Dehydration is water deficiency in the body. In dehydration, fluids from the blood and the space between the cells (together called extracellular space) are lost first, which is followed by loss of fluid from the cells (intracellular space).hen proportionally the same amount of water and sodium is lost from the body, the sodium concentration of the extracellular fluid and hence its tonicity will not change — this is isotonic dehydration. Statistically, in most cases (~ 80%) dehydration is isotonic 2.