Re: What are the physiological effects of dehydration on the brain?

The body attempts to maintain the levels of nutrients, ions, and gases at 
constant levels appropriate for the functioning of the body (homeostasis). 
 Severe dehydration can have adverse affects on the brain such as cell 
death and seizures which can lead to increased cell death in the brain.  
This may be due to a change in the ionic composition of the fluid 
surrounding brain cells (neurons) which causes neurons to be more 'active' 
than usual and more sensitive to stimuli.

Mild dehydration can lead to headaches.  This is due to the actions of the 
autonomic nervous system.  The autonomic nervous system causes the vessels 
in the brain to become dilated to keep more blood in the brain while 
vessels in the rest of our body become constricted to keep our blood 
pressure up.  The pressure from the dilated vessels puts pressure on the 
brain (because it's encased in the skull) and we get a headache.

Luckily, there are many compensatory mechanisms in place to protect the 
brain above all other body parts.  Groups of cells in the aorta and in the 
hypothalamus work to sense the fluid volume and the osmolarity (how 
concentrated or dilute the blood is) and orchestrate hormonal signals to 
compensate for the loss of water.

You can find this information in a basic physiology book ( I used one by 
Lauralee Sherwood) and many neuroscience texts in your library.