Homeostasis is the process by which the body regulates vital functions such as temperature, energy levels, pH and fluid balance in order to maintain its internal equilibrium. Fluid balance is maintained through the process known as water homeostasis. Water homeostasis is largely carried out by the kidneys. The role of the kidneys is complemented by a degree of regulation coming from the adrenal glands, and overall control of fluid balance is maintained by the brain. As part of water homeostasis, the concentrations of various important chemicals called electrolytes, which are dissolved in water, are also regulated.
The body's internal environment contains water in two different compartments. Most of the water in the body, around two-thirds, is located inside cells. The remaining third exists outside cells in the circulation. Important electrolytes such as sodium and potassium are concentrated in body fluids and, if levels of these electrolytes fall or rise too much, this can affect the body's ability to function normally. As well as the more complex systems of regulation going on in organs such as the kidneys, the body uses more obvious mechanisms such as feeling thirsty or having an urge to eat salty foods, which contribute to water homeostasis.
Water can be lost from the body by not drinking enough, sweating a lot, passing feces and urinating. Some fluid is also lost from the lungs during breathing. The kidneys are able to influence water homeostasis most of all, by controlling how much urine is produced.
When a person is dehydrated, the concentration of sodium in the circulation is relatively high and this is detected by the brain. A hormone called antidiuretic hormone is released from the pituitary gland in the brain. It travels in the blood to the kidneys and they respond by reducing the amount of water that leaves the body in urine. This results in there being more water in the body and in the circulation, and sodium levels return to normal.
If there is too much water in the body, special receptors in the heart are stretched and activated, and this stops the production of antidiuretic hormone. Larger than usual amounts of water in the body mean that the concentration of sodium in the blood falls. The brain detects this and a hormone called aldosterone is produced by the adrenal glands. In response to aldosterone, the kidneys reduce the amount of sodium that leaves the body in urine, increasing sodium levels in the blood once more.