Our bodies are about 60 percent water. Water regulates our body temperature, moves nutrients through our cells, keeps our mucous membranes moist and flushes waste from our bodies. Our lungs are 90 percent water, our brains are 70 percent water and our blood is more than 80 percent water. Simply put, we can't function without it. Most people sweat out about two cups of water per day (0.5 liters). Each day, we also lose a little more than a cup of water (237 ml) when we exhale it, and we eliminate about six cups (1.4 l) of it. We also lose electrolytes -- minerals like sodium and potassium that regulate the body's fluids. So how do we replace it?

We can get about 20 percent of the water we need through the food we eat. Some foods, like watermelon, are nearly 100 percent water. Although the amount of water that we need each day varies, it's usually about eight cups (2 l). But instead of worrying about getting in those eight cups, you should just drink when you start to feel thirsty. You can get your water by drinking other beverages -- but some beverages, like alcohol, can make you more dehydrated.

If your urine is dark yellow, you might not be drinking enough water. Of course, you need more water when you're exercising; ill with diarrhea, vomiting or fever; or in a hot environment for a long time. Most people can survive only a few days without water, although it depends on a number of factors, including their health and environment. Some have gone as long as two weeks. Followers of a Buddhist boy meditating in Nepal claim that he has gone two years without food or water, but doctors have not been able to substantiate this.

When you don't get enough water, or lose too much water, you become dehydrated. Signs of mild dehydration include dry mouth, excessive thirst, dizziness, lightheadedness and weakness. If people don't get fluids at this point, they can experience severe dehydration, which can cause convulsions, rapid breathing, a weak pulse, loose skin and sunken eyes. Ultimately, dehydration can lead to heart failure and death.

Dehydration caused by diarrhea is a major cause of death in undeveloped countries. Nearly 2 million people, mostly children, die from it each year [source: WHO]. Consuming water polluted with biological contaminants and not having access to adequate sanitary facilities can lead to diseases like malaria and cholera and parasites like cryptosporidiosis and schistosomiasis. Water can be also be contaminated with chemicals, pesticides and other naturally occurring substances.

Source: http://science.howstuffworks.com

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