Respiratory System

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 term respiration 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 synthesize sugars and other components. Animals consume plants or other organic material to obtain chemical compounds, which are then oxidized to sustain vital processes.

This article considers the gaseous components of air and water, the natural respiratory habitats of animals, and the basic types of respiratory structures that facilitate gas exchange in these environments.

Although the acquisition of oxygen and the elimination of carbon dioxide are essential requirements for all animals, the rate and amount of gaseous exchange vary according to the kind of animal and its state of activity. In the Table the oxygen consumption of various animals is expressed in terms of millilitres of oxygen per kilogram of body weight per hour, reflecting the gas demands of different species at rest and in motion. A change in the chemical composition of the body fluids elicits a response from the central nervous system, which then excites or depresses the machinery of external respiration.