How is water lost from the human body?

The most water is lost through exhalation (breathing), sweating and evaporation, and excretion (in urine).
You can lose water from your body through sweating

Another way to loose liquid is evaporation through breathing, urinating, bleeding, and vomiting. These factors greatly influence emergency survival in arid areas and ones survival strategies should focus on preventing these.

Eating food when water is scarce also draw water out of the body reserves to digest the food increasing dehydration. Drinking salt water also drains water from the body to process the salt and in a dehydration situation it can be potentially fatal to drink salt water..

When you reach the point where you stop sweating the body have no way of regulating body temperature and heatstroke is imminent.