And I'd mentioned that the key first step for both of them

is this change in volume, going up in volume

or going down in volume.

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

so I thought I would jump into that now.

And let me begin by telling you that in the middle

of your chest, you have this enormous kind of bone that

goes down.

And I'm drawing it out of proportion just

to make it very clear where this bone is.

But you can go ahead and feel on your own body

this bone, which we call either the breastbone,

or the more technical name is the sternum.

So I'll write that down here.

The sternum is this middle bone, and all the ribs on both sides

attach there.

So you've got a total of 12 ribs and seven pairs of them.

Actually, I should say 12 pairs of ribs.

I don't want you to think there are 12 total.

We actually have 24 total and seven pairs of the ribs.

So 14 ribs actually attach directly to this sternum bone.

So in white, these are the ribs.

And between the ribs, you actually have muscle.

So I'm going to draw in some of these muscles between the ribs.

And these muscles are all going to have their own nerve that

allows them to contract.

So these muscles are controlled by your brain,

and their name-- let me just jot down here

on the side-- is intercostal muscle.

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

And costal refers to the ribs.

So when you see that word costal,

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

So what's between the ribs is these muscles,

intercostal muscles.

And they are going to start moving outwards when

your brain says, hey, I want to take a deep breath.

So these muscles are going to contract.

The ribs, I should say, are going to move outwards.

So these go out.

And you also have-- let me just make

a little bit of space on this canvas-- another muscle

that kind of rides down here and has

kind of an upside down U shape to it.

So I'm drawing it kind of like a dome.

You can think of it as a dome.

And this dome is the floor-- if you remember,

we talked about the floor of the thorax.

So this is, of course, our diaphragm muscle.

So we've got our diaphragm muscle.

And this one when it contracts, instead of going out,

it's going to go down.

So it's going to kind of flatten out.

And I can actually draw this.

If you can now just stick with me for a moment,

I'm going to erase this dome-like shape.

And I'm going to draw what it looks like as it contracts.

So when it contracts, it's actually going to be more flat.

And this flat diaphragm, as you can see,

is now further down than it used to be.

And as it goes down, all of the structures that

are inside this space-- so the two lungs.

And of course, I didn't draw the heart here.

But the heart would be kind of in this cardiac notch.

If you want, maybe I could even draw that heart here.

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

So this is our heart and our lungs.

They're physically going to be kind of drawn

downwards and out.

They are going to also move out as the intercostal muscles move

out.

So you have expansion of these lungs.

That's basically the idea.

And if you were to kind of zoom in on this to kind of see

exactly what this expansion looks

like, when I say you have more volume in the lungs,

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

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

let's say this is another branch.

And this is another alveoli right here.

All these alveoli, they are actually expanding.

And you have about 500 million alveoli.

If you can just kind of fathom how big a number that is.

It's an enormous number of alveoli.

And if I was actually drawing them,

I would be here drawing forever.

It would take forever to write out

this many different alveoli.

But basically what happens is that when the ribs go out

and the diaphragm moves down, these alveoli are actually

being pulled out.

They're actually being pulled outwards,

so they are actually going to be getting larger in size.

They literally look like they've grown in size,

and this is what they look like.

And actually, if you were to take an even closer look,

you'd see that these alveoli have around them

a bunch of protein.

The cells around them have a bunch of protein,

and this protein is called elastin.

And you can guess what elastin might do.

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

And elastin is basically kind of like a rubber band,

so you can kind of think of elastin as a rubber band.

And when the muscles move down and out

and the alveoli are pulled open--

let me actually now scroll up, because you can kind of go back

to the idea of inhaling-- what is happening, then?

Well, you have a couple things happening.

One, you have muscles-- I'll just write muscles contracting.

And when I say muscles, you know I'm

talking about all those intercostal muscles

in your diaphragm.

And as a result of the muscles contracting,

you have now the alveoli are stretched open.

So those rubber bands, those elastin proteins,

are literally physically being stretched opened.

And keep that in mind, because what's going to happen then

is when the muscles relax, which is

what happens when you exhale, what do you

think is going to happen to that elastin?

Well, if it's like a rubber band,

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

then the alveoli are going to recoil.

And that's actually the driving force

for why the volume goes back down.

So if you have a bunch of rubber bands

that you're stretching out-- let me actually

bring up the picture.

You'll see it really clearly.

If you're physically kind of using your muscles to help

pull this stuff open, then the moment

that you stop pulling open, the moment

that you stop contracting those muscles now

that you have a nice big volume, what's going to happen?

Well, all these elastin molecules

are going to snap back.

Let me do it with a different color.

Let's do this color.

They're going to snap back like this.

All that protein is going to want

to snap back into the original size.

And when they do, this thing gets smaller.

So my alveoli goes back to its original size,

which was much smaller than this.

Let me actually just quickly show that and show you

that even though contraction is what opened up things,

it's the recoil that brings things

back down to their normal size.

And let me erase this to make it kind of a neater drawing.

So you can see it now.

Inhaling, the way that we actually increase the volume,

is by pulling things open through contraction.

And this actually requires energy.

Remember, you can't contract a muscle

without spending chemical energy.

So this takes chemical energy, and we usually

think of this molecule ATP as the specific type of chemical

energy we're going to use.

And to exhale, when you reduce the volume,

it's going to be driven by this elastic recoil.

So that's a type of elastic potential energy.

So this process of inhaling and exhaling

is really a little different from each other.

On the one hand, you're using ATP.

You're actually burning through these molecules.

And then when you exhale, you're actually

not using chemical energy anymore.

You're just using that elastic potential energy,

kind of the same sort of energy that you

can imagine you would have if you snap a rubber band.

So let's stop there, and we'll pick up in the next video.