Define the Measurement of Extracellular Fluid Volume 

This method involves the use of dilution principle. A substance that can readily diffuse through the entire extracellular fluid chamber, which passes easily through the capillary membranes but as little as possible, passes through the cell membranes into the cells, is injected. After half an hour or more of mixing, a sample of extracellular fluid is obtained by removing blood and separating the plasma from the cells by centrifugation. Plasma, which is actually a part of extracellular fluid, is then analyzed for injected substance.