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Voiceover: In our last video we saw a cross section through a bronchiole ...
Bronchioles remember are those tubes that allow air to pass into the lungs
and they're the ones that branch off of the bronchi.
These guys are no longer held open at a fixed diameter
by the rings of cartilage
but these guys have smooth muscle in their walls
so they're able to change diameter.
We also saw that lining these tubes
there was a little bit of mucus and that that mucus
was good for trapping particles that might enter with the inhaled air.
One thing that I didn't mention is that there are also cells
in these bronchiolar walls that contain a chemical called histamine.
Histamine is a chemical that if it's released in the tissues
it causes swelling of the tissues.
Okay, now let's look at the way in which we can understand
what happens to somebody who suffers from asthma
during an asthma attack.
This is a bit of a simplification
but it's a good way to understand what's going on.
There are some receptors in these walls
and this type of receptor is called an alpha 1-receptor.
The alpha 1-receptors send messages to the smooth muscle
in the walls of the bronchioles and tell them to constrict.
Alpha 1-receptors can be turned on in people with asthma by triggers
and these are things like
cigarette smoke or dust or pollen or pet dander.
Whatever triggers that person's asthma attack
that will basically turn on the alpha 1-receptor.
What's going to happen then is that the smooth muscle
in the wall of that bronchiole is going to shorten or contract
and because it's contracting or shortening
it's going to reduce the diameter of the tube.
Another thing that's going to happen in response
to a trigger binding to those receptors
is that the histamine in the histamine-containing cells
is going to spill out into the wall of the bronchiole
and the result of that is going to be edema or swelling
of the bronchiolar wall.
You can see that this wall is quite a bit thicker than the original wall.
The third thing that's going to happen during an acute asthma attack
is that there is going to be increased secretion of mucus
into the lumen or the hole of this tube.
That's simply an attempt to try and trap more of these offending particles.
But the problem you can see is that if we compare the diameter
of this tube during the asthma attack
and the diameter of the original bronchiole
you can see that there's not much space here for air to pass.
Airflow is severely restricted during an asthma attack like this.
Luckily there's another kind of receptor.
I'm going to draw it as the circle so you can see
that it's different from the alpha 1-receptor.
This receptor is called a beta-2 receptor.
It basically tells the smooth muscle in the walls of the bronchioles
to do the opposite.
It tells the smooth muscle here to relax
and to return to the original relaxed open state of the bronchiole.
The thing that binds to that receptor is called a beta agonist.
Beta agonists are the medicine that's contained
in the fast-acting inhalers that a person with asthma will carry with them
so that if they're in a situation like this
they can quickly inhale the beta agonist medicine
and get back to a relaxed state where they can breathe more easily.
If we had to summarize, the three factors that cause narrowing
of the bronchioles during an asthma attack are:
number one muscular constriction of the smooth muscle cells in the walls.
The second one is edema of the walls that's mediated by histamine.
And the third thing is the increased mucus secretion
that also reduces the size of the lumen
or the pathway for air going into and out of the lung.