Comparative analyses of capsular fluid and interstitial fluid.

HALJAMXE, H., A. LINDE, AND B. AMUNDSON. Comparative analyses oj capsular fluid and interstitial @id. Am. J. Physiol. 227(5) : 11991205. 1974.-Interstitial fluid from rabbits was collected from implanted capsules and with a “liquid-paraffin cavity” technique. The fluids were analyzed for K+, Naf, Cl-, proteins, and glycosaminoglycans. The rate of transvascular exchange of albumin-1311 to the fluids was determined. The results show that the electrolyte composition is different from that of an ultrafiltrate of plasma due to the existence of negatively charged macromolecules in the interstitium. Thus the K+ and Na+ concentrations are higher and the Clconcentration is lower in interstitial fluid than in plasma. The observed protein content of the fluids was in agreement with expected interstitial protein levels. Furthermore, significant differences in electrolyte, protein, and glycosaminoglycan content between the capsular fluid and the directly sampled interstitial fluid are demonstrated. The exchange of labeled albumin was also extremely slow to capsular fluid as compared to interstitial fluid proper. These differences between the two fluids are discussed on the basis of current concepts of the interstitium and a suggestion of interstitialand capsular-phase submicroscopic structure is presented. It is concluded that the compositional and functional characteristics of the directly sampled fluid fit the requirements of a hypothesized, true interstitial fluid far better than does capsular fluid.