Primary dehydration-

a) Simple deprivation of water from desrt travel, extreme weakness and mental patients refusing to drink causes dehydration. It occurs more quickly in fever or in high temperature of the environment.
b) Excessive water loss due to vomiting, prolonged diarrhea, excretion of large quantity of urine or sweat.
In water depletion, the concentration of extracellular fluid increases. Water is drawn from the cells and both the extracellular and intracellular compartment shrinks. Extreme thirst result, the individual complains of hot and dry body, dry tongue.