Circumventricular Inspiration

Thirst is a sensation aroused by a need for water, and relief from it sought by drinking water. The natural stimuli for thirst can be divided into two types: hypovolemic and osmotic (Fitzsimons, 1972). The hypovolemic stimulus occurs when there is volume loss such as from slow dehydration over a period of time, massive vasodilation or rapid haemorrhage. The second natural stimuli is an increase in plasma osmotic pressure. Loss of cell water is detected by osmoreceptors, located mainly in the hypothalamus, and their stimulation gives rise to thirst.
Hypovolemia is detected by vascular stretch receptors which are found in the walls of heart and vasculature (Gauer & Henry, 1963). During hypovolemia, angiotensin II levels in plasma increase due to low pressure in the renal artery that stimulates renin release. The brain is then informed about the changes in volume by inputs from the heart and baroreceptors located on the blood vessels relayed through the nucleus tractus solitarii in the dorsal part of the medulla (Unger, 2001). The neural inputs stimulate the release of angiotensin II in the parts of the brain that arouse thirst sensation. These areas are the circumventricular organs devoid of a BBB such as the SFO and the OVLT, including median preoptic nucleus (MnPO), which is also found within the BBB. MnPO is vital to induce thirst and if destroying
In the PVN, angiotensin II acts as a neurotransmitter is released and binds to its receptors, AT1, to stimulate the thirst and vasopressin release. Additionally, the AT1 receptor antagonist, losartan, abolished the drinking and vasopressin response to a hyperosmotic