Water is the most abundant constituent of the body, accounting for 50%–60% of its mass. It is an inorganic (carbonless) substance composed of two hydrogen atoms (H2) bonded to one oxygen atom (O). Water is intricately involved in numerous functions of the body (see Figure 1), including the transport of oxygen, nutrients and waste products into and out of the cells. Drinking water contains several electrolytes (substances in solution that conduct an electric current), including calcium, chloride, fluoride, magnesium, potassium and sodium. Water is necessary for all digestion and absorption functions, and it lubricates mucous membranes in the gastrointestinal and respiratory tracts.
Even though it contains no calories, water is the medium for most chemical reactions in the body, especially those metabolic reactions involved in energy production. The body uses water as a coolant, helping to regulate body temperature during exercise, when fever is present and in hot environments. Water also serves as a cushioning component between joints, in the spinal cord and in the brain.