The process of Respiration.

... Respiration Respiration is the process by which human beings and other living things obtain and use oxygen. Except for certain microorganisms, all living things require oxygen to live. Respiration also involves the elimination of carbon dioxide, a gas produced when cells use oxygen. Respiration may be divided into three phases: (1) external respiration, (2) internal respiration, and (3) cellular respiration. In external respiration, or breathing, a plant or animal takes in oxygen from its environment and releases carbon dioxide. In internal respiration, oxygen is carried to the cells of the organism and carbon dioxide is carried away from them. In cellular respiration, oxygen is used in chemical reactions within the cells. These reactions release energy and produce carbon dioxide and water as waste products. Organisms carry out external respiration in various ways, depending on their size and environment. For example, single-celled organisms, such as diatoms, and amoebas, exchange oxygen and carbon

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