 in ureter ligated cats. Perfusate concentration of tracers was adjusted to equal
Cutler RW, Lorenzo AV, Barlow CF. Sulfate and Iodide Concentration in BrainThe Influence of Cerebrospinal Fluid. Arch Neurol. 1968;18(3):316–323. doi:10.1001/archneur.1968.00470330106012