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PRESENTER: Hello, and
welcome to Byte Size Med.

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This video is on
the breathing cycle.

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The breathing cycle
involves air going

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into the lungs during
inspiration and air

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leaving the lungs
during expiration.

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During this cycle, there are
pressure and volume changes.

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In this video, we're going to
put pressure and volume together

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and see what happens during
one cycle of respiration.

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There are three phases.

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There's rest where there's
no airflow, inspiration

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where air enters, and
expiration where air leaves.

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Now, we're going to
use this schematic lung

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to try and understand it.

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The lungs are surrounded
by pleural cavities

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lined by pleura.

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There's an inner
visceral layer, which

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is sort of attached
to the lungs,

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and the outer parietal layer,
which is towards the chest wall.

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Now, the pleural cavity
is filled with fluid.

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This fluid acts like a lubricant
and helps the lungs move

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during respiration.

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The pressure in
the pleural space,

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that's the intrapleural
pressure, or just simply

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pleural pressure.

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Now, the air is going to
enter through the airways

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into the alveoli.

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The pressure in the alveoli
is the alveolar pressure.

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So we've got the pleural
pressure and the alveolar

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pressure.

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Now, the difference between
these two pressures--

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that is the pressure
across the organ.

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That's the transmural pressure.

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Since we're talking
about the lungs,

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it's the transpulmonary
pressure.

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The pressures are in
centimeters of water

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and are in relation to
atmospheric pressure.

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To understand it, we consider
atmospheric pressure to be 0.

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So that's our
reference pressure.

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Air moves along a pressure
gradient from high pressure

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to low pressure.

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So that's what drives
the air to move

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between the lungs
and the atmosphere.

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First, let's look
at the volumes.

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There are four lung volumes--

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the inspiratory reserve
volume, the tidal volume,

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the expiratory reserve volume,
and the residual volume.

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The air that enters or
leaves the lungs just

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while quietly breathing
in and breathing out,

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that's the tidal volume, which
is around 500 milliliters.

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So what's left behind after
the tidal volume leaves?

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The expiratory reserve volume
and the residual volume,

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which together form the
functional residual capacity,

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the FRC.

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So that is the air
that gets left behind

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after quietly
breathing out, and it's

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the air that's in the
lungs in a state of rest.

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So at rest, the
volume in the lungs

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is at functional
residual capacity.

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During inspiration, 500 mL
of air enters the lungs,

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and during expiration, that
500 mL leaves the lungs.

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For this to happen,
pressures have to change.

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And now we're going to
add in the pressures.

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So let's start at rest.

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The volume is at FRC,
like I said before.

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The chest wall, it
has a natural tendency

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to want to pull outwards,
and the lungs have a tendency

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to want to collapse inwards.

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At this point, these two forces,
they balance each other out.

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The pressure in the
pleural space at rest

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is slightly negative at
minus 5 centimeters of water.

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That keeps the lungs open.

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The pressure in
the alveoli is 0,

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equal to that of the atmosphere.

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Remember, we consider
the atmosphere to be 0,

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and that's our reference.

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So now there's no gradient.

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There's no airflow.

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And the system is
at equilibrium.

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So this is the
situation at rest.

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Now, when inspiration
begins, the diaphragm

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is going to contract.

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The lungs expand.

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And so the alveolar pressure
becomes slightly negative.

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It's going to go down to
minus 1 centimeters of water.

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So now there's a gradient
between the atmosphere

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and the alveoli.

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And the alveolar pressure is
lower than the atmosphere.

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So the air is going
to enter the lungs.

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The fact that the chest
wall is expanding,

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that makes the pleural
pressure more negative.

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So it goes down
from it's negative 5

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to minus 7.5
centimeters of water.

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At the end of inspiration,
the alveolar pressure

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goes back up to 0.

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So now 500 mL of air
has entered the lungs,

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and the volume in
the lungs would now

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include both the functional
residual capacity

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and the tidal volume.

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Now, unlike
inspiration, which was

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active from muscles contracting,
expiration is passive.

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It's from elastic recoil.

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So the alveolar
pressure now becomes

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slightly positive at plus
1 centimeters of water.

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Now you can see that the
gradient has reversed.

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So air is going to move
in the opposite direction.

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From high to low pressure,
it moves from the lungs

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to the atmosphere.

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So it leaves the
lungs, and what's

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left behind is now the
functional residual capacity

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again.

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And at the end of expiration,
the pleural pressure

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comes back up to minus
5 centimeters of water.

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So now we're at rest again.

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So we've completed
one breathing cycle.

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And the cycle is
going to repeat again.

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So this is rest, the
phase of inspiration,

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and the phase of expiration.

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What happened to the volume?

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The volume of air that
entered, or the volume change,

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was from 0 to 500 milliliters.

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And then that 500 mL left
and it came back to 0.

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We're talking about
a volume change.

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The actual volume at rest was
the functional residual capacity

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and not 0, but the change that
happened, that was by 500 mL.

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For that 500 mL to
enter, what happened

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to the alveolar pressure?

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It went from 0 at
rest down to minus 1,

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came back to 0 at the
end of inspiration.

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Then for the 500 mL to
leave, it went to plus 1

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and then came back to 0 again.

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The pleural pressure went from
minus 5 at rest to minus 7.5

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at the end of inspiration and
then came back up to minus 5

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again.

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But there's one more pressure,
the transpulmonary pressure.

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It's the alveolar pressure
minus the pleural pressure.

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So if we take rest,
end of inspiration,

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and end of expiration,
at rest you

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can see it's 0 minus of minus 5.

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That's plus 5 centimeters
of water at rest.

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By the end of inspiration,
it's plus 7.5.

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And then it comes back to plus 5
again at the end of expiration.

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So throughout the breathing
cycle, it's positive.

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As long as the transpulmonary
pressure remains positive,

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the airways stay open.

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When the transpulmonary
pressure becomes negative,

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airways collapse.

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And that's what happens during
one cycle of respiration.

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Thanks for watching, and
I'll see you in the next one.