They do this with about one million tiny structures called nephrons. These nephrons are kind of like workers on a conveyor belt, actively adding things and taking things away, things like sodium, glucose or sugar, calcium, amino acids and water, to make sure the body maintains homeostasis, or balance. If levels of anything get too high or too low, then that can be harmful. And it is the job of the kidney s to keep the levels of many of these substances in the just-right zone. Substances that aren't needed leave the kidney and head to the bladder, where you excrete them in the form of urine.

腎臟通過一百萬個(gè)微小的結(jié)構(gòu)來完成上述工作， 這種微小結(jié)構(gòu)叫腎元。腎元有點(diǎn)像流水線旁的工人， 不停地往里加點(diǎn)什么或者拿走點(diǎn)什么，比如納、葡萄糖、糖、鈣、氨基酸和水，保證身體處于平衡的狀態(tài)。如果任何東西過多或者過少，都是有害的。腎臟的作用就是將多種物質(zhì)的含量保持在恰當(dāng)?shù)膮^(qū)間內(nèi)。不需要的物質(zhì)就會(huì)離開腎臟，流向膀胱，在那里以尿液的形式排出體外。

So where do eight glasses of water a day fit in? They don't. Noticed that I did not say that the k idney function is improved with excess water. Imagine that you're sweating a lot, so you're losing water from your blood. The kidneys know your blood volume is dropping ever so slightly and that your blood is getting ever so slightly saltier. They compensate by absorbin g more water back into the blood, making the urine more concentrated.

這中間哪一步需要一天8杯水呢? 哪都不需要。請(qǐng)注意我剛剛并沒有說， 腎功能會(huì)因?yàn)轭~外的水份而得到改善。想象一下， 你出了很多汗，因此你血液里的水份減少了。你的腎臟注意到你的血量下降了一點(diǎn)點(diǎn)， 因此導(dǎo)致血液里的含鹽量上升了一點(diǎn)點(diǎn)。它們會(huì)通過多吸收一些水回血液來進(jìn)行補(bǔ)償， 這樣會(huì)導(dǎo)致你的尿更濃一些。

If the kidney s sense enough fluid can't be reabsorbed from?the urine, you're signaled to drink, meaning you get thirsty.?If you don't have fluid available, the thirst message gets?stronger and stronger. A person facing real dehydration?won't be unsure if they need water. They'll do whatever?they need to get it. It's one of our most basic instincts that's?evolved over a very long time, in environments where clean?water wasn't nearly as readily availableas it is today.

如果腎臟發(fā)現(xiàn)尿液里的水也不夠它們重新吸收的， 它們就會(huì)提醒你喝水， 這時(shí)你就會(huì)覺得渴。如果你沒有喝水的話， 這種口渴的感覺就會(huì)越來越強(qiáng)烈。一個(gè)真的面臨脫水狀況的人， 是不可能拿不準(zhǔn)自己是不是需要水的。他們會(huì)想盡一切辦法去搞到水。這是我們最基礎(chǔ)的本能之一， 是經(jīng)歷了漫長的進(jìn)化過程得來的， 當(dāng)時(shí)的環(huán)境并不像今天， 清潔的水唾手可得。