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Cells cannot remain alive outside certain limits of temperature and much narrower limits mark the boundaries of effective functioning. Enzyme1 systems of mammals and birds are most efficient only within a narrow range around 37C; a departure of a few degrees from this value seriously impairs2 their functioning. Even though cells can survive wider fluctuations3 the integrated actions of bodily systems are impaired4. Other animals have a wider tolerance5 for changes of bodily temperature.
For centuries it has been recognized that mammals and birds differ from other animals in the way they regulate body temperature. Ways of characterizing the difference have become more accurate and meaningful over time, but popular terminology6 still reflects the old division into "warm-blooded" and "cold-blooded" species; warm-blooded included mammals and birds whereas all other creatures were considered cold-blooded. As more species were studied, it became evident that this classification was inadequate7. A fence lizard8 or a desert iguana-each cold-blooded----usually has a body temperature only a degree or two below that of humans and so is not cold. Therefore the next distinction was made between animals that maintain a constant body temperature, called home0therms, and those whose body temperature varies with their environments, called poikilotherms. But this classification also proved inadequate, because among mammals there are many that vary their body temperatures during hibernation9. Furthermore, many invertebrates10 that live in the depths of the ocean never experience change in the depths of the ocean never experience change in the chill of the deep water, and their body temperatures remain constant.
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1 enzyme | |
n.酵素,酶 | |
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2 impairs | |
v.损害,削弱( impair的第三人称单数 ) | |
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3 fluctuations | |
波动,涨落,起伏( fluctuation的名词复数 ) | |
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4 impaired | |
adj.受损的;出毛病的;有(身体或智力)缺陷的v.损害,削弱( impair的过去式和过去分词 ) | |
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5 tolerance | |
n.宽容;容忍,忍受;耐药力;公差 | |
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6 terminology | |
n.术语;专有名词 | |
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7 inadequate | |
adj.(for,to)不充足的,不适当的 | |
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8 lizard | |
n.蜥蜴,壁虎 | |
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9 hibernation | |
n.冬眠 | |
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10 invertebrates | |
n.无脊椎动物( invertebrate的名词复数 ) | |
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