The importance of thirst in maintenance of fluid balance

Plasma osmolality is maintained within very narrow limits by the control of water intake via thirst and water output via secretion of vasopressin. Osmoreceptors are situated in the brain, but on the blood side of the blood-brain barrier in a circumventricular organ. These regions are stimulated by an increase in plasma osmolality and form the most important input to cause thirst and drinking. Cardiopulmonary and arterial baroreceptors sensitive to blood volume and blood pressure also can be important, so hypovolaemic events such as haemorrhage can stimulate thirst. Both raised plasma osmolality and reduced blood volume contribute to thirst and vasopressin secretion following water deprivation. The importance of the nucleus medianus in the neural circuitary involved in integrating thirst should be emphasized. Mechanisms which stop drinking are different from those which initiate it, and oropharyngeal metering of the volume of fluid consumed provides the important input. There are a number of situations in humans where thirst thresholds and sensitivities are altered. The elderly have higher thirst thresholds and this can cause symptoms of dehydration. Increased drinking is seen in congestive heart failure, renal hypertension and certain cerebral lesions. Thirst thresholds are set at lower levels in pregnancy and in the luteal phase of the menstrual cycle and may contribute to fluid retention in these situations.