One of questions which is raised first of all at a research of far-out planets and their satellites it is a question of existence or absence waters there. Only where there is water, there is a hope to find life.
Will not be exaggeration to tell that Earth in such look as it is, created the planet water. Liquid water occupies ¾ the surfaces of the planet, solid water (snow and ice) covers 1/5 terrestrial land, the atmosphere is saturated with water vapor. Thanks to the high thermal capacity of water, Earth does not manage neither to cool down in a night, nor "to overheat" in the afternoon, fluctuations of temperature are rather small. Such climate allowed to arise and remain lives on Earth, so, and to the person.
Water in living cells
Life arose in water. In the ancient seas the first living beings – monocelled appeared. From that water environment in which they were these cages absorbed substances necessary for them in the form of water solutions. Whatever steps evolution did since then, this principle remains: all chemical reactions in cages happen between the substances dissolved in water. It is right also for plant cells, both for animals, and for monocelled, and for those cages which make a metaphyte – including human.
Thus, water in a human body provides the metabolism forming the activity basis. But it is not the only function of water at the cellular level. In close proximity to cellular membranes it gains the stickiness comparable to ice. So water "cements" a cage and creates for it a protective barrier. A special role is played by water in nervous cages. Passing of signals between them is connected with transfer of ions of potassium and sodium through their membranes, and this transfer is provided too by water.
Extracellular water
Water in an organism contains not only in cages. It is a part of intercellular liquid, plasma (liquid part of blood) and a lymph. Intercellular liquid surrounds cages which absorb from it nutrients and allocate in it exchange products. It is possible to tell that human cages "live" in intercellular liquid as ancient monocelled lived in the primitive sea. In blood plasma water becomes peculiar "vehicle" for blood cells, proteins and other substances which are a part of plasma. Not only blood and a lymph, but also all liquids of an organism represent water solutions. For example, saliva consists of water for 99%. Water promotes removal from an organism of metabolism products, harmful to it, urine is water solution too. One more important function of water – thermal control. Evaporating water with breath and from the surface of skin in the form of sweat, the human body gives excess of heat that protects it from overheating. At such abundance of functions the amount of water in a human body has to be quite big. And this is true. The average content of water in an organism is 75%. This indicator differs depending on age, weight, a constitution, a floor. At men the percentage of water is higher, than at women; children have more, than at elderly people. Also water content in different fabrics differs. Least of all it in bones (10-12%), and most of all – in blood (up to 92%). Water content in a brain is quite high – to 85%.