 When chronic obstructive pulmonary disease, we always have to distinguish between two different pathophysiological mechanisms that are going on here. One of them is emphysema and the other one, chronic bronchitis. So let's look at emphysema first. So emphysema is caused by in-health pollutants and most commonly that is smoke or any kind of other environmental pollutants such as smoke, living close to factories, a lot of agriculture, those kind of things. And these in-health pollutants over time destroyed the elastin that the alveoli are made of. Therefore, when the elastin is destroyed, the alveoli lose their recoil ability after exhalation. So basically what that means, I've drawn it out here. So you have the airway and on the alveoli at the end and typically the air comes in, the O2 comes in, gas exchange happens at the alveoli and then CO2 is exchanged. Well when the alveoli here lose their elasticity, that elastic recoil, they're kind of a little bit more distended to begin with and because they can't really kind of contract as they're trying to get rid of the CO2, they're not able to exhale or eliminate the CO2. And that means that the CO2 gets trapped here in the alveoli. And that's right here, the small airways then collapse and leading to alveolar distension and that air trapping that I just explained over here. Which means that the CO2 can't exit and the O2 can't enter and therefore the patient will become hypoxemic, so less oxygen concentration in the blood. In comparison chronic bronchitis is also a chronic obstructive pulmonary disorder, but it works a little bit differently. The causative organism or mechanism again is chronic exposure to smoke with the environment. But in chronic bronchitis, the bronchi and bronchioles get inflamed. Now this inflammation as we know typically anywhere, especially in the lungs causes increase in mucus production and therefore the airway walls thicken causing an airway obstruction. So now we're having the problem where the air also can't escape because of the mucus and the thickened airway walls. And a chronic bronchitis diagnosis is usually made if there's a daily productive cough for three months for two consecutive years. So that's a long time exposure to any of these pollutants or smoke that cause all these things going on over time. And keep in mind both of these are chronic diseases. So nothing happens really quickly here. This is a chronic exposure and these changes happen very slowly over time.