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...work. Complex, multicellular life on Earth needs an atmosphere for either respiration or photosynthesis. Even aquatic species rely on dissolved oxygen from the air. Photosynthesis, in contrast, requires carbon dioxide, also from the atmosphere. To survive, both animals and plants need a considerably larger mass of air per day than they do food or drink, typically by about...

...workings of the human body. Diagnosis of illness is aided by the ability to obtain detailed information about the structure of a particular organ or part of the skeleton to see if it is abnormal, damaged or malfunctioning in some way. For example, a growth may prevent the passage of food and waste through the gut, or fatty deposits may cause problems with blood...