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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鍵是一個謎；一個富有吸引力的理論認為我們對鹽的依 賴可從寒武紀海洋的化學變化中得到線索。五億年前，就是在那里，微小的后生動物首先 進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物體內液體的化學原型--一個無論外界環(huán)境如何改變，其細胞活動仍將繼續(xù)的環(huán)境。這個設想是建立這一事 實基礎上：即使是在今天，物種迥異的眾多動物血清非常相似。蜥蜴、鴨嘴獸、綿羊和人類，在解剖學和飲食習慣上完全不同，但細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)域并比較終向陸地移動的過程中，鹽始終是它們生存環(huán)境--如果不是外部環(huán) 境，其碼也是內部環(huán)境中的關鍵成份。 比較高級的哺乳類動物為保持所需的鹽濃度而進化出 了高效的內分泌系統(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，盡管有各種的補償機制，后果將馬上極其危險。