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Some of the issues that are important

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if you want to have people in space

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for long periods of time.

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One is that people will tend to lose

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bone and muscle mass.

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We know this.

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If you have to put a cast on your leg,

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and you take the cast off after a few weeks,

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you'll see that your muscles have shrunk in size.

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And if you measured the bone strength,

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you'd also see that might have gone down a little bit, too.

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And so, it's very interesting that our body has that ability

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to adapt to the loads that are put on it,

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so that bones and muscles aren't static,

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they're always changing.

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While we think of bone as being a solid thing

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that doesn't change very much,

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it changes too.

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And it turns out that in weightlessness,

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you lose bone.

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And then you also cause the muscles

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that work against gravity,

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what are called the postural muscles,

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they'll start to shrink and lose strength.

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There are other things in the cardiovascular system,

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the heart and blood vessels.

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And if you think about it,

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standing up in gravity

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means you have to work against gravity

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in order to keep blood pumping to your head.

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So, if you couldn't keep blood pumping in the head,

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you'd pass out every time you stood up

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because when you're lying down,

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you don't have to push against gravity.

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But when you stand up,

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you got to work against gravity

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to keep blood flowing to your head.

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And your heart and blood vessels

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have a really nicely worked-out system

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to make that happen every time.

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But that system can also change in weightlessness.

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And then the other area that changes

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is the system that has to do with balance.

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Again, maintaining your balance

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is something that you're doing against gravity, right?

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If you didn't have gravity present,

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you wouldn't have to worry about falling.

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But you obviously do have to worry about falling,

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and we have a very highly developed sense of balance

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to keep us upright

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and to prevent us from falling.

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And when you see what skaters do,

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you realize just how exquisite a system it is.

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But when you go into weightlessness,

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your balance system changes.

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You don't really notice it

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while you are in weightlessness,

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but when you come back,

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you do notice it,

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that your balance has changed

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and you have a little bit of trouble

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maintaining your balance.

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And what it shows is that while you're in space,

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your brain is trying to allow you

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to function in weightlessness.

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And so, it re-adapts you to be weightless,

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which you don't notice until you come back

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and find out that you're now back on Earth

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with a balance system that's been adapted to space.

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You know, all life developed here on Earth

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with gravity being present,

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so life evolved under the influence of gravity,

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and then we grow up with gravity being present,

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so we learn how to walk

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and catch a ball

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and ice skate

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or whatever,

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all with gravity being present.

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And what if you were to grow up without gravity?

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What about the systems that depend on gravity,

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like your muscles

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or your balance system

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or the heart and blood vessels?

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Would they develop normally,

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or would they be different in some way?

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One reason why you might think

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that it would go down a different pathway

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is from an experiment

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that was done some time ago

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by two neuroscientists

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called Hubel and Wiesel.

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And what they did is they had a kitten,

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and they put a patch over the eye of the kitten.

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And then the kitten grew up to be a cat,

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and they removed the patch.

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And so, the question is,

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can the cat see out of that eye?

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Now, there's nothing wrong with the eye, right?

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But it just hasn't seen anything,

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there hasn't been any light coming in.

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And the answer is that the cat can't see out of that eye

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because what happens is that the brain

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goes down a different pathway when it develops

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and the connections that would ordinarily develop

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to that eye don't develop.

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And that can't be undone,

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that's a permanent change.

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So, the brain of that cat

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is fundamentally different from the brain of a cat

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that grew up seeing out of that eye.

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That cat grew up with a different brain, in essence.

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So, then you wonder,

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well, what about gravity?

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What if you don't have the forces

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that gravity produces?

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Is your balance organ going to develop

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in the same way,

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or will it be different?

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If somebody grew up in space,

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could they come back to Earth and function,

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or would they really be a different person?