In a short search I found this:
"Water, as a universal solvent, supplies nutrients and hormones to the body, regulates body temperature, and lubricates our eyes and joints," explains Mitchell Moffit and Greggory Brown of AsapScience.
And as educator Mia Nacamulli explains in a new animation for TED-Ed, when your body becomes dehydrated, the sensory receptors (nerve endings) of the brain's hypothalamus — found in what scientists call the "thirst center" (OCPTL) - send signals for the release of an antidiuretic hormone.
This hormone reaches the kidneys and causes aquaporins, proteins in cell membranes that can carry water molecules, allowing the blood to hold more water in your body.
And, when this happens, the urine becomes darker and has a stronger odor.
We will also feel, during this dehydration process, that we have less desire to urinate and that we secrete less saliva.
And we are likely to feel dizzy because, in the meantime, our brain is trying to adjust to the lack of its essential fluid.
Adaptation mechanisms
A dehydrated brain shrinks due to lack of water, and it must work harder to achieve the same thing as a brain that has enough water.
This process, however, can only last a few days: if you stop drinking water completely, your body will begin to experience more serious effects and, in the end, it will completely stop working.
Stopping drinking water for days (chronic dehydration) can lead to other disorders, such as diabetes, high cholesterol, skin and digestive problems, fatigue, and constipation.
The survival time varies, depending on each person, between three and five days, although cases of people who managed to survive longer have been reported.