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Measurements indicate that the ECF can be modelled as consisting of: What is measured is not the ‘true’ ECF so it is conventional to refer to the compartment measured not as ECF but as a spaceĭefined by the tracer used and the equilibration time (eg ‘20 hour bromide space’). They do not enter cells but the lack of fullĮCF distribution results in a low estimate of ECF. The crystalloids are larger and less diffusable throughout the ECF. The ionic tracers are small and distribute throughout the ECF but there is some entry into cells.
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Throughout all compartments occurs during a 3 to 4 hour equilibration period. It is a weak beta emitter making it easy to measure in a liquid scintillation counter. This is estimated by measuring the volume of distribution of isotopes of water. The volume of distribution can be determined by extrapolation back to zero time.
Measuring body fluid compartments series#
The tracer is metabolised, a series of measurements can be made and assuming exponential decline (first order kinetics), If the tracer is excreted in the urine, then the loss can be determined and corrections made in the calculation. not interfere with body fluid distribution.not be excreted (or excretion is able to be corrected for) during the equilibration period.be rapidly and evenly distribute throughout the nominated compartment not enter any other compartment.