Commonly, the processes by which living organisms take up oxygen and eliminate carbon dioxide in order to provide energy. A more formal, comprehensive definition is the various processes associated with the biochemical transformation of the energy available in the organic substrates derived from foodstuffs to energy usable for synthetic and transport processes, external work, and, eventually, heat. This transformation, generally identified as metabolism, most commonly requires the presence of oxygen and involves the complete oxidation of organic substrates to carbon dioxide and water (aerobic respiration). If the oxidation is incomplete, resulting in organic compounds as end products, oxygen is typically not involved, and the process is then identified as anaerobic respiration. The oxygen required in aerobic respiration is ultimately derived from the atmosphere or from oxygen dissolved in water. The process of the exchange and transport of oxygen from an animal's environment to the intracellular sites involved in metabolism, and the reverse transport and exchange of the end product carbon dioxide, is often called respiration, although it is only a part of the overall process.