Breathing Cycle Physiology | Pressure & Volume Changes During Respiration | Respiratory Physiology

PRESENTER: Hello, and
welcome to Byte Size Med.

This video is on
the breathing cycle.

The breathing cycle
involves air going

into the lungs during
inspiration and air

leaving the lungs
during expiration.

During this cycle, there are
pressure and volume changes.

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

and see what happens during
one cycle of respiration.

There are three phases.

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

where air enters, and
expiration where air leaves.

Now, we're going to
use this schematic lung

to try and understand it.

The lungs are surrounded
by pleural cavities

lined by pleura.

There's an inner
visceral layer, which

is sort of attached
to the lungs,

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

Now, the pleural cavity
is filled with fluid.

This fluid acts like a lubricant
and helps the lungs move

during respiration.

The pressure in
the pleural space,

that's the intrapleural
pressure, or just simply

pleural pressure.

Now, the air is going to
enter through the airways

into the alveoli.

The pressure in the alveoli
is the alveolar pressure.

So we've got the pleural
pressure and the alveolar

pressure.

Now, the difference between
these two pressures--

that is the pressure
across the organ.

That's the transmural pressure.

Since we're talking
about the lungs,

it's the transpulmonary
pressure.

The pressures are in
centimeters of water

and are in relation to
atmospheric pressure.

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

So that's our
reference pressure.

Air moves along a pressure
gradient from high pressure

to low pressure.

So that's what drives
the air to move

between the lungs
and the atmosphere.

First, let's look
at the volumes.

There are four lung volumes--

the inspiratory reserve
volume, the tidal volume,

the expiratory reserve volume,
and the residual volume.

The air that enters or
leaves the lungs just

while quietly breathing
in and breathing out,

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

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

The expiratory reserve volume
and the residual volume,

which together form the
functional residual capacity,

the FRC.

So that is the air
that gets left behind

after quietly
breathing out, and it's

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

So at rest, the
volume in the lungs

is at functional
residual capacity.

During inspiration, 500 mL
of air enters the lungs,

and during expiration, that
500 mL leaves the lungs.

For this to happen,
pressures have to change.

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

So let's start at rest.

The volume is at FRC,
like I said before.

The chest wall, it
has a natural tendency

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

to want to collapse inwards.

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

The pressure in the
pleural space at rest

is slightly negative at
minus 5 centimeters of water.

That keeps the lungs open.

The pressure in
the alveoli is 0,

equal to that of the atmosphere.

Remember, we consider
the atmosphere to be 0,

and that's our reference.

So now there's no gradient.

There's no airflow.

And the system is
at equilibrium.

So this is the
situation at rest.

Now, when inspiration
begins, the diaphragm

is going to contract.

The lungs expand.

And so the alveolar pressure
becomes slightly negative.

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

So now there's a gradient
between the atmosphere

and the alveoli.

And the alveolar pressure is
lower than the atmosphere.

So the air is going
to enter the lungs.

The fact that the chest
wall is expanding,

that makes the pleural
pressure more negative.

So it goes down
from it's negative 5

to minus 7.5
centimeters of water.

At the end of inspiration,
the alveolar pressure

goes back up to 0.

So now 500 mL of air
has entered the lungs,

and the volume in
the lungs would now

include both the functional
residual capacity

and the tidal volume.

Now, unlike
inspiration, which was

active from muscles contracting,
expiration is passive.

It's from elastic recoil.

So the alveolar
pressure now becomes

slightly positive at plus
1 centimeters of water.

Now you can see that the
gradient has reversed.

So air is going to move
in the opposite direction.

From high to low pressure,
it moves from the lungs

to the atmosphere.

So it leaves the
lungs, and what's

left behind is now the
functional residual capacity

again.

And at the end of expiration,
the pleural pressure

comes back up to minus
5 centimeters of water.

So now we're at rest again.

So we've completed
one breathing cycle.

And the cycle is
going to repeat again.

So this is rest, the
phase of inspiration,

and the phase of expiration.

What happened to the volume?

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

was from 0 to 500 milliliters.

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

We're talking about
a volume change.

The actual volume at rest was
the functional residual capacity

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

For that 500 mL to
enter, what happened

to the alveolar pressure?

It went from 0 at
rest down to minus 1,

came back to 0 at the
end of inspiration.

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

and then came back to 0 again.

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

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

again.

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

It's the alveolar pressure
minus the pleural pressure.

So if we take rest,
end of inspiration,

and end of expiration,
at rest you

can see it's 0 minus of minus 5.

That's plus 5 centimeters
of water at rest.

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

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

So throughout the breathing
cycle, it's positive.

As long as the transpulmonary
pressure remains positive,

the airways stay open.

When the transpulmonary
pressure becomes negative,

airways collapse.

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

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