Hypoxic pulmonary vasoconstriction

I want to start us out by orienting us to what you see here. We've got a couple of lungs here-- the left lung and the right lung. And we have a heart at the bottom, right? And specifically I actually divided this up into the four chambers of the heart. And I'm going to show you the four chambers. This is the right atrium. This is the right ventricle. Then we have the left atrium and the left ventricle. So these are the four chambers of the heart. And I kind of cut away a lot of the stuff that comes into and out of the heart, a lot of the vessels, because I want to highlight one particular vessel. I'm actually just going to label it for you. And it's the pulmonary artery. And I've drawn it in blue just to kind of point out the fact that it's full of blood that has no oxygen in it. But it's called an artery, you remember, because arteries take blood away from the heart. So this is the pulmonary artery right here. And of course there's a left and right pulmonary artery. This would be the right pulmonary artery. And this would be the left pulmonary artery. It's actually kind of difficult to say quickly. You can see I'm tripping over the words a little bit. In any case, so that's the way the blood goes out of the right ventricle. And let's say about five liters-- I'm just going to label it right here, five liters per minute. So a lot of blood is kind of gushing through that pulmonary artery going to the right and left lung. And let's say we have some blood kind of going up this way into the left long and some blood going this way into the right lung. And this is kind of a normal thing that's happening. Now, let's say you're eating some food. Let's say you're eating some peanuts, and you accidentally choke on one of the peanuts. So obviously this would be a terrible thing that would happen, but let's say you choke on a peanut. And that peanut kind of goes down this way. And it has to either go down to the right lung or the left lung through what we call the main bronchus, right? So is it going to go down the left main bronchus or the right main bronchus? And just by looking at it you might remember that gravity is going to push more towards the right main bronchus. So things kind of have a tendency of getting stuck on the right main bronchus a little bit more just because it's more vertical. You can see the shape-- it's going to attract more things like food. And so if a peanut get stuck there, our question or my question is, what would happen next? So let me actually have you put on your x-ray goggles, and let's see if you can actually-- I'm going to kind of just clear up some of this stuff, and see if we can kind of see what would happen in our lung. I'm actually just going to clear out both sides and reveal to you what things might look like if you could look inside of them. So you can see there's a little alveoli here, right? That's the first thing I want you to notice. This is a little alveoli. And I'm just going to label it on this side. But you can see both pictures are kind of the same. And we have a pulmonary arterial and a capillary. So this purple one is a capillary. And I drew it in purple just to kind of let you know that gas exchange is happening. So some of the carbon dioxide is leaving, and some of the oxygen is kind of getting into the blood at that point. So it's kind of a purplish color or that's kind of how we think of it anyway. And right before the capillary, the blood again is kind of coming this way. I should do it with a white line. Blood is going that way. Right before the capillary is the arterials. Let me actually write that in here. This is the arterial or pulmonary arterial-- you might hear that phrase as well. And all that means is kind of the arterial in the lungs. So this is the arterial and the capillary that are coming up very near an alveoli. And in our peanut situation, what's happening? Well, our left lung is actually doing pretty well, right? It's pretty happy. This little alveoli is really happy because it's full of oxygen. And that's kind of the key idea I want to present today is that there's a difference in the amount of oxygen that's getting into the lungs and, of course, all the alveoli within the lungs, right? So in the right lung, what's happening? Well, this alveoli is not too happy at all. Not too happy because there's very little oxygen getting in there. And when little oxygen gets into the alveoli, when there's not too much oxygen there, an interesting thing happens. And I'm actually just going to kind of show you using this arterial. This arterial has a lot of smooth muscle, and this smooth muscle, it can tighten down. Like any muscle, it can actually contract. What happens is that instead of being this nice large arterial, because the smooth muscle starts to contract down-- and remember, the reason that's contracting down, I should point this out, is that there's actually a little signal that gets sent from the alveoli's low oxygen. Because there's low oxygen in there, a signal gets sent. And this is actually a signal that is heavily researched upon exactly how it works. So suffice to say, there is a signal. And this little arterial gets a little smaller. So the size of the tube, if you think of it as a tube, is now kind of tinier than it was before. And so blood is still going through, but obviously there's a lot more resistance. So really the big change is that the alveoli had very little oxygen, it sent a signal, and as a result of the signal, the size of that arterial got smaller. And because we know that when size goes down resistance goes up-- I'm going to write increase resistance here. So basically, the amount of resistance goes way up as a result of having very little oxygen in that area. So you might be thinking, well, that's not a huge deal, right? Because this is just one little alveoli and who cares if a little resistance goes up. Will that really affect anything? And the truth is that it does. It really does. Because remember there isn't just one alveoli having this problem, you have about 250 million alveoli-- let's say about that many in the right lung. And let's say a very similar number of alveoli in the left lung. So you have these large numbers of alveoli all having kind of similar problems. And as a result, what happens is that it's not just one little unhappy face on this right lung. You actually have millions of them. I can't really draw millions. But you get the idea that this entire lung is really without oxygen. It's really not doing so well. And on the other side, things are actually really, really awesome, right? This side, the alveoli are really happy because they're full of oxygen. They're doing really well. So things are good on the left side, but not on the right. And if all these alveoli are doing the exact same kind of trick, then the resistance is going to go up in this vessel. So this vessel right here, the right pulmonary artery, that vessel is actually going to have lots of resistance, lots and lots of resistance. And as a result, if blood has a choice-- and of course, it does, right? In a sense, it's not thinking but, of course, it has a choice in terms of whether to go to the right or the left. Now, a lot more blood is going to go to the left because it's going to say, why the heck would I go to the right when there's all that resistance over there? It's going to go to the left. So you have a lot more blood coming out of the pulmonary artery on the left and a lot less blood going to the right pulmonary artery. So if you were to think about it in terms of blood flow, flow goes up. In this lung, blood flow goes up. And similarly you could also say, well, obviously it's not like the amount of tissue on the left or right lung changed. So if there's more blood flow, there's also going to be more perfusion. So you'll often hear this word, perfusion. And that really refers to the idea that there's more blood, you could say, perfusing the left lung. Now, this whole trick, the idea of oxygen going down and blood kind of as a result going to the opposite lung-- there's a name for this trick. I'm going to write it out here. It's called hypoxic, which just means low oxygen. Hypoxic-- pulmonary, which of course just refers to the lungs because this trick is happening in the lungs, hypoxic pulmonary vasoconstriction. Remember, we said vasoconstriction just means kind of making the blood vessels smaller. So it's kind of a fancy name, hypoxic pulmonary vasoconstriction, but all it means is kind of what we described happening in this side where the alveoli has very little oxygen and as a result it sends a signal out to the arterioles to tighten down. Resistance goes up and blood goes flowing the other way. So an easy way to remember this is I always think of blood chasing oxygen. You can think of it that way. And it makes it kind of an easy idea to remember if you think of it in these terms.