In addition to the information above :
Most of our bodies will consume at least a liter of water daily to meet the demands of life, but some people's demands can be as high as eight liters. That is at least four eight-ounce glasses for us normal people, but our demand changes on a daily basis. Every day the intake needed for basic survival may vary depending on diet, perspiration, ambient humidity levels due to respiration, and an amazing two-thirds of total body water output is accounted for in urination alone.
Thirst is the primary signal of dehydration, this is the body’s way of saying you're starting to run low on water. The condition of your skin is a good way to determine when dehydration has progressed. Skin may feel dry, itchy, and non-elastic; when pinched skin will not have normal bounce back. The face may become red and flushed, white spit can collect in the corners of the lips and the mouth will be dry. Lips may be pale in color, or even, dried out and cracked. Somebody with any of these signs needs to be given a glass of water, they will have an insatiable thirst to be quenched. For some, this might also cure that headache that never seems to go away.*
Loss of appetite and fatigue are a couple more early signs of dehydration. Water is needed in digestion or the process will slow down to a dead stop without it. Water is also absorbed to soften the fiber in a person's stool. Regular bowel movements are a sign that your body is receiving adequate amounts of water. Water helps in the absorption of nutrients during digestion, and the prevention of nutrient loss during urination. A good rule - clear urine means the body is receiving adequate amounts of water, dark odorous urine is bad. Dehydration can lead to the loss of electrolytes, which can be hard to replenish with water alone.
Becoming severely dehydrated, the levels of sodium and other nutrients will drop along with the water. Sport drinks and such will have sodium, potassium, and sugar. These are key in helping your cells absorb water again through osmosis.
Severe dehydration is different with each individual person and situation, but the most common symptoms are: weakness, muscle cramps or spasms (the shakes), lightheadedness, vision blackouts or visual snow, confusion or delirium and fainting due to Orthostatic hypotension, or a sudden drop in blood pressure. This occurs when about 10% of the bodies water is lost and the plasma levels in your blood are lowered and thus the pressure. Prolonged dehydration can lead to death from kidney or heart failure, but there are plenty of signs warning of possible dehydration before becoming dehydrated to such a degree.
It's best to drink water prior to dehydration because a ridiculous percentage of our bodies are Water! By drinking the right amounts of water you might just live a longer life, and feel better while doing so. It is a vital substance for your body to stay alive, so, drink up.
* Headaches are the sign of hypoanemia and hyperanemia (decreased or elevated sodium levels) as well as dehydration.
Sources :
http://www.webmd.com/diet/news/20040227/how-much-water-do-you-need
http://www.medicinenet.com/dehydration/article.htm
http://heartspring.net/symptoms_of_dehydration.html
http://en.wikipedia.org/wiki/Dehydration
Pages viewed 3/21/08 - 4/7/08