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So we've talked about the pressure in the left ventricle and the aorta. And I thought it would be fun to sketch it out again. But I wanted to go into some details, because when we did it last time, I brought up some issues but quickly glossed over them. And you may be wondering about them now. So I thought we would get into some of the details and finer points of the pressure in the heart, the left side of the heart. So you know we usually think of pressure in millimeters of mercury. So that's why I want to just write that out so it's clear. And I'm going to sketch out the pressure tracing we said happens with the aorta and the left ventricle. So first, the left ventricle. We said that the pressure rises slowly, and then eventually, the left ventricle is going to contract really hard. And that's the real cause of why the pressure rises abruptly, and then it relaxes. And eventually as it relaxes more, it falls. And then blood falls into that chamber from the left atrium, and that's why the pressure goes back up again. So that's a very quick way of thinking about the left ventricle. And then of course, you have the aorta. And I'm actually going to do the aorta a little bit more slowly. The aorta, of course, there's blood leaving the aorta, going to different parts of the body, and then let's stop it right there and now talk about exactly what happens when the pressure of the aorta and the left ventricle cross. So let me draw for you, I'm going to draw in the side what this looks like in the body. So you've got the left atrium here. And then you've got the left ventricle coming off, and it is enormous. So just draw it that way. And then there's the aorta, also a large vessel, coming off of the left ventricle. And the aorta wraps around and goes down, but it leaves a few branches. One there, one there, and let's draw a third one. So classically, three branches coming off of the arch. We call that the arch of the aorta. And then that's the aorta there. So let me label this stuff. This is my aorta, this is my left atrium, and my left ventricle. So what is happening? I'm going to draw a little blue arrow then to start us off. What is happening right there? Well at that point, we know that we have a couple valves. This is, let's say, our mitral valve here. And I'll label that mitral. And while I'm at it, I'll draw in the aortic valve here. And this is my aortic valve. So these are my two valves. The mitral valve is closed. And remember, it closed actually a long time ago. I shouldn't say a long time ago, because not a lot of time has passed between these two points. But it closed over there. So if I'm going to show it closing, I'm going to have to block it off, just to make it very clear that it's closed. Really, no blood can pass through that gate. So, we're in the middle of contraction. And I'm going to draw pressure with yellow arrows. So you've got a lot of pressure coming in here because of the contraction of the left ventricle. A lot of contraction there, a lot of pressure. And the question is, is the aortic valve opened? Well, if I'm talking about my blue arrow on my figure, well, not really. It's still hasn't gotten the same pressure that the aorta has. So the valve is still closed. So really, both valves are closed, and pressure is building in the left ventricle. Now, as the left ventricle pressure increases even more and rises, let's say we get to this point. Well, all of a sudden, now the pressure is greater than the aorta. So let's actually show that by drawing a little opening there. So now, that valve opens. And you actually have blood flowing this way. And actually, of course, it flows into all of these different places. So it starts flowing through the aorta, through all the vessels. But one thing that often is forgotten is that the aorta is actually compliant. Let's actually write that out, because that's the big key for understanding how the aortic pressure tracing looks. So compliance. And what does compliance mean? Basically, I think of it as stretchability. So if you can actually stretch these vessels, and you can, then you can begin thinking about them almost like you would think about a balloon. They're actually stretchable, the vessels themselves. And so while there's blood rushing around, it's also pushing off into the walls. The walls themselves are going to stretch, because I said it's a little bit like a balloon. And really, what you get is you actually get these walls are no longer going to be as they look now. They're going to stretch out. You get something like this. You basically get stretched out vessels, something like that. So this is actually quite interesting because now it begins to look balloon-like. And what's actually stretching it out? Well, blood is stretching it out, right? It's not like you're using a puff of air or something, like you would for a balloon. Literally, blood is pushing into the walls and stretching them out. So you've got blood in here. That's what's causing the stretching. So while you're stretching the walls, you actually have increase of aortic pressure. So the aortic pressure starts to rise. And that's what the tracing looks like. So it rises to that point. So that's the most pressure that's in the aorta at any given time. So this is the point when the walls are actually maximally stretched. So this is when there's maximal stretching of the walls, right here. And then, you can see the left ventricle pressure starts falling. And as it falls, so does the aortic pressure, because it's all a continuous space. But as this is falling, as this starts to fall, what's happening? This is my question. What's happening to the walls? Well, at that moment when it begins to fall again, the walls, remember, they're like rubber bands. The moment that you put less pressure on them, they're going to start to recoil. Immediately, they start to recoil. So they start doing this. They're recoiling. And so you've got two things happening. As these yellow arrows in the aorta are being created, and those yellow arrows represent recoil, the yellow arrows in the ventricle are disappearing. Remember, that's contraction. So you have this passive process. We think of it as a passive process here. Passive recoil. Remember, that is this elastic energy. And I say passive because it's elastic and it's not chemical energy. And in the ventricle, you've got this active contraction. So I just want to contrast the two. Active contraction is using chemical energy. When the ventricle contracted, we were basically burning up a bunch of ATP. That's how we got that. So remember, that's ATP. When I say chemical, I'm talking about ATP. Whereas when I say passive recoil, it's passive because it's not using chemical energy, but it is using elastic energy. And that was actually stored up, remember? That was stored up energy. And when was it stored up? It was stored up when the left ventricle first forced blood into the aorta. So you stored up some energy, and now those rubber bands are recoiling. And so it's a very natural process, it makes sense. And so, while we're on the downhill of our pressure slope, the bands are recoiling. And now eventually what's happening is that pressure here is rising just slightly from the fact that you've got some recoil, and pressure here is falling quickly, because basically, now this process of contraction has gone away. On the down swing of the left ventricular pressure, these arrows go away. So at this point now, you've got slight increase in pressure in the aorta, slight decrease in pressure in the left ventricle. So what's going to happen? Well, at some point, the pressure in the aorta is actually going to slightly exceed the pressure in the left ventricle. So again, this point where you actually have more pressure in the aorta than the left ventricle. You might be like well, how can that ever be? Aren't they continuous? Right, they are continuous. But a different process is happening here than here. Here, you've got less pressure in the left ventricle, less pressure because the contraction has stopped. And in the aorta, you've got more pressure, because you've got recoil of the walls. So as the pressure gradient switches, now you've got more pressure in the aorta, what happens? Well, you've got a little bit of blood that pushes out this way and pushes out this way, because of course, the walls are coming back in. And the moment that there is slightly more pressure on the aortic side, this valve snaps shut. Aortic valve snaps shut. So now that valves snaps shut, and that's very interesting. And that's actually a very critical moment Let me actually make some space here. And this is all happening, of course, within fractions of a second. That valve snaps shut. And there's blood that literally wanted to go in, and basically gets the door slammed in its face. And it actually kind of recoils and goes back around. So it wanted to go towards the left ventricle. The aortic valve snapped shut and pushed that blood back into the aortic space. And of course, as all these blood snaps back, what's it going to do? It's going to push again. It's going to push back, just like it did the first time, out on those walls. So these pressure arrows reverse. And you actually get pressure going the other way because of this snap back. So you might think, well, that's such a small amount of blood. How can it make such a big difference? But it does. That tiny amount of blood that hits off of the aortic valve pushes back into the aorta and actually increases the pressure in the aorta. So, let me actually write that out very formally here. You actually have a snap back. That's just my own terminology of blood bouncing off the aortic valve. OK, so now it's really clear where that pressure's coming from. It's coming from the snap back of blood. And as a result, you actually have a slight pick up of pressure in the aorta. So now, blood is going to reenter the walls here. And now finally, you've got your aortic valve closed, and you've got blood in the walls and blood leaving, as it always has, through all these different routes. Leaving through all the vessels, going to the different parts of the body. So at the end of this, what's going to happen? Well eventually, all this blood is going to want to leave. And the walls will basically empty out. So all this blood will finally leave, empty out, and the walls go back to their original shapes. So let me just show them in their original shape, and the whole process can start over again. So, these walls go back to where they were when I first drew them, something like that. And this is, of course, due to the fact that the rubber bands are basically back in their natural position. And as that's happening, blood is draining off to the different parts of the body, and you basically get something like that. So, this point and this point are the same. So you really come full circle. And what you get here, what it looks like, is a notch, because pressure goes up here and it comes down here. So it basically looks like a little notch. Now before, I drew it just more straight. I didn't really draw that notch, because it was a little bit clearer to show the simpler way, but truly this is actually what happens. And this notch is called the dicrotic notch. So I'm going to draw this again for you with the dicrotic notch, and you'll see how it differs from the way we drew it before. The way we drew it before, this is the inaccurate way, was something like this. I said well, it goes down like that. The more accurate way would be to say it goes down, but then there's actually a notch that comes in there. So this increase, this difference, is because of the compliance of the aorta.