Hypoxic vasoconstriction

- [Voiceover] Our lungs have this neat little trick. It's called hypoxic vasoconstriction. It's basically a way of describing how the lungs behave to maximize their own function. I've drawn you this little diagram here to show you how the blood vessel system interacts with the air spaces in the lungs. This little phenomena actually have to do with how they interact, as well. The hypoxic part is referring to the fact that it's low in O2. So the lungs' job is to carry O2, or oxygen, to our bodies, right? When it's low in oxygen, it's hypoxic. Vasoconstriction is basically a stricture. If you have your vessel going like this, vasoconstriction is when they get restricted and the diameter actually decreases. On this diagram, the green thing is a terminal unit in the lung. This is an alveoulus. Air comes in here. This is where gas exchange happens. The air in here is higher in oxygen content. So the oxygen goes from here into the blood and, at the same time, the carbon dioxide, which is the waste that the blood is carrying to the heart and lungs, it gets into our lungs, then gets breathed out. The hypoxic part is really referring to what happens in this green air space. For some reason, that has to do with the lungs were low in oxygen there. Then the vasoconstriction has to do with the blood vessel system. Do you see how it kind of flows around the lung tissue? Like I said before, it's a cause and effect relationship because of hypoxia. Something happens to our blood vessel. First let's look at how this usually works. If this is a very healthy lung, healthy blood vessel, all the airways are open, it would filled with air in both these sacs. We'd have deoxygenated blood flowing in here. My blue stream is going to be the deoxygenated blood flow. Carbon dioxide. It flows both ways. The fork is going to come in down here. This is going to be a gradient, but I'm going to have to draw it roughly because I don't know how to draw a gradient. Now the gas exchange is happening between this air sac and the blood vessel. And as the blood flows through, it eventually becomes red, full of oxygen. At this point, it's going to be going back into the heart to be pumped to the body. This is how the normal relationship works. I'm going to erase everything I just drew. Let's look at what happens if one of these air sacs is out of commission. I'm going to block off this side. Now, as the blood is coming in here, the lungs realize that, if you go to this side, there's not going to be much gas exchange. It realizes that there's hypoxia within this little pocket. And the body literally comes in here at this point and restricts this path leading to the hypoxic area of the lung. This changes the diameter of the lumen this side to be much smaller, and blood is directed, or redirected, I should say, more of it over to this side. So it's now, it's going to be mostly on this side. Here, assuming this airway's still nice and open, we still get the same thing. Blood picks up oxygen, becomes red, and it flows out. On this side, of course, some blood still gets through. It's not going to be closed off completely. But the blood coming this way is going to be significantly less because there's higher pressure. Blood, remember, wants to go the path of lowest resistance. So it goes like that. We see that this actually maximizes the amount of oxygen we can still deliver, because we've shunted more of the blood over to the side that works better. This here, the color might be a little purplish, because we have some mixing with the blue blood, but it's still mostly red. The restriction here allows us to stay red like this by giving most of the blood over to the side that works. I've drawn this in a very zoomed-in scale, showing you how this happens in one alveolus versus another. But this can be more global. I'll just draw a pair of lungs really quickly. I have the trachea leading to the main stem bronchi. The lungs arise like that. We have our left and right sides. Yep. Now remember that the blood supply follows the path of the airway. Just like the airway branches, the blood supply does, too. It goes into millions of different branches. Same on both sides. Like we said before, because the blood supply is distributed according to how well that area is receiving oxygen, we might see a lot of unevenness on a global scale. Let's say down here, there's good air, all these branches get a lot of blood. I'll fill that in. Looks like chicken feet. But for some reason, on this side of the lung, up here, we're not getting a lot of air. So the blood supply will be thinner here. There's restriction of the vessels. Same with this side. It might be completely different. Where the hypoxia is depends on what's causing it. Maybe down here, there's not as much good blood. So it's thinner. Now, this whole mechanism was intended to be an emergency way of dealing with sudden hypoxia, or at least, it's a temporary measure. It also assumes that only some areas of the lungs are affected. But what happens if you have a condition that affects all the areas of the lungs, if there's global, chronic hypoxic vasoconstriction? Think about it. Then a lot of the vessels would be affected, and it'll be constricted day after day, month after month. Over time, this will give us a new condition. You might have guessed this is where we're going. We're going to get pulmonary hypertension, because the blood pressure in these vessels are so elevated all the time that it becomes a condition on its own. Remember that this tends to happen when it becomes a chronic problem and also when the restriction, the hypoxia we're talking about, tends to be global, meaning not just in one alveolus but in larger sections of the lungs. Because if there's not good air anywhere, then everywhere we're going to have vasoconstriction. That in itself directly leads to hypertension. When it's so chronic, and tension doesn't come back down, then we have a pulmonary hypertension chronic problem on our hands. What can lead to low oxygen hypoxia everywhere? Again, we come to some of the usual pulmonary problems, emphysema, as an example, or chronic bronchitis, a lot of the diseases that can result from smoking or chronic exposure to irritants. The lungs become less effective at moving air. We're hypoxic everywhere. Another one might be long exposure to high altitudes. When the altitude is high, the air is thinner. It would literally have less oxygen in the air. So hypoxia is going to be there all the time. The other long chronic diseases, we have asthma. A lot of people who have asthma eventually develop pulmonary hypertension. These are just some examples to show you that hypoxic vasoconstriction is basically a pathophysiology, or a story of how lung problems that had nothing to do with the blood vessels in the first place can eventually cause a blood vessel problem in the lungs. It will lead to a whole host of other problems that we'll call pulmonary hypertension.