Water is of major importance to all living things; in some organisms, up to 90% of their body weight comes from water. Up to 60% of the human adult body is water (source: water.usgs.gov).

According to H.H. Mitchell, Journal of Biological Chemistry 158, the brain and heart are composed of 73% water, and the lungs are about 83% water. The skin contains 64% water, muscles and kidneys are 79%, and even the bones are watery: 31%.

According to the Institute of Medicine (IOM), an adequate intake for men is approximately 3 liters a day and for women around 2.2  liters. 

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. ( Source: science.howstuffworks.com).

                (source: water.usgs.gov).

Essential functions of water in our body:

• A vital nutrient to the life of every cell, acts first as a building material.
• It regulates our internal body temperature by sweating and respiration
• The carbohydrates and proteins that our bodies use as food are metabolized and transported by water in the bloodstream
• It assists in flushing waste mainly through urination
• acts as a shock absorber for brain, spinal cord, and fetus
• forms saliva
• lubricates joints

Some common beliefs of drinking Water

1. Drinking water first thing in the morning helps regulate stomach acid.
2. Water helps to boost energy and also prevents the onset of many diseases. It flushes out toxins and improves blood circulation. 
3. Water is extremely important for a healthy heart. Dehydration can lead to elevated cholesterol and blood pressure as the blood becomes thicker and the flow is affected when the body is dehydrated. Enough amount of water will help your heart pump your blood more effectively.
4. Water helps fuel your muscles, so drinking before or during exercise will boost energy, as well as reduce the likelihood of cramps and sprains.
5. Research has shown that drinking half a litre of water within 30-40 minutes can increase how fast your body burns calories by up to 30%.
6. Drinking water helps to fight against colds and flu and prevents salts accumulating in the kidneys which form kidney stones.
7. Water helps with weight loss
8. Water is a natural wrinkle-buster
9. Drinking water can help fight infections all over your body, not only because it flushes out toxins but because when you're dehydrated you're more likely to catch a bug. 

Source: water.usgs.gov, science.howstuffworks.com, cosmopolitan.co.uk, goodtoknow.co.uk, in.style.yahoo.com