Substances Used In Measuring Extracellular Fluid Volume

Substances used in measuring Extracellular Fluid Volume - The Concept of "Fluid Space"

Substances that have been used for measuring extracellular fluid volume are:

• Radioactive sodium
• Radioactive chloride
• Radioactive bromide
• Thiosulphate ion
• Thiocyanate ion
• Inulin, and
• Sucrose

Some of these, especially sucrose and inulin, do not diffuse readily into all out-of-the way places of the extracellular fluid compartment. Therefore, the volume of extracellular fluid measured with these is likely to be lower than the actual volume of the compartment. On the other hand, other substances such as radioactive chloride, radioactive bromide, radioactive sodium and thiocyanate ion are likely to penetrate into the cells to a slight extent and therefore, are likely to measure a space somewhat excess of the extracellular fluid volume. Because no single substance can measure the exact extracellular fluid volume, one usually speaks of the 'sodium space', 'thiocyanate space', 'inulin space' etc.