Asthma pathophysiology

- Before we talk about what asthma looks like, let's take a look at a normal airway. So, we've taken a cross section of somewhere along the airway. Asthma is a small airway disease, so this is not the trachea or the stuff in your throat, but in your chest. And one of the important players in the caliber of the structure of the airway that I want to start with is the smooth muscle layer. So, we'll use red for muscle, and this layer controls the diameter of the opening, diameter of the lumen. Of course, outside the muscle there's other things too. There's connective tissue and cartilage, depending on where-- If you're high up enough on the airway. Those things are not really as affected by asthma, so we'll just leave it kind of like this. All right, so we've got our smooth muscle, which smooth muscle, remember, you can't control consciously. It reacts to the environment, and usually it keeps our airway nice and open like this. I'll label that here, smooth muscle. Now, inside that we have a layer of the mucosa. So, we call it the mucosal layer. Let me just label that, the mucosa. That basically coats the inside opening of the airway, and within this layer it's very important that we have these glands that can secrete mucus into the lungs. They're there for lubricating the opening and keeping our lungs moist, enough so when there is inflammation, or there are foreign bodies we have to get rid of, the mucus that they secrete is important in clearing out the area. So when we cough, and we cough up the phlegm, the phlegm comes from these glands that secrete the mucosa. So, they're kind of everywhere in the mucosal layer. And, of course, here we have the lumen. This is where the air actually moves in and out. It is dry but nice and lubricated by the mucus. So, this is a quick look at what the normal lung, what the normal cross section of the airway looks like. And now, let's look at what happens when this person is having asthma. As you know, asthma comes in attacks, so most of the changes happen during an attack. Depending on the severity, this person might get it more often than the next patient. So, first we still have our muscular layer, but now the lung is in spasm. The muscles are spasming, which means it's constricting, and the opening is going to be much smaller. The connective tissue, like the cartilage and other things, are still out there, so I'll still draw it like this, but the important thing to focus on right now is the fact that the muscular layer has thickened and is clamping down on this opening. In addition, the mucus layer, which is nice and round here, that I've drawn, has swelled up in reaction to the inflammation. So, instead of a nice round opening like this, now we have this amorphous shape that further clamps down on the opening. So now look at what has happened to our lumen. It's much smaller, and to make matters worse, don't forget the glands. So, the glands are still here, but this time they're also reacting to the immune response, and as a result, they're filling up with mucus, and they're secreting it into the lumen that's already narrow and blocked off. So, now we've got this swirling mucus in here. That only makes matters worse. So, look at the difference between air going through this nice big opening and the air trying to get through this. So, between the constriction and all the fluid in there, we're going to have little bubbles. That's why you hear the popping and the wheezing when you listen to a person having an asthma attack. The wheezing comes from the obstruction, both from mechanically narrowing this area and the extra fluid in there from these glands going crazy. So, that's how it happens, but why does this whole thing take place? What triggers asthma? Well, that is sort of the million dollar question, because there's so many things in the environment. What I'm drawing here are, it could be pollution. It could be smoke. It could be food or dander. Anything that this particular person reacts to. These are the allergens in the air. And when they get into the body, our body has an immune response that reacts to anything that's foreign. Some people react more than others. And we have these antibodies that kind of look like a Y-shaped molecule. So these immune reaction tend to be IGE. That's just the name of these antibodies. Now, the body has a tricky memory system. If it's the first time that it's seeing these allergens, you might only get two IGEs, because they're working hard to make new ones, to recognize them, but the third or fourth time, every time you get exposed, because the memory's still there, it's easier to make the IGE each time. It's like fighting an enemy that you've already fought before. So, the IGE actually increases in number. So, the size of this IGE response is proportional to the size of this immune response or to this allergic response, I should say, because when the immune response goes haywire, responding to something that's foreign and trying to fight it off, that's called an allergy. Okay, so we've got these foreign allergens, whatever the trigger is, with these IGE. The job of the IGE is to recognize and pick up these foreign particles. The IGE is in the same family of antibodies that help us fight off infections, but in this case, it goes to search for a cell we call the mast cell. It's spelled mast, like the mast on a ship, and inside the mast cell, it's just floating around our body, carrying little pockets of a molecule we call histamine. And histamine is the main player in any allergic reaction. If you remember, if you've ever taken anything to fight off an allergy, there's a class of drugs called the antihistamines, right? So, histamine is just usually walled off in the mast cell, not in our system, but the IGE antibodies we just talked about, they are friends with the mast cells, and only when they're carrying a foreign body like this do they find the mast cell and attach to it. So, every pair of IGE that has picked up an allergen will go then look for a mast cell to attach to it like this. This attachment right here kind of wakes up the mast cells. So, all these little pockets inside it open up. The histamine then flows out of the mast cell like this and kind of floods into our system, into our bloodstream, causing allergic reactions everywhere. This is why you sneeze, and your eyes water. You can get hives. The whole cascade of that allergic reaction is thanks to these little histamine molecules, and in the lungs, if this person has asthma, this is exactly what happens as a result of the histamine. Let me just draw a few more particles here to show you. And while we get this constriction, we get the mucosal swelling, and we get the wheezing. So, asthma is really an immune response going overboard, as all allergies are is a part of our body's natural response to a foreign body that then causes us harm by reacting too strongly and releasing things that cause us discomfort. So, to sum up the pathophysiology of asthma, remember first we've got the muscular layer thickening and constricting. Then we've got the mucosal layer swelling up, and third we have the glands overproducing the mucus that then floods the already constricted opening.