Your body fluid compartments are NOT Intracellular, Extracellular and Vascular. They are:
1. Intracellular - or the fluid inside the cells.
2. Extracellular - which has subsets of Intravascular and Interstitial
3. Transcelluar - which are the fluid compartments which carry very little fluid at any given time, such as Synovial fluid, Peritoneal fluid, etc.
Then the importance is to remember that normal serum osmolarity is approx. 290. Anything 50 below or above this (240-340) is considered isotonic. (> than 340 is a hypertonic ) (less than 240 is hypotonic) This means that an isotonic fluid is just like, or similar to plasma. So, a pt who has lost fluid volume, lets say from N/V may benefit greatly from an isotonic fluid into the vascular space, or blood vessel.
Interstitial fluid is fluid surrounding the cells.
Intracellular (ICF) fluid is the fluid outside the vein, and inside the cells. When you pt has cellular dehydration, a fluid, like a hypotonic solution, would be great to assist in this as fluid will leave the vascular space and go into the interstitial and the cells.
In contrast, a Hypertonic solution would be helpful to give a patient when you have someone who has just come out of surgery and has third space fluid shifts, as fluid will be mobilized when a hypertonic fluid is given and fluid will leave the cells and interstitial spaces and come into the vascular space, thereby decreasing the fluid trapped.. and then the kidneys can get rid of this fluid.
P.S. Albumin can be a 5% solution, or 25% SOLUTION. 5% is isotonic, whereas 25% is hypertonic and one must give cautiously and observe for fluid overload.