A comparison of sodium, chloride, thiocyanate, and sucrose spaces as estimates of extracellular fluid volume.

Sodium, chloride, thiocyanate, and sucrose spaces were measured in whole body and different tissues of rat. For each analytical method, when all tissue spaces were added the sum was in good agreement with corresponding total body fluid space obtained by dilution methods. There was surprisingly good agreement among the three ionic space measures in the total body, when one considers that there were discrepancies as large as 2- to 8-fold within certain of the different tissues. Some differences could be explained by the facts that sodium is a constituent of bond mineral and that there are marked differences between these four measures when observed in the water of the gastrointestinal contents.