背景介绍

【背景】

水中的鱼要不要喝水呢？也许你会说，在水里，鱼的嘴巴常常一开一合，那当然是在喝水。其实，那是为了让水快速流过它的鳃，以便吸取水中的氧气，放出体内的二氧化碳。
事实表明，生活在淡水中的鱼，终生都不喝水；而生活在海水中的鱼，却经常需要喝水。这是为什么呢？

这里，我们不妨从渗透压谈起。渗透压是由溶解在水里的物质形成的；溶解在水中的物质越多，渗透压也就越高。鱼的血液和组织液中含有很多盐和蛋白质，溶解浓度要比周围的淡水高，渗透压也就比淡水大。由于水是从渗透压低的溶液向渗透压高的溶液渗透的，因而周围的淡水会从四面八方渗入淡水鱼的体内。结果，这些淡水鱼不但不必喝水，还得设法把多余的水份排出体外，不然它们就会被水活活涨死。相比之下，海水中的鱼就不同了。海水中的含盐量要比这些鱼体液中的含盐量高得多，海水中的渗透压也比鱼体内大得多。大海拼命地吮吸着它们体内的水份，使这些鱼不得不经常喝水。海水鱼喝水的方式是：水份先由口进入肠内，通过肠壁的渗透作用再进入体内，增加体液中的水份。
是不是所有的海水鱼都需要喝水呢？也不是。鲨鱼和鳐鱼，是用另外的方式适应环境的：它们在体液中保留了尿素。尿素是一种有害的物质，要是别的动物会很快地把它排出体外。为了保存尿素，鲨鱼和鳐鱼的鳃用一种特殊的粘膜遮盖起来，使尿素不会渗出。这样，它们体内的渗透压就要比海水的渗透压高得多，它们便能象淡水鱼摄取淡水那样，从大海中得到水。同时，它们也会把多余的水分排出体外。
无论是大多数海水鱼的喝水，还是淡水鱼和鲨鱼、鳐鱼的不喝水，都是对环境的一种适应。

英文原文

【原文】

Fish live in water, but do they drink it? The answer is yes, but how fish drink water depends on where they live.

Water gets into a fish's body through osmosis, the process in which water diffuses from a higher to a lower concentration. For example, if there is more water outside of a cell than inside, water will try to flow into the cell until there is the same concentration of water on either side of the cell's membrane. The body of a fish acts the same way, either absorbing or losing water depending on its surroundings.

Whether a fish absorbs or loses water is based on the fact that all fish must maintain a certain amount of salt in their bodies to stay healthy. Fish that live in fresh water have a higher concentration of salt in their bodies than the surrounding water. Consequently, water continuously flows into the fish's body to attempt to dilute the amount of salt in the fish until it is equal to the amount of salt in the surrounding water. Since fish cannot allow their salt content to be diminished, their kidneys work overtime to expel excess water in the form of urine.

Ocean fish have the opposite problem. Surrounded by salt water, their bodies contain a relatively lower concentration of salt than the ocean water. In this case osmosis causes the fish to constantly lose water in order to equalize salt concentration inside and outside the fish. to partially compensate for the water loss, ocean fish actually drink water through their mouths. And to get rid of the excess salt they take in by drinking seawater, they excrete some salt through cells in their gills.

1.depend on依赖(依靠, 取决于, 随...而定)

如： A lot will depend on how she responds to the challenge.
在很大程度上将取决於她对这一难题的反应。

Depend on me.
相信我。

2. diffuse 散播, 使(气体或液体)慢慢混合

如： The light was diffused by the leaves.
树叶使光线漫射。

3. absorb吸纳, 吸引...的注意, 吞并

如： Dry sand absorbs water.
干沙吸收水份。

Plants can absorb carbon dioxide and release oxygen.
植物可以吸收二氧化碳释放氧气。

4. diminish减少, 变小, 减损

如： He likes to diminish the skills of others.
他喜欢贬低别人的技能。

He was expelled from the school.
他被学校开除了。

(可可英语版权所有,未经允许请勿转载)