It is the most abundant substance in our bodies. On the average, an adult's body weight is 55 to 75 percent water. That means each of us contains about 10 to 12 gallons of water. How much water comprises each of us varies from person to person. For example, the leaner a person is, the higher the proportion of water in her or his body.
Males carry a higher percentage of water than females, because males bodies are more muscle.
Muscle tissue is about 73 percent water, while body fat is about 25 percent water. Even bones are 22 percent water. Water plays a number of important roles in the body. It regulates body temperature, carries nutrients and oxygen to cells, removes waste, cushions joints and protects organs and tissues. Since the brain is 70 percent water, blood is 82 percent water, and the lungs are nearly 90 percent water, it is easy to see how even mild dehydration can cause problems.