Maintain Body Water BalanceCredit:
Water intake must equal water output for the body to be properly hydrated. Every person’s water intake is different, being influence by diet, lifestyle and habit but to give a median number, an adult needs to consume roughly 2500 ml of water a day. We all have an idea that most of the water we need comes from the fluids we drink and the foods we eat. Did you know that cells also produce water? The water that is produced by cells is called water of oxidation or metabolic water.
Water Intake and Output
Balancing Body Water Intake and OutputCredit: the needed 2500 ml per day, about 60% or 1500 ml comes from ingested liquids. Foods that we eat, when digested and absorbed gives about 750 ml of water or 30% and the remaining 10% or 250 ml is produced by the cell or the ones we call metabolic water.
Most of our water output is through urination, which constitutes to 60% of the average daily output. 28% is loss is by the skin and lungs, known as the insensible fluid loss. Sweat and feces combine for 12% of the total fluid loss per day. 
Regulation of Water Intake
Metabolic Water Intake RegulationCredit: body is very sensitive when it comes to fluid loss. Even a mere 2-3% of increase in the osmolality of the plasma excites the hypothalamus. The hypothalamus is the thirst center of the body and when it is excited, it turns on the thirst mechanism which tells the body to consume more water. What an increase in the osmolality of the plasma is it raises the plasma oncotic pressure, driving the fluids to leave the bloodstream. A simple manifestation of this is the common dry mouth when we are thirsty. 
Another mechanism employed by the body, although less potent is triggered by the decline in the blood volume or a decline in blood pressure. This mechanism is less potent since it needs a substantial decrease of about 10-15%. 
Maintaining ECF osmolalityCredit: interesting thing to note is that out thirst is quenched almost as soon as we being to drink water, even though water that we just drank has not reached the blood yet. This is explained because stretch receptors in the intestine and the stomach are activated, which provides feedback to the hypothalamus and then inhibits the thirst mechanism. This is a vital action because it prevents us from consuming too much water, overdiluting our body fluids. This premature quenching of thirst also allows time for osmotic changes to come to play as regulatory factors.
Regulation of Water OutputWater and Electrolyte Balance in the BodyCredit:
Obligatory water loss helps to explain why we are not able to survive for long without drinking water. The output of water is unavoidable; hence it is needed that we replace the water we lost. Even a herculean task by the kidneys will be futile if we have zero fluid intake. Included in this obligatory water loss are the fluids we loss during defecation, urination and the insensible water losses from the skin and lungs.  In a day where you perspire a lot, you may notice a decrease in the amount of your urine. This is done by the body to ensure balance in water loss. On normal occasions, the kidney begins to eliminate excess water in around 30 minutes, giving time to inhibit the release of ADH.