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Diffusion and respiration Typeit
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Respiration is an exothermic reaction used to transfer energy in organisms. It can occur as aerobic respiration or as respiration, depending on the availability of oxygen. When there is insufficient oxygen, muscles produce , although this can be converted into carbon dioxide and when oxygen levels increase, so it can eventually be converted back to , the preferred energy source for most organisms.
Gas exchange in the lungs takes place in small sacs called . These are well-adapted for gas exchange because their walls are only one thick, they have a high surface-area-to- ratio, and there is a steep gradient of oxygen and carbon dioxide between them and the capillaries.