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- [Lecturer] Even though the
entire respiratory system

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is used for gas exchange, the
actual exchange of the gases

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occurs within the lungs.

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Now lungs are within your thoracic cavity.

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You can think of the
thoracic cavity as everything

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above your diaphragm.

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You're probably saying
where is the diaphragm?

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Find you ribcage right in
the middle where it meets

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there's that thing called
the xyphoid process.

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The diaphragm's right about there.

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It's right where you ribs
kind of meet in the middle

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of your chest, that little indent.

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So from there up, that's
the thoracic cavity.

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We already mentioned it's
surrounded by a double membrane,

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these pleural membranes meant to protect,

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help reduce friction.

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The bronchioles come after the
bronchi, but the way you get

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the gas into the lungs is
from the trachea to the brochi

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to the bronchioles down to the alveoli.

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Now the alveoli themselves,
think of tiny little

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beach balls.

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Inside is completely hollow.

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Outside is a thin wall.

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Now give or take about a
million or two of these

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beach balls, and jam them into one space.

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That's your lungs.

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All these tiny little air-filled sacs.

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They're constantly undergoing
gas exchange, the alveoli.

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But then around the alveoli you have the

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pulmonary capillaries.

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Their job is to bring
blood as close as possible

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to the actual alveoli.

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This way it makes it
easier and more efficient

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for the oxygen to leave
the alveoli and diffuse

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into the blood, and the carbon
dioxide to leave the blood

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and diffuse into the alveoli.

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So here's our picture of the lungs.

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Now if you notice, the
lungs are not equal.

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Your left lung is gonna have two lobes,

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while the right has three.

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The reason for the difference
is the heart itself.

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The heart is a little bit offset
to the left, if you notice,

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causing the left lung to
be a little bit smaller.

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So these pulmonary capillaries,
I already mentioned that

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it's bringing blood into close contact,

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bringing it as close as
possible with the air.

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So what ends up happening
here is you look at Figure A,

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top left here.

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You can see the bronchiole
going down into three

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individual alveolar sacs.

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Well, if you take that one
and blow it up at the bottom

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right corner, you can see
that it's trying to show three

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individual alveoli cut in cross-section.

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You can see the red and
blue vessels in there.

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So blue is the oxygen-poor
blood coming in.

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The red is the oxygen-rich blood leaving.

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Those alveoli allow for gas diffusion.

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Now if you look in that little
small rectangle from there,

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it shows part of the alveolar wall

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and part of the capillary.

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You zoom in to the top
right, you can see the air in

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alveolus is most of the
left third of that figure.

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But then you can see there is an O2 going

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into the capillary.

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CO2, it's heading out.

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So this is showing the different
gas exchanges occurring

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across membranes, the diffusion.

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So how do we breathe?

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Well, it's not just a simple
oh, you think about breathing

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and it does it.

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It is that simple, but
it's not that simple.

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Yes, all you do is think about
it, but no, it's not simple

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because you have to use
several different muscles.

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So when you're in a relaxed
state, that means air is not

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entering, air is not leaving.

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This is the point right
in between a breath.

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The diaphragm is relaxed, it's
in that kind of dome shape.

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These intercostal muscles
in between the ribcage

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are just in their relaxed state.

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But when you start to
inhale the diaphragm is

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going to contract.

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As the diaphragm
contracts, it flattens out.

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Now if you take a deep
breath, you not only use

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the diaphragm, you're also
gonna use the intercostal.

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You use the external
intercostal muscles to help pull

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the actual ribs out.

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Take a deep breath.

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You can feel that your
chest kind of expands.

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That expansion is the ribcage opening up,

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the diaphragm contracting down.

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What this allows is for
an increase in volume.

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You now have a larger
volume inside your ribcage.

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This larger volume ends up
creating a lower pressure.

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Well, we know that diffusion
goes from high to low,

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so the low pressure
means the air gets sucked

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into your lungs.

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So you have this low
pressure pulling the air in.

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But the flip side, expiration.

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In this case when you breathe
out, you don't really have

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the diaphragm pushing.

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All the diaphragm does is relax.

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The diaphragm relaxes.

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It goes back up to its dome shape.

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That's a simple, normal
everyday breathing relaxation.

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But if you're trying to force the air out,

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so you're exhaling very
forcibly, very quickly,

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you're gonna have these
internal intercostals actually

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contract and cause the
chest to decrease in size.

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Take that deep breath in again.

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(deep breath)

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And force it out.

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Exhale very quickly.

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You'll notice that your
ribcage is going from a very

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large position down to
a much smaller position.

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It's compressing the lungs,
increasing the pressure

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in the lungs causing there
to be a higher pressure.

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The air then flows from
the high pressure in

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to low pressure out.

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You exhale.

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So really breathing is all about pressure,

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going from a high pressure
to a low pressure.