- [Lecturer] Even though the entire respiratory system
is used for gas exchange, the actual exchange of the gases
occurs within the lungs.
Now lungs are within your thoracic cavity.
You can think of the thoracic cavity as everything
above your diaphragm.
You're probably saying where is the diaphragm?
Find you ribcage right in the middle where it meets
there's that thing called the xyphoid process.
The diaphragm's right about there.
It's right where you ribs kind of meet in the middle
of your chest, that little indent.
So from there up, that's the thoracic cavity.
We already mentioned it's surrounded by a double membrane,
these pleural membranes meant to protect,
help reduce friction.
The bronchioles come after the bronchi, but the way you get
the gas into the lungs is from the trachea to the brochi
to the bronchioles down to the alveoli.
Now the alveoli themselves, think of tiny little
beach balls.
Inside is completely hollow.
Outside is a thin wall.
Now give or take about a million or two of these
beach balls, and jam them into one space.
That's your lungs.
All these tiny little air-filled sacs.
They're constantly undergoing gas exchange, the alveoli.
But then around the alveoli you have the
pulmonary capillaries.
Their job is to bring blood as close as possible
to the actual alveoli.
This way it makes it easier and more efficient
for the oxygen to leave the alveoli and diffuse
into the blood, and the carbon dioxide to leave the blood
and diffuse into the alveoli.
So here's our picture of the lungs.
Now if you notice, the lungs are not equal.
Your left lung is gonna have two lobes,
while the right has three.
The reason for the difference is the heart itself.
The heart is a little bit offset to the left, if you notice,
causing the left lung to be a little bit smaller.
So these pulmonary capillaries, I already mentioned that
it's bringing blood into close contact,
bringing it as close as possible with the air.
So what ends up happening here is you look at Figure A,
top left here.
You can see the bronchiole going down into three
individual alveolar sacs.
Well, if you take that one and blow it up at the bottom
right corner, you can see that it's trying to show three
individual alveoli cut in cross-section.
You can see the red and blue vessels in there.
So blue is the oxygen-poor blood coming in.
The red is the oxygen-rich blood leaving.
Those alveoli allow for gas diffusion.
Now if you look in that little small rectangle from there,
it shows part of the alveolar wall
and part of the capillary.
You zoom in to the top right, you can see the air in
alveolus is most of the left third of that figure.
But then you can see there is an O2 going
into the capillary.
CO2, it's heading out.
So this is showing the different gas exchanges occurring
across membranes, the diffusion.
So how do we breathe?
Well, it's not just a simple oh, you think about breathing
and it does it.
It is that simple, but it's not that simple.
Yes, all you do is think about it, but no, it's not simple
because you have to use several different muscles.
So when you're in a relaxed state, that means air is not
entering, air is not leaving.
This is the point right in between a breath.
The diaphragm is relaxed, it's in that kind of dome shape.
These intercostal muscles in between the ribcage
are just in their relaxed state.
But when you start to inhale the diaphragm is
going to contract.
As the diaphragm contracts, it flattens out.
Now if you take a deep breath, you not only use
the diaphragm, you're also gonna use the intercostal.
You use the external intercostal muscles to help pull
the actual ribs out.
Take a deep breath.
You can feel that your chest kind of expands.
That expansion is the ribcage opening up,
the diaphragm contracting down.
What this allows is for an increase in volume.
You now have a larger volume inside your ribcage.
This larger volume ends up creating a lower pressure.
Well, we know that diffusion goes from high to low,
so the low pressure means the air gets sucked
into your lungs.
So you have this low pressure pulling the air in.
But the flip side, expiration.
In this case when you breathe out, you don't really have
the diaphragm pushing.
All the diaphragm does is relax.
The diaphragm relaxes.
It goes back up to its dome shape.
That's a simple, normal everyday breathing relaxation.
But if you're trying to force the air out,
so you're exhaling very forcibly, very quickly,
you're gonna have these internal intercostals actually
contract and cause the chest to decrease in size.
Take that deep breath in again.
(deep breath)
And force it out.
Exhale very quickly.
You'll notice that your ribcage is going from a very
large position down to a much smaller position.
It's compressing the lungs, increasing the pressure
in the lungs causing there to be a higher pressure.
The air then flows from the high pressure in
to low pressure out.
You exhale.
So really breathing is all about pressure,
going from a high pressure to a low pressure.