The human body is 65 percent water. How much water a person needs depends largely on the volume of urine and sweat lost daily. Water needs are increased if a person suffers from illness or undergoes heavy physical exercise. Water is replenished by drinking liquids, preferably those without caffeine or alcohol, both of which dehydrate the body.

Water circulates through our blood and lymphatic system, transporting oxygen and nutrients to cells and removing wastes through urine and sweat. Water also maintains the natural balance between dissolved salts and water inside and outside of cells. Our joints and soft tissues depend on the cushioning that water provides. Without water in our diets we could not digest or absorb the foods we eat or eliminate the body's digestive waste.