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公共英语三级作文:Salt and Metabolism

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盐是如何变得对我们的新陈代谢如此重要的还是个谜;一个吸引人的理论将我们对它的依赖追溯到寒武纪晚期海洋的化学家。5亿年前,正是在那里,微小的后生动物首次进化出了隔离和循环液体的系统。因此,早期海洋的水可能已经成为所有动物生命液体的化学原型——无论外部环境如何变化,细胞都可以在其中继续运作。这种推测是基于这样一个事实,即即使在今天,完全不同物种的血液血清也非常相似。蜥蜴、鸭嘴兽、绵羊和人类在解剖学和饮食习惯上几乎没有什么不同,但他们血液细胞周围液体中的盐含量却几乎相同。在早期海洋生物进入淡水并最终进入陆地的过程中,钠元素即使不是它们的外部环境,也是它们内部环境的关键组成部分。最成功的哺乳动物物种应该是那些发展出有效的荷尔蒙系统来维持所需的钠浓度的物种。例如,人体使用肾素、血管紧张素和醛固酮等激素来维持或释放组织液和血浆。结果,在有利的条件下,液体体积和钠浓度都不会有太大的波动。但是,如果身体没有盐,尽管有补偿机制,这种影响很快就会变得危险。


Just how salt became so crucial to our metabolism is a mystery; one appealing theory traces our dependence on it to the chemist ry of the late Cambrian seas. It was there, a half billion years ago, that tiny metazoan organisms first evolved systems for sequestering and circulating fluids. The water of the early oceans might thus have become the chemical prototype for the fluids of all animal life - the medium in which cellular operations could continue no matter how the external environment changed. This speculation is based on the fact that, even today, the blood serums of radically divergent species are remarkably similar. Lizards, platypuses, sheep, and humans could hardly be more different in anatomy or eating habits, yet the salt content in the fluid surrounding their blood cells is virtually identical. As early marine specics made their way to fresh water and eventually to dry land, sodium remained a key ingredient of their interior, if not their exterior, milieu. The most successful mammalian species would have been those that developed efficient hormonal systems for maintaining the needed sodium concentrations. The human body, for example, uses the hormones renin, angiotensin, and aldosterone to retain or release tissue fluids and blood plasma. The result, under favorable conditions, is a dynamic equilibrium in which neither fluid volume nor sodium concentration fluctuates too dramatically. But if the body is deprived of salt, the effects soon become dangerous, despite compensatory mechanisms.
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