大众英语80篇背诵阅读带翻译(三十二)
"64 Salt and Metabolism
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.盐与推陈出新盐为何成为人类推陈出新的关键是一个谜；一个富有吸引力的理想以为我们对盐的拜托可从冷武纪陆地的化学转变中获得线索。五亿年前，便是在那边，巨大的后活泼物首先退化成与外隔尽的轮回液体系统。因此，早期陆地里的水年夜概是全部动物体内液体的化学原型--一个无论外界情况若何窜改，其细胞活动仍将继承的情况。这个设想是建立这一毕竟基础上：即便是在本日，物种悬殊的众多动物血清极度类似。蜥蜴、鸭嘴兽、绵羊和人类，在解剖学和饮食习尚上完全不合，但细胞附近的液体中的盐含量却根基上是相称的。在陆地生物向海水区域并最终向陆地挪动的历程中，盐一直是它们糊口生活情况--如果不是内部情况，其码也是内部情况中的关键成份。最后级的哺乳类动物为连结所需的盐浓度而退化出了高效的内排泄系统。例如，人体为了留存或释放构造液体和血浆而利用高血压卵白原酶、血管重要素、醛固酮等激素。如许在有利的情况下，液体与其中盐浓度之间组成的静态平衡，两者均不致出现年夜起年夜落。但如果身段里丧失落了的盐分，虽然有各种的补偿机制，结果将马上极度危险。