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So we talked about inhaling and exhaling.

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And I mentioned that the key first step 
for both of them is kind of

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the change in volume: going up in volume 
and going down in volume.

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But I didn't really talk about how that happens exactly.

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So, I'll kind of jump into that now.

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And let me begin by telling you that 
in the middle of your chess,

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you have this enormous kind of bone that goes down.

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I'm drawing it out of proportion 
just to make it very clear

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where the bone is being.

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Go ahead and feel on your own body 
this bone which we called

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either the breast bone or the more 
technical name is sternum.

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Write that down here.

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The sternum is this middle bone and

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all the ribs on both sides attached there.

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So, you've got a total of 14 ribs and 7 pairs of them.

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Actually, I should say 14 pairs of ribs.

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I don't want you to think there're 14 in total.

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There are actually 14 in total and 7 pairs of the ribs.

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So 14 ribs actually attached directly 
to this bone, the sternum bone.

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So in white, these are the ribs.

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And between the ribs, you actually have muscles.

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So I'm gonna draw some of these muscles 
between the ribs.

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And these muscles are all going to have 
their own nerve

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that allows them to contract.

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So, these muscles are controlled 
by your body or your brain.

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And their name, let me just jot down here on the side, 
is "intercostal muscle".

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And "inter" just means "between". 
So, this is the name of the muscle.

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And "costal" refers to the ribs.

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So, when you see that word "costal",

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you'll know we're talking about the ribs.

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So what's between the ribs is these muscles: 
intercostal muscles.

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And they're gonna start moving outwards

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when your brain says, 
"hey I want to take a deep breath!"

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So, these muscles are going to contract

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and the ribs are gonna move outwards.

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So these go out. And you also have...

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Let me just make a little bit of space on this xxx.

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You also have another muscle 
that kind of right down here.

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And it has kind of a an upside down U shape to it.

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So I'm drawing it kind of like a dome. 
You can think of it as a dome.

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And this dome is the floor. If you remember,

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we talked about the floor of the thorax.

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So this is of course, our diaphram muscle.

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So, you've got a diaphragm muscle.

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And this one when it contracts, it's gonna go...

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instead of going out, it's gonna go down.

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So it's gonna kind of flatten out.

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And I can actually draw this if you just 
stick with me for a moment.

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I'm gonna erase this dome-like shape.

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And I'm gonna draw it when it looks like as it's contract.

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So when it contracts, this is actually 
going to be more flat.

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And this flat diaphragm, as you can see, is now

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further down that it used to be.

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And as it goes down, all of the structures

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that are inside the space, so the two lungs,

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and of course, I didn't draw the heart here,

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but the heart would be this kind of this cardiac notch.

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If you want me to, I can even draw that heart here.

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This is our heart here.

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They're all going to kind of physically moved down.

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So this is our heart and our lungs.

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They are physically gonna be kind of drawn downwards and out.

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They're gonna also move out as 
the intercostal muscles move out.

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So you have expansion of these lungs, 
that's basically the idea.

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And if you kind of zoom in on this,

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to kind of see exactly what this expansion looks like,

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when I say, you know, you have more volume, the lungs,

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really what I should be saying, 
if I want to be more exact,

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is that all the alveoli, if these are the alveoli,

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let's say this is another branch,

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this is another alveolar right here;

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all these alveoli, they are actually expanding.

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And you have about 500 million alveoli.

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You can just kind of fathom how big a number that it is.

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It's an enormous number of alveoli.

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If I was actually drawing them here... drawing forever, right?

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It'll take forever to kind of write out 
this many different alveoli.

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But basically what happens is that, when the ribs go out,

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the diaphragm moves down.

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These alveoli are actually being pulled out. 
They are actually pulled outwards.

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So they are actually gonna be getting larger in size.

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They literally look like they've grown in size.

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And, this is what they look like.

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And actually, if you take even a closer look,

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you'll see that these alveoli have around them, 
a bunch of protein.

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The cells around them have a bunch of protein.

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And this protein is called "elastin".

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And you can guess what elastin might do.

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It has kind of a similar sound to the word "elastic".

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Elastin is basically kind of like a rubber band.

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So you kind of thinking of elastin as a rubber band.

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And, when the muscles move down and out,

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and the alveoli pulled open,

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let me actually kind of scroll up,

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because you can kind of go back to the idea of inhaling.

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What is happening? Well, we have 
a couple things happening.

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One, you have muscles contracting. Muscles contract.

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And when I say "muscles", you know I'm talking about

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all those intercostal muscles in your diaphragm.

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And as a result of the muscles contracting,

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you have now the alveoli, alveoli are streched open.

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So those rubber bands are elastin, protein,

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are literally physically being stretched open.

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So the alveoli are stretched open.

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And keep that in mind, because 
what's gonna happen then is,

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when the muscles relax, which is 
what happens when you exhale,

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when the muscles relax,

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what do you think is going to happen to that elastin?

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Well, it's like a rubber band,

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if that's what I'm saying, it's gonna be like,

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then the alveoli are going to recoil. 
They are gonna recoil.

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And that's actually the driving force for 
why the volume goes back down.

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So if you have a bunch of rubber bands 
that you're stretching out...

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Let me actually bring up the picture 
so you'll see really clearly.

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If you're physically kind of using your muscles 
to help pull this stuff open,

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then the moment that you start pulling open, 
the moment that you,

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you know, stop contracting those muscles,

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now you have a nice big volume.

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What's gonna happen?

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All these elastin molecules are gonna snap back.

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We'll do it with a different color.

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I'll do it with this color.

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They are gonna snap back like this.

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All that protein are gonna snap back into 
kind of the original size.

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And when they do, this thing gets smaller.

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So my alveoli kind of goes back to its original size,

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which is much smaller than this.

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Let me actually show you that

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even though contraction is what opens up things

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it's the recoil that kind of brings things back down 
to their normal size.

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Let me erase this to make it kind of neater drawing.

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So you can see it now, inhaling, the way that 
we actually increase the volume,

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is by pulling things open through contraction.

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And this actually requires energy, right?

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Remember, you can't contract a muscle 
without spending chemical energy.

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So this takes chemical energy.

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And we usually think of this molecule ATP

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as the specific type of chemical energy 
we're gonna use.

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And to exhale, when you reduce the volume,

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it's gonna be driven by this elastic recoil.

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So that's a type of elastic potential energy.

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So, this process of kind of inhaling and exhaling

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is really a little different from each other.

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On the one hand, you're using ATP;

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you're actually burning through these molecules.

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And when you exhale,

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you're actually not using chemical energy anymore.

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You're just using that elastic potential energy,

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kind of the same sort of energy that you can imagine

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you would have if you snap a rubber band.

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So, let's stop there. We'll pick up the next video.