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.
If this video helped you, give it a thumbs up,
and subscribe to my channel.
Thanks for watching, and I'll see you in the next one.