Respiratory System

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respiratory system, the system in living organisms that takes up oxygen and discharges carbon dioxide in order to satisfy energy requirements. In the living organism, energy is liberated, along with carbon dioxide, through the oxidation of molecules containing carbon. The termrespiration denotes the exchange of the respiratory gases (oxygen and carbon dioxide) between the organism and the medium in which it lives and between the cells of the body and the tissue fluid that bathes them.

With the exception of energy used by animal life in the deep ocean, all energy used by animals is ultimately derived from the energy of sunlight. The carbon dioxide in the atmosphere in conjunction with the energy of sunlight is used by plants to
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The amount of oxygen available in natural waters is also limited by the amount of dissolved salts. This factor is a determinant of oxygen availability in transitional zones between sea and fresh water. Pure water, when equilibrated with oxygen at 0° C, for example, contains about 50 millilitres of oxygen per litre; under the same conditions, a solution containing 2.9 percent of sodium chloride contains only 40 millilitres of oxygen per litre. Bodies of water may have oxygen-poor zones. Such zones are especially evident in swamps and at the lower levels of deep lakes. Many animals are excluded from such zones; others have become remarkably adapted to living in them.

The Earth’s atmosphere extends to a height of many miles. It is composed of a mixture of gases held in an envelope around the globe by gravitational attraction. The atmosphere exerts a pressure proportional to the weight of a column of air above the surface of the Earth extending to the limit of the atmosphere: atmospheric pressure at sea level is on average sufficient to support a column of mercury 760 millimetres in height (abbreviated as 760 mm Hg—the latter being the chemical symbol for mercury). Dry air is composed chiefly of nitrogenand inert gases (79.02 percent), oxygen (20.94 percent), and carbon dioxide (0.03 percent), each contributing proportionately to the total pressure. These percentages are relatively constant to
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