Correlation between body fluid volumes and body weight in the rat

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Abstract
Red cell, plasma and extracellular fluid volumes were measured in 141 rats ranging in weight from 100 to 300 g. Volumes were measured as the volume of distribution of radioactive indicators (Cr51-tagged red cells, radio-iodinated serum albumin and Na22Cl) utilizing multiple sampling and extrapolation to zero time to avoid errors due to loss of tracer. Results expressed as milligrams per 100 g body wt, were correlated with body weight and expressed in milliliters per 100 cm2 with body surface; significant correlations were found only between volumes (in ml/100 g) and body weight. A negative correlation was observed for all measured spaces; the differences in volumes between a rat weighing 100 g and a rat weighing 300 g were, on the average: for red cell volume, 1.34 ml/ 100 g; for plasma volume, 1.12 ml/100 g; and for extracellular water, 15.44 ml/100 g. It was also observed that corporeal hematocrit decreased with increased weight, as can be determined from the different slopes of red cell and plasma volumes. A similar observation was made for interstitial fluid volume, determined from the different slope of plasma and extracellular volumes.

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