The exchange of sodium between plasma and cerebrospinal fluid.

This study is concerned with the mechanism of exchange of sodium between plasma and cerebrospinal fluid (CSF). Several workers (1, 2) have reported that there is no significant difference between the concentration of sodium in plasma and CSF. Merritt and Fremont-Smith (3) reported that the average concentration of sodium in CSF, in man, exceeded the plasma concentration by about four mEq. Davson (4) has determined the distribution ratio of sodium between CSF and plasma as 0.975 in the dog, correcting for the difference in water content of these fluids. Despite the similarity in sodium concentration of these fluids and the superficial resemblance of CSF to a protein-free filtrate of plasma, intravenously administered radio-sodium reaches equilibrium in the CSF far more slowly than in the extracellular fluid (5, 6). The delay in exchange between plasma and CSF has been attributed to the existence of a blood-CSF barrier for sodium (5-7). The present experiments have been devised to note the effect of a variety of physiological and pharmacological alterations on the exchange of sodium between these fluid compartments.