Osmolality of brain tissue and its relation to brain bulk

Abstract

Changes in the osmolality of the blood plasma of guinea pigs were induced by peroral hydration, coupled with injections of Pitressin, by peritoneal dialysis, and by hypertonic intravenous infusions. Using freezing-point determinations on homogenates the effects of these changes upon the osmolality of the tissue fluids of the brain were investigated. The water content of the brain and its sodium and potassium content were also measured. It was found that a rise or fall in plasma tonicity exceeding about 45 mosmoles/kg water was accompanied by a change of osmolality of the tissue fluids of the brain in the same direction and by a corresponding loss or gain of water. The water movements were sufficient to produce variations in the bulk of the brain which might critically affect intracranial volume-pressure relationships. In untreated animals the osmolality of the brain fluids appeared to be somewhat greater than that of plasma.