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A simple compound that is essential for all life. Its molecular structure is H2O (2 atoms of hydrogen bonded to 1 of oxygen). Water is the most common substance in the body, accounting for about 99 per cent of all molecules, but a smaller percentage of total body weight. Approximately 2 thirds of the body’s water content is contained within the body cells, and the remaining third is extracellular (found, for example, in the blood plasma, lymph, and cerebrospinal and tissue fluid).
Water provides the medium in which all metabolic reactions take place (see metabolism), and transports substances around the body. The blood plasma carries water to all body tissues, and excess water from tissues for elimination via the liver, kidneys, lungs, and skin. The passage of water in the tissue fluid into and out of cells takes place by osmosis.
Water is taken into the body in food and drink and is lost in urine and faeces, as exhaled water vapour, and by sweating (see dehydration). The amount of water excreted in urine is regulated by the kidneys (see also ADH). Extra water is needed to excrete excess amounts of substances, such as sugar or salt, in the blood, and high water intake is essential in hot climates where a large amount of water is lost in sweat.
In some disorders, such as kidney failure or heart failure, insufficient water is excreted in the urine, resulting in oedema.