Water is one of the fundamental ingredients for life on Earth and our bodies are mainly composed of it. When we are forced to go without water, things can turn nasty very fast indeed. On average, water makes up about 60-70% of the human body, largely depending on your age. Our bodies lose water through our urine, sweat, feces and breath, so we have to continually replace this by drinking and eating. If we don't do this, our bodies can become dehydrated. When exercising in a hot environment, the human body can lose between 1.5-3 liters of water every hour due to sweat. Another 200-1,500 ml can be lost as moisture in exhaled breath, depending on the humidity of the surrounding air. As we lose more water, our ability to cool down through sweating also decreases, making overheating more of a risk. With no water in a climate of 50 degrees Celsius (122℉), coupled with extreme exercise, dehydration can quickly become lethal. The first stage of dehydration is thirst when we lose body fluid equivalent to just 2% of our body weight. "When thirst kicks in, your body clings to all remaining moisture," says Dileep Lobo, professor of gastrointestinal surgery, who researches fluids and electrolyte balance. "Your kidneys send less water to your bladder, darkening your urine. As you sweat less, your body temperature rises. Your blood becomes thicker and sluggish. To maintain oxygen levels, your heart rate increases." The rate at which dehydration occurs varies. At the second stage, when we lose water 4% of our body weight, blood pressure drops and fainting can occur. With more water leaving our bodies than coming in, our blood starts to thicken and become more concentrated, meaning our cardiovascular system has to work harder to keep our blood pressure up. Our kidneys try to compensate by retaining more water through reduced urination, and water rushes out of our cells into our blood stream, causing them to shrink in size. The third stage, when 7% of body weight is lost, is organ damage. "Your body is having trouble maintaining blood pressure," says Lobo. "To survive, it slows blood flow to non-vital organs, such as your kidneys and gut, causing damage. Without your kidneys filtering your blood, cellular waste quickly builds up. You're literally dying for a glass of water."