Not having enough water in your system can start out as simply uncomfortable, but if it continues, it can be fatal. The first sign of dehydration is thirst: If you feel thirsty, you should drink water. Severe dehydration can make your pulse weak and cause rapid breathing, convulsions, loose skin and sunken eyes. Untreated dehydration can eventually lead to heart failure and death.
What makes us thirsty?
Not having enough water in your system can start out as simply uncomfortable, but if it continues, it can be fatal. The first sign of dehydration is thirst: If you feel thirsty, you should drink water. Severe dehydration can make your pulse weak and cause rapid breathing, convulsions, loose skin and sunken eyes. Untreated dehydration can eventually lead to heart failure and death.
http://curiosity.discovery.com/question/why-people-need-water