Thirst

If the water volume of the body falls below a certain threshold or the osmolite concentration becomes too high, structures in the brain detect changes in blood constituents and signal thirst.

This condition is called isotonic and occurs when the same levels of solutes are present on either side of the cell membrane so that the net water movement is zero.

The loss of blood volume is detected by cells in the kidneys and triggers thirst for both water and salt via the renin-angiotensin system.

When these cells detect decreased blood flow due to the low volume they secrete an enzyme called renin.

[3] Destruction of this part of the hypothalamus in humans and other animals results in partial or total loss of desire to drink even with extremely high salt concentration in the extracellular fluids.

[4][5] In addition, there are visceral osmoreceptors which project to the area postrema and nucleus tractus solitarii in the brain.

[2] Because sodium is also lost from the plasma in hypovolemia, the body's need for salt proportionately increases in addition to thirst in such cases.

[7][8][9] In 2009, the European Food Safety Authority (EFSA) included water as a macronutrient in its dietary reference values for the first time.

[10] Recommended intake volumes in the elderly are the same as for younger adults (2.0 L/day for females and 2.5 L/day for males) as despite lower energy consumption, the water requirement of this group is increased due to a reduction in renal concentrating capacity.

[2] The median preoptic nucleus and the subfornical organ receive signals of decreased volume [clarification needed] and increased osmolite concentration.

The subfornical organ and the organum vasculosum of the lamina terminalis contribute to regulating the overall bodily fluid balance by signalling to the hypothalamus to form vasopressin, which is later released by the pituitary gland.