An approach to the measurement of body fluid compartment volumes in non-steady conditions in the rat.

A method has been developed by which body fluid volume changes can be assessed frequently after only one initial injection of the appropriate radioactive tracers. This method is based on the assumptions that, 1. following their intravenous injection, the temporal behaviour of tritiated water or radio-sodium can be adequately modelled by the kinetic behaviour of an open, interconnected two-compartment system; 2. known amounts of tracer are added to or irreversibly removed from the system only via the first compartment. In the measurement of body fluid volume changes in rats after isotonic blood volume expansion, the rapid urinary tracer excretion was treated as a series of negative tracer "injections" made instantaneously into the first tracer compartment at the mid point of each short urine collection period. The effect of these "injections" on the first compartment was regarded as diminishing with time in accordance with the steady state rate constants. The vaolues for non steady state changes in total body water volume and functional extracellular fluid volume obtained by such a mathematical treatment, agreed closely with directly measured changes where such direct comparisons could be made.