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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.