JOURNAL OF NUQ..EAR MEDICINE 7:290-303, 1966 Selenate as a Substitute for Sulfate in the Measurement of Extracellular Fluid Volum&'5

Extracellular fluid ( E.C.F.) volume constitutes approximately 20 per cent of total body weight. Three-fourths of this volume of fluid ( 15 % of body weight) is distributed in the interstitial space which is in constant equilibrium, across the capillary membrane, with the intravascular water content (five per cent of body weight) and the cellular membrane with the intracellular fluid space (1,2,34), (Fig. 1). In recent years, great efforts have been made to measure extracellular fluid (E.C.F.) volume and to determine its significance in clinical medicine and the management of fluid therapy (23,31,42). E.C.F. volume is a reservoir of water and electrolytes. A major portion lies in the interstitial space between the cellular elements and the intravascular space. This latter volume of fluid serves to main tain tissue hydration and circulating blood volume. The transfer of nutrition and excretion of metabolites between the cellular elements of tissues and the blood stream takes effect through this buffer zone of the extracellular extravascular fluid space. In recent years, various ions (3,4,5,6,7,8,9) and different molecular-size compounds (10,11,12,13,14,15) have been utilized to measure the E.C.F. volume by the dilution principle. Walser et al (16) introduced the sulfate ion labeled with 35S as a tracer which rapidly equilibrates with a volume of fluid, designated as a sulfate water space, that seems to approximate the accessible portion of the E.C.F. volume