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A knowledge of the distribution of different fluids given by intravenous infusion is basic to the understanding of the effects of fluid therapy. Therefore, a mathematical model was tested to analyze the volume kinetics of three types of fluids.The authors infused 25 ml/kg of Ringer acetate solution, 5 ml/kg of 6% dextran 70 in 0.9% NaCl, and 3 ml/kg of 7.5% NaCl over 30 min in 8 male volunteers aged from 25 to 36 years (mean, 31 years) and measured the changes in total hemoglobin, serum albumin, and total blood water over time. The changes were expressed as fractioned dilution and then plotted against time. The curves were fitted to a one-volume and a two-volume model, which allowed an estimation of the size of the body fluid space expanded by the fluid (V) and the elimination rate constant (kr) to be made.The changes in blood water concentration indicated a mean size of V of 5.9 l (+/- 0.8, SEM) for Ringer's solution, 2.6 (+/- 0.3) l for dextran, and 1.2 (+/- 0.1) l for hypertonic saline. The corresponding values of kr were 94 (+/- 42), 12 (+/- 6), and 30 (+/- 4) ml/min, respectively. Blood hemoglobin indicated a degree of dilution similar to that indicated by blood water. Serum albumin indicated a more pronounced dilution, which resulted in a larger expandable volume and a greater mean square error for the curvefitting. The larger volume obtained for serum albumin can probably be explained by a loss of intravascular albumin into the tissues along with the infused fluid.The distribution of intravenous fluids can be analyzed by a kinetic model adapted for fluid spaces, but slightly different results are obtained, depending on the marker used to indicate dilution of the primary fluid space. Analysis and simulation of plasma volume expansion by this model is a tool that can help the anesthetist to better plan fluid therapy.