A flow-limited simulation model of isotopic water dilution.

A model of the kinetics of water metabolism is not synonymous with an anatomic model of body water pools. Quantitating the anatomic relations of pools of body water that are separable on a kinetic basis is very difficult, and these relations are influenced by the physiological state of the animal. A fifteen-pool, flow-limited simulation model of body water distribution and mixing was constructed based on literature values to study the relationship between anatomical and kinetic pools of body water. The blood dilution curve predicted by the model agrees well with dilution curves in the literature. Altering rumen water volume in the model does not alter the blood dilution curve. Segmenting anatomical pools of water on the basis of blood deuterium oxide (D2O) dilution curves is not a realistic goal. Total body water, independent of the physiological state of the animal, can be measured by D2O dilution after isotopic equilibration has been established if corrections are made for the amount of D2O that is lost prior to attaining equilibrium. The model can be used to search for dosing and sampling schemes that have the potential for fractionating anatomical pools of body water. Although the model is not predictive of any particular situation, it does provide a good base for researching kinetics of body water distribution and body composition.