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Respiration has two common meanings in biology.
Respiration uses oxygen and fuel (food) to produce energy for cells. The products of respiration are carbon dioxide and water. A demonstration that carbon dioxide is a product of respiration is often shown in schools. See School science experiment- Huff & Puff Apparatus for details.
Respiration in animals is divided into:
Respiration can be measured using a device called a respirometer.
Plant respiration is limited by the process of diffusion. Even a baobab[?] tree is mostly dead because air can penetrate only skin deep. However, most plants are not involved in highly metabolic activities like hunting, i.e. they do not need the energy necessary for predators[?], and thus their breathing is limited.
Insects use a system of tracheae[?], thin channels, through their exoskeleton, to improve on simple diffusion and let air flow more freely throughout the organism. However this simple system limits their size. No modern insect exists that is larger then a foot or so (in metric units[?], about half a meter). Many people fear big bugs, and they should be comforted by this fact, which has to do with respiration. A bug, however annoying, cannot be large, although it can be long. Nevertheless a large number of insects like locusts can do a lot of damage.
In tissue engineering[?], respiration is an essential problem. The small depth of diffusion respiration sufficient to support the metabolism of an average human cell is less than a milimetre in metric units[?], or less than a quarter of a quarter of an inch in Imperial units. Various substances can be used to enhance this depth, essentially having a haemoglobising role.