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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

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to lose 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 this 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 your 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 keep 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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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 are 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