Water homeostasis

Under normal circumstances, the maintenance of water balance is a question of balancing urine output against oral water intake, after allowance for the largely unregulated loss of water through other routes (respiratory, transcutaneous, and via the gastrointestinal tract). Normally, this is managed by the feedback mechanisms controlling thirst and diuresis, but in a medical context it is important to allow for other forms of administration that may not be under the control of the patient, and other routes of fluid loss, such as haemorrhage and drains. Electrolyte and water homeostasis are closely interrelated: the major trigger for both antidiuretic hormone (vasopressin) release (and hence renal water retention) and thirst is plasma osmolality. Sodium and chloride are the major solutes in extracellular fluid so are major determinants of body water content and circulating volume.