Humans can live for weeks and sometimes months without food. Going without water, however, is a different story…
Two-thirds of the human body (by weight) consists of water. Humans need water for circulation, respiration, and converting food to energy. After oxygen, water is the body's most important nutrient. Quite simply, you need water to live.
Your body loses water constantly through sweat, urine, and even breathing. You must replace the water your body loses for your organs to continue to work properly. Dehydration occurs when your body doesn't have enough water, because you're losing more water than you're taking in.
In extreme heat, an adult can lose almost half-a-gallon of water through sweat alone. Without water, dehydration can set in within an hour in severe heat. The combination of physical overexertion and extreme heat — without water — can lead to death in as little as several hours.
Surprisingly, it's also easy to become dehydrated in very cold environments. Since cold air cannot hold much moisture, it dehydrates your body with every breath you take. Even if you aren't sweating, you still need to replenish fluids even in cold weather.
So how long can you survive without water? Humans in average shape and perfect conditions (not too hot or cold) can probably live for three to five days without any water if they're not physically exerting themselves. Healthier people can live a day or so longer, while those who are unhealthy or exposed to particularly hot or cold weather may not survive as long.
To stay healthy, you need to continually replenish your fluid supply. Experts recommend drinking approximately two quarts (64 ounces or about eight glasses) of water each day. Of course, if you live in an extremely hot or cold area — or if you exercise a lot — you may need to drink a gallon or more of water every day.
If you love animals, you may already know that camels are famous for being able to go long periods of time — in the desert heat — without drinking. Camels in the Sahara Desert have been known to survive six months or more without drinking during the winter. In the extreme heat of a desert summer, they can still go five to ten days without drinking.
So how do they do it? Some people believe it's because they store water in the humps on their back, but that's not true. Their humps are filled with fat, which allows them to go a long time without food.
Camels can go a long time without drinking because their internal body temperature can vary much more than a human's, which allows them to withstand high temperatures without needing to sweat to cool their bodies. Camels also have specially-shaped elliptical blood cells, which promote blood flow and don't allow blood volume to drop when fluid levels drop. These factors allow them to survive in harsh desert climates without water for days at a time.