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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. 
　　全文翻譯：鹽與新
　　陳代謝鹽為何成為人類新陳代謝的關(guān)鍵是一個(gè)謎；一個(gè)富有吸引力的理論認(rèn)為我們對(duì)鹽的依 賴可從寒武紀(jì)海洋的化學(xué)變化中得到線索。五億年前，就是在那里，微小的后生動(dòng)物首先 進(jìn)化成與外隔絕的循環(huán)液體系統(tǒng)。因此，早期海洋里的水可能是所有動(dòng)物體內(nèi)液體的化學(xué)原型--一個(gè)無(wú)論外界環(huán)境如何改變，其細(xì)胞活動(dòng)仍將繼續(xù)的環(huán)境。這個(gè)設(shè)想是建立這一事 實(shí)基礎(chǔ)上：即使是在今天，物種迥異的眾多動(dòng)物血清非常相似。蜥蜴、鴨嘴獸、綿羊和人類，在解剖學(xué)和飲食習(xí)慣上完全不同，但細(xì)胞周圍的液體中的鹽含量卻基本上是相等的。在海洋生物向淡水區(qū)域并最終向陸地移動(dòng)的過(guò)程中，鹽始終是它們生存環(huán)境--如果不是外部環(huán) 境，其碼也是內(nèi)部環(huán)境中的關(guān)鍵成份。 最高級(jí)的哺乳類動(dòng)物為保持所需的鹽濃度而進(jìn)化出 了高效的內(nèi)分泌系統(tǒng)。 例如，人體為了保留或釋放組織液體和血漿而使用高血壓蛋白原酶、血管緊張素、醛固酮等激素。這樣在有利的環(huán)境下，液體與其中鹽濃度之間形成的動(dòng)態(tài)平衡，兩者均不致出現(xiàn)大起大落。但如果身體里喪失了的鹽分，盡管有各種的補(bǔ)償機(jī)制，后果將馬上極其危險(xiǎn)。