Water composes half to four-fifths of you, depending on how much body fat you have. It makes up about 85 percent of your brain, 80 percent of your blood, and 70 percent of your lean muscle. The human body, like all living organisms, survives by means of an ongoing flow of energy. You are an engine, and water is your fuel, coolant, and lubricant. Water keeps you running," states Suzanne Winckler in the UC Davis publication Groundwater.