Thirst

The body contain four major fluid compartments, one is Intra Cellular the other three are Extra Cellular fluid compartments. The compartments which is found within the cell is called Intra Cellular. The fluid portion of the cytoplasm of the cell or the fluid contained within the cell.
The rest of fluid is known to be Extra Cellular fluid or the fluid outside the cell. It is divided into three major compartments.

I) Inter Stitial Fluid.
It means that baths. The cell filling the space between the cells of the body.
II) Blood Plasma/ Intra Vascular Fluid.
This fluid is found within the Vessels.
III) Cerebro Spinal Fluid.
It is similar to blood plasma that is found in the ventricular system of the brain and the subarchnoid space surrounding the brain and spinal cord.

The intra cellular fluid is controlled by the concentrations of Solutes in the inter stitial fluid.

Solutes

A solid compound that is desorption in liquid or solution normally inter-stitial fluid is "Isotomic" which means equal to osmotic pressure to the contents of the cell. A cell placed in isotomic solution neither loses nor gain water, due to this aspect water does not move into or outside the cell, while the inter-stitial loses water. The losing of water is known to be "hypertonic" and another hand if the inter-stitial fluid gains water is called "hypotonic".

Hypovolmia

Reduction in the volume of the intra vascular fluid. Litrarily ,earns low volume of blood. Due to this problem Heart can no longer pump the blood and Heart failure can occur. In other words there are two important characteristics of the body fluid which maintain the body system.
1) The solute concentration of intra cellular compartments.
2) Volume of blood.
Water regulates with above methods.