One is that people will tend to lose bone and muscle mass. 其中之一就是骨质疏松与肌肉萎缩。

　　We know this. If you have to put a cast on your leg, 我们都知道，如果你的腿打上石膏，

　　and you take the cast off after a few weeks, you'll see that your muscles have shrunk in size. 几周之后取下石膏后，你会看到肌肉体积萎缩了。

　　And if you measured the bone strength, you'd also see that might have gone down a little bit, too. 并且如果你测量骨骼的强度，也将看到些许下降。

　　And so, it's very interesting that our body has that ability to adapt to the loads that are put on it, 而且相当有意思的是，我们的身体可以适应加在之其上的负重，

　　so that bones and muscles aren't static, they're always changing. 所以我们的骨骼和肌肉不是永恒不变的，它们在不停发生变化。

　　While we think of bone as being a solid thing that doesn't change very much, it changes too. 即使我们认为很牢靠的骨骼并不怎么变化，但其实它也会变化。

　　And it turns out that in weightlessness, you lose bone. 事实上，在失重状态下骨质会发生流失。

　　And then you also cause the muscles that work against gravity, 并且我们用以对抗重力的“姿势肌肉”

　　what are called the postural muscles, they'll start to shrink and lose strength. 也会开始萎缩并失去力量。

　　There are other things in the cardiovascular system, the heart and blood vessels. 再来看看心血管系统：心脏及血管。

　　And if you think about it, standing up in gravity means you have to work against gravity 你想象一下，你在重力环境下站起来就必须对抗重力，

　　in order to keep blood pumping to your head. 以保证血液流向大脑。

　　So, if you couldn't keep blood pumping in the head, 所以，如果你不能保证血液流向大脑，

　　you'd pass out every time you stood up because when you're lying down, you don't have to push against gravity. 你每次躺下后起身会就感到头晕，这是因为你躺下来的时候不需要对抗重力。

　　But when you stand up, you got to work against gravity to keep blood flowing to your head. 但是当你站起来的时候，你必须再次对抗重力以保障血液流向你的大脑。

　　And your heart and blood vessels have a really nicely worked-out system to make that happen every time. 每当上述情况发生，你的心脏和血管有一套自发的应付机制。

　　But that system can also change in weightlessness. 但是在失重条件下，这套机制也会变化。

　　And then the other area that changes is the system that has to do with balance. 另一个会发生变化的地方就是平衡系统。

　　Again, maintaining your balance is something that you're doing against gravity, right? 同样的，保持平衡是在重力环境下做的事情，对吧？

　　If you didn't have gravity present, you wouldn't have to worry about falling. 如果没有重力存在，我们也不用担心跌倒了。

　　But you obviously do have to worry about falling, 但是很明显你需要担心跌倒，

　　and we have a very highly developed sense of balance to keep us upright and to prevent us from falling. 并且我们已经有相当发达的平衡感知，使得我们保持直立并防止跌倒。

　　And when you see what skaters do, you realize just how exquisite a system it is. 而且等你看过别人玩滑板之后，你就会意识到这个系统有多么精妙了。

　　But when you go into weightlessness, your balance system changes. 但是当你进入失重环境时，你的平衡系统跟着变化。

　　You don't really notice it while you are in weightlessness, but when you come back, you do notice it, 在失重状态下你可能无法意识到这一点，但当你回到正常状态时，

　　that your balance has changed and you have a little bit of trouble maintaining your balance. 就会注意到你的平衡能力已经发生改变，这时候想保持平衡就会有点儿困难了。

　　And what it shows is that while you're in space, your brain is trying to allow you to function in weightlessness. 当你置身太空时，大脑会设法让你在失重环境下活动。

　　And so, it re-adapts you to be weightless, which you don't notice until you come back 这样，你再次适应了失重环境，直到你回到正常状态之前才会意识到这些，

　　and find out that you're now back on Earth with a balance system that's been adapted to space. 落地之后你会发现你的平衡系统已经适应了太空环境。