Importance of bulk flow in the exchange of compounds across the blood cerebrospinal fluid barrier

THE failure of many compounds, particularly those which are highly ionized, insoluble in lipid, or of high molecular weight, to achieve diffusion equilibrium across the blood cerebrospinal fluid barrier¹ suggests that some mechanism exists which results in the removal and return to the blood stream of these compounds. Pappenheimer et al.² have recently described a stereo-specific transport system which aids in the removal of organic acids from the cerebrospinal fluid. It seems unlikely, however, that this mechanism could effect the removal from cerebrospinal fluid of inulin or similar inert large molecules. In this communication we wish to present evidence that bulk flow of cerebrospinal fluid into blood can be an important means by which the former is cleared of foreign compounds.