What mechanisms does the body use to maintain fluid balance?

The body gains water almost entirely through food and
drink. Water is lost primarily by urination; perspiration, respiration, and defecation
also remove lesser amounts of fluid from the body. Maintaining proper hydration is
important to keep blood volume constant and to allow for the creation of urine to remove
nitrogenous wastes.

When fluid levels drop, sensors in the
ventricles of the brain cause the pituitary to secrete vasopressin (also known as ADH,
or anti-diuretic hormone), which slows the kidneys and causes them to make more
concentrated urine to conserve water.

Aldosterone, a
hormone secreted by the adrenal glands, has a similar function, but is secreted in
response to blood pressure in the chest and abdominal cavity. Under normal conditions it
functions in parallel with the brain sensors, but has a much faster response time to
large changes, so it is also important during sudden events like bleeding or large
environmental temperature fluctuations.

The major mechanism
for adding fluid to the body is thirst. The exact physiology of thirst is not well
understood, but it's generally agreed that the same brain sensors that cause te
secretion of vasopressin are responsible for creating the sensation of thirst when fluid
volumes drop.