 We know that the fluid in our body is distributed in various compartments that is inside the cells known as the intracellular fluid and outside the cells which is extracellular fluid which is further divided into various compartments inside the blood vessels and the interstitial fluid. The interstitial fluid is the surrounding the fluid surrounding the cells and that inside the blood vessels is plasma. Now the distribution of body fluid in various compartments can be measured by applying principle of conservation of mass. It is a very simple principle, so suppose I have a known amount of a substance say mass M, if I mix it in a solvent with a known volume say V1, after mixing the concentration of the substance can be determined by a formula mass upon volume 1. Now if you mix it in another container which is having another volume say D2, the new concentration will be mass upon volume 2. Now since mass remains the same, if it is not being destroyed then the mass will remain the same. In that case we can write M is equal to M, so we know that M value can be calculated like C1 V1, C1 into V1 right, so here it will be we can write as C1 V1 is equal to C2 V2. So we already told that if we know the initial volume V1 right, we can determine C1 by this formula M by V1. Now suppose we do not know how much is V2, how can we determine the volume V2 by this principle of conservation of mass? What we do is that we can take out a sample of the solution and determine that concentration of the substance in this particular solution. So initially we already know C1, we already know V1 by determining the concentration in this new solution we will know C2, thus by solving this equation we can determine this unknown volume V2. This very principle we use for determination of distribution of body fluids but which volume would have be determined? Will it be total body water? Will it be intracellular fluid? Will it be ECF or within ECF it will be plasma or interstitial fluid? Now it depends where the substance can freely go. So what we are doing we are injecting the substance, the known concentration of a substance in a known volume into a vein by which it is going into the circulation right. If a substance can freely cross capillary membrane and also freely enter into the cells. That means after some time it would have been present in all body fluid compartments. That means such a substance will determine total body water. One such substance is radioactive water or heavy water. So for determining total body water we use heavy water because when we inject it it will go into all body fluid compartments and by determining the concentration back that is the C2 we will be able to determine the volume, new volume. Now if a substance can cross capillary membrane but cannot enter into the cells so it will remain in two compartments see in the blood vessel and in the interstitial fluid. So in this case we will be able to determine the extracellular fluid compartment. What substances are ions? So radioactive sodium ions chloride ions can be used to determine ECF compartment. Now if a substance remains only in blood vessel and cannot go here neither enter into the cells that means it is remaining only into the plasma. So after determining its concentration we will be able to determine the plasma volume. Radiolabelled albumin is used for this purpose because this protein is not able to cross the capillary membrane and enter into any other compartment. In summary we use principle of conservation of mass where M1 remains equal to M2 and if we know the known concentration of a substance in a known volume and we can determine the new concentration when the substance gets distributed in a new volume we will be able to determine the new volume and for this purpose substances used are based on which compartment they are able to go. So we saw that we can determine total body water by heavy water, heavy water is nothing but radiolabelled water. Then extracellular fluid compartment can be determined by radiolabelled ions and the plasma volume can be determined by radiolabelled albumin. How much is intracellular fluid that we have not determined? It is simple we can calculate intracellular fluid by subtracting extracellular fluid amount from total body water and similarly the amount of interstitial fluid we can determine by subtracting plasma from extracellular fluid volume. So this is the way how we determine the various body fluid compartments.