Some of the issues that are important

if you want to have people in space

for long periods of time.

One is that people will tend

to lose bone and muscle mass.

We know this.

If you have to put a cast on your leg

and you take the cast off after a few weeks,

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

And if you measured the bone strength,

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

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

to adapt to the loads that are put on it,

so that bones and muscles aren't static,

they're always changing.

While we think of bone as being this solid thing

that doesn't change very much,

it changes too.

And it turns out that in weightlessness,

you lose bone.

And then you also cause the muscles

that work against gravity,

what are called the postural muscles,

they'll start to shrink and lose strength.

There are other things in the cardiovascular system,

the heart and blood vessels.

And if you think about it,

standing up in gravity

means you have to work against gravity

in order to keep blood pumping to your head.

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

you'd pass out every time you stood up

because when you're lying down,

you don't have to push against gravity.

But when you stand up,

you got to work against gravity

to keep blood flowing to your head.

And your heart and blood vessels

have a really nicely worked-out system

to make that happen every time.

But that system can also change in weightlessness.

And then the other area that changes

is the system that has to do with balance.

Again, maintaining your balance

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

If you didn't have gravity present,

you wouldn't have to worry about falling.

But you obviously do have to worry about falling

and we have a very highly developed sense of balance

to keep us upright

and to keep us from falling.

And when you see what skaters do,

you realize just how exquisite a system it is.

But when you go into weightlessness,

your balance system changes.

You don't really notice it

while you are in weightlessness,

but when you come back,

you do notice it,

that your balance has changed

and you have a little bit of trouble

maintaining your balance.

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

your brain is trying to allow you

to function in weightlessness.

So it re-adapts you to be weightless,

which you don't notice until you come back

and find out that you are now back on Earth

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

You know, all life developed here on Earth

with gravity being present,

so life evolved under the influence of gravity,

and then we grow up with gravity being present,

so we learn how to walk

and catch a ball

and ice skate

or whatever,

all with gravity being present.

And what if you were to grow up without gravity?

What about the systems that depend on gravity,

like your muscles

or your balance system