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In biology, the word respiration is used in two slightly different ways. Cellular respiration, which takes place in mitochondria, is the release of energy from the breakdown of food molecules in the presence of oxygen. Without oxygen, cells lose much of their ability to produce ATP. Without ATP, cells cannot synthesize new molecules, pump ions, or carry nerve impulses.
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The blood carries oxygen from the lungs to the body's tissues, and carries carbon dioxide—a waste product of cellular respiration—in the opposite direction. At the level of the organism, respiration means the process of gas exchange—the release of carbon dioxide and the uptake of oxygen between the lungs and the environment.
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