ABSTRACT
Water is the most critical nutrient for the survival and well being of a person. One can survive without the intake of other nutrients for days, weeks, and even months but one cannot survive without water for more than a few days. In a 70 Kg (154 lb) person, the water content is about 40 liters (10.56 gallons) (i.e., 60% of body weight). Most of the water (25 liters) is inside cells; nevertheless, about 15 liters reside outside of cells. The blood volume is about 5 liters (1.32 gallons) and the maintenance of this volume is critical to a person's survival, despite the fact that daily fluid intake can vary from 1-8 liters. Excess fluid intake can easily be regulated; however, a problem arises when fluid intake is below one liter per day and blood volume starts to decrease below 5 liters (for example, a blood volume of 4 liters or less can cause death). Under sedentary conditions the skin and the kidneys (i.e., urine output) are the most important regulators of body water. Under conditions of hot weather and exercise (despite fluid intake in many cases), the skin (because of sweating) becomes the only important regulator of body water, as well as body temperature. Loss of water in a heavy, prolonged exercise (e.g., a 3-hour marathon) can increase from 0.1 to 5 liters.
