Encyclopedia of Earth   Content Cover Image  Bronchial anatomy
detail of alveoli and lung circulation. Source: Patrick J.Lynch
Respiration is the gas exchange effected by living organisms for
the purpose of sustaining vital metabolic processes. In the case
of most animals, oxygen is taken into the organism, and carbon
dioxide is expelled. In the case of plants, the inverse process
occurs of consuming carbon dioxide and expelling oxygen as a
waste gas.  Respiration may also be viewed at a cellular level,
examining gas exchange at the cell wall; for very simple
organisms, such as unicellular lifeforms, the process of gas
exchange with the environment is simplified, so that cellular
wall gas exchange is the totality of respiration for such an
organism.  In the case of some bacteria and archaea, respiration
sometimes occurs without any oxygen, and alternative molecular
gases such as hydrogen sulfide or methane may participate in
respiration and subsequent cellular metabolic reactions. Often
such organisms are cl