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  • So we talked about inhaling and exhaling.

  • 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.