Analysis of three compartments of extracellular fluid in dog

The distribution kinetics of an extracellular volume tracer 51Cr-EDTA was analysed from the results of continuous monitoring of plasma radioactivity. 51Cr-EDTA was injected intravenously into spleno-nephrectomized dogs by a single injection, and the radioactivity of circulating blood and physiological parameters were continuously monitored for 150 min. The dilution curve of radioactivity was resolved into 4 components, and expressed as c(t) = Ae-alpha t + Be-beta t + Ce-gamma t + D. The half-times of each term were 0.51, 2.41, and 19.26 min, respectively. Based on this result, 51Cr-EDTA space was divided into 3 compartments--plasma volume (V0), rapidly diffusing interstitial fluid (V1), and slowly diffusing interstitial fluid (V2). To explain the distribution of the tracer among these fluid spaces, two possible models were proposed. In the "tandem model" the volume of each compartment was calculated as 59.2, 121.7, and 101.7 ml/kg of body weight, respectively, and the mobility index of the tracer from V0 to V1 was 5.1 times as great as that from V1 to V2. In the "parallel model" the volumes were 59.2, 72.3, and 151.1 ml/kg of body weight and the mobility index of the tracer from V0 to V1 was 3.3 times as great as that from V0 to V2. The characteristics of each model were discussed in relation to the properties of the interstitial space.