water

: Normal regulation of body fluid volume also depends up on a balance between water loss and water intake.
If increased drinking is not compensated by increased urine loss, body water must increase, end result over hydration of the cells.

On the other hand, if increased drinking does not compensate urine loss, body water decreases with resulting cellular dehydration.

The stimulation to drink is generated in, what is called the primary thirst centre of the brain, and the basic stimulation for the primary thirst centre is intracellular dehydration.

Stimulation of the primary thirst centre may also be triggered by volume and pressure receptors located in some of the larger blood vessels.
A reduction of 8% in blood volume or pressure can induce thirst and stimulate the release of an anti diuretic hormone (ADH).

While a 2% change in the extra cellular fluid volume will also cause ADH release and cause the kidneys to re-absorb water and concentrate the urine.

In addition to water re-absorption, ADH also increases the re-absorption of Urea.
This is important, as Urea influences the ability of the kidneys to re-absorb water.

However, stimulation of the primary thirst centre is not the only mechanism that determines water intake in normal animals. Food intake as well as exercise also triggers thirst, in anticipation of possible water needs, before any actual cellular deficiencies can occur.

Sodium balance is closely regulated and maintained within narrow limits regardless of large variations in the dietary intake of sodium. Although sodium is excreted from both the gastrointestinal tract and the kidneys, it is the kidneys that primarily regulate sodium balance.

Several factors influence this function; this includes a mineralocorticoid called Aldosterone secreted by the Adrenal Cortex, the volume of blood flow through the kidneys, and the availability of ADH and Urea.

In a normal healthy dog nearly 75% of the fluid that passes through the kidneys is re-absorbed.
Essential to this function are Aldosterone, ADH, Urea, and Sodium.

Anything that affects the available levels of these substances will affect the ability of the kidneys to function normally.

So called acid neutralizers, alkalising diuretics, some infections (viral and bacterial), toxic substances and kidney disease, all reduce the ability of the kidneys to re-absorb water, and causing various levels of dehydration and loss of essential substances, such as potassium.