Chronic bronchitis (COPD)
Chronic bronchitis pathophysiology
Voiceover: Before we get to exactly how chronic bronchitis happens, I just wanted to review a quick rule of thumb about the body. So this is any opening, the body has to the outside. Let's label this as the outside. And anytime you have an opening and irritants get into the body, it could be, by irritants I mean anything that doesn't belong there that inflames a tissue. A common first defense of the body is to call on the glands which are embedded in tissue. To secrete either liquid or mucous into the space. And the goal is to have this fluid, this mucous mixed with the irritants and eventually flush them out or bring them out. I'm sure you can think of examples of what I'm talking about, like tears. If things get in your eye, it makes your eyes tear, right? So this is a mechanical thing the tears actually flush out the irritant in your eye. And we have snot. I'm sorry this is kind of gross, but if things get in your nose and it's not suppose to be there, the snot has to bring it out. And the third thing which has to do with what we're going to talk about today is of course phlegm. So when things get into your throat or your lungs, the walls of the lungs secrete the mucous which forms a phlegm. Now the system in the lungs also triggers a cough reflex, which so the walls here if you can me drawing in red here. The walls here are lined with smooth muscle, and when the area is being irritated, the smooth muscle spasm and that's how we get the cough reflex to bring the phlegm out. Now keep in mind, I said this is the body's first line of defense. So these are good things to have, you want the body to do this. So chronic bronchitis is basically too much of a good thing. So we have over production of mucous, such that the body cannot clear it too much of a good thing, that's when we have chronic bronchitis. And of course itis just means it's being irritated, and bronch refers to the lungs. So chronic inflammation and irritation of the lungs. Alrighty, let's draw some airway. So we have our trachea, the big stem bronchus branch off into their branches. Here you go, just keep going. There are 20 or 30 branches to this, you get the idea, they keep getting smaller and smaller. It's how our lungs expand. And as you know oxygen goes into our lungs and carbon dioxide from the body gets out. So you have irritants in there flooding what is usually a clear path, then the lungs are not going to like that. One of the most common chronic irritants that we have in our lungs is of course cigarette smoking. So, here we go, we're going to light this cigarette. And where you see a smoke rising from the cigarette is actually billions and billions of particles that can enter and lodge into our lungs. As a response the glands in the side of the walls, they start secreting the mucous. Remember it's part of our normal response to irritation. But of course chronics, we're talking about years and years of constant. Everyday it's like this. Now eventually, what's suppose to be a clear pathway becomes convoluted with the mucous. It's like, if you're drinking through a straw from a cup, instead of having a clear straw, you're sucking on a milkshake. So this milkshake is thick and it's clogging the straw. So you'll have to suck harder and work harder to get through. Likewise, with the mucous, sometimes there's so much build up that it can clog off one little branch of the lungs. So no more gas exchange can take place here and depending on the severity of the disease you can have different areas of the lungs being out of commission because of the mucous. Now of course we always talk about smoking, but there are other things that can lead to chronic bronchitis. One important thing is pollution. Again pollutants are also billions of particles in the air all the time. In some countries that's especially a big problem and chronic bronchitis is very common there. Or we can have coal dust, for people who work in coal mines, or people who work with asbestos. Shipyard workers. The common theme here is it happens when we can inhale irritant and it happens everyday. Now if I just smoke for a few days and I have a lot of mucous, that doesn't qualify me for having chronic bronchitis. The pathophysiology here talks about the overgrowth of the glands. So we actually look for tissue change. Now in a patient, to diagnose chronic bronchitis is usually a clinical diagnosis, which means there are no test, it's just from the story. But we know from examining lungs of people who have passed away, the tissue change actually happens. So I just blew up one branch of the airway here. We'll call this the lumen. The lumen just means the inside of the tube. So the lumen is here, where the air is getting through. So the black space on both sides are the walls of the bronchi. So basically if we take a cross section of our airway, in the middle here is the lumen. And right now I'm going to talk about the wall space of the bronchi. Okay so we have some smooth muscle around the walls. Again this spasm in this is what gives us the cough reflex. And right near the surface close to the lumen we have these layers of glands. They're responsible for the mucous production we've been talking about. As you can see they are right under the surface. The job is to secrete things into the lumen. Okay just for completeness let me draw some irritants in the lumen. Okay. So that's what a normal wall usually looks like. There is a way for us to put a number on this. And we only do this, we've only seen this in people who have passed away and their lungs get caught up in the lab, and we can look under the microscope to see the Reid index. Which is a way to calculate the ratio of the glands, the thickness of the glands to the whole wall. Now normal Reid index is less than 40%. So we can see the thickness here versus the thickness of this whole wall. It should be less than 40% in you or me. And of course in the middle here there are other things like cartilage or connective tissue. We're just going to fill it in a little bit. Now what happens in chronic bronchitis is, the actual thickness of the gland layer grows. So the glands start proliferating, because we need so much mucous that the original number is not enough, so the body is recruited to make more glands and they extend towards the smooth muscle. So in chronic bronchitis, the Reid index is usually over 50%. So definitely more than half of the thickness of the whole wall is made of glands. Now remember we cannot do this in living people because we have to cut away their lungs. So just a way for us to put a number on this disease, but usually the patients are diagnosed and treated according to their symptoms. And the symptoms all come from this tissue change and the over production of mucous.