In healthy adults, total body water remains remarkably constant.
An increase or decrease in water intake brings about an appropriate increase or decrease in water output to maintain the balance.
Primarily water enters the body through food and directly by drinking liquid. Some water is produced in the body by metabolic processes (e.g., during the breakdown of energy nutrients such as fat and carbohydrates).
Water is lost from the body mainly through urine and by respiration (lungs) and sweating (skin). Faeces normally contain only little water (around 3-4%of total excretion).
The example below refers to an healthy adult living in a temperate climate.
In the state of water balance approximately 2,500 ml (2.5 l) per day are required to maintain body functions. The same volume (2,500 ml) is excreted through the previously mentioned routes.
The body is equipped with a number of mechanisms for regulating body water within narrow limits.
Important among these mechanisms are nerve centres in the brain, which control the sensation of thirst and water output by the kidneys.
Stimulation of the thirst centre creates the desire for water, while stimulation of other centres causes the release of antidiuretic hormone (ADH) from the pituitary gland.
Release of ADH results in the excretion of smaller volumes of highly concentrated urine by the kidneys, thereby conserving body water.
In addition, as fluid volume drops in the bloodstream, blood pressure falls. This fall initiates a sequence of events which finally trigger the adrenal glands to release the hormone aldosterone. Aldosterone signals the kidneys to retain more water.
These mechanisms cannot work by themselves to maintain water balance unless a person drinks enough water.
This is because the body must excrete a minimum of about 400 ml of water each day as urine, i.e., enough to carry away the waste products generated by the day's metabolic activities.
Therefore, under ordinary conditions, intake of water should amount to about 1.5 litres per day for healthy adults.
But as much as 10 litres per day may be needed during strenuous exercise in hot climates.