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Video transcript
So you're pretty familiar with this image of the heart. We have the four chambers. And I'm just going to start by labeling them-- the right atrium and right ventricle on this side and the left atrium and left ventricle on this side. And the question is this. What happens if we choose two spots? I'm going to choose this one right here, with the little x in left ventricle and some other spot over here in the aorta, let's say, with the purple x. And this is, of course, our aorta up here. What happens if I follow the pressure at those two spots? So of course, I was going to say maybe if I'm the red blood cell sitting there, but we know red blood cells move around. But let's say I'm just following the pressure at those two spots, those locations, over time. So this will be time over here. What does the pressure look like at that location? And let's say I'm following for, let's say, one second. So, if my heart rate was somewhere like 60 beats a minute, that would basically mean one second would be one beat. This is 60 beats a minute. And just keep in mind, 60 beats a minute is actually pretty low. So that would be like if I'm reading a book or sitting around relaxing. And on this side, let's do pressure. And I'll do 0 to, let's say, 100 up here. And remember, the units of pressure are millimeters of mercury. Now actually, before I start, let me even jump into some naming, just so we don't have to stop later. These are the valves. This is the mitral valve. And below it, I've drawn the tricuspid valve. And put together, we would call these the AtrioVentricular valves. This is the AtrioVentricular valves. And AtrioVentricular, I've capitalized A and V because sometimes you'll see the word AV valves. AtrioVentricular valves are also known as AV valves. And the other two valves, here, this is the pulmonary valve down here. And on the other side of it right here is the aortic valve. Put together, we would call these the semilunar valves. And actually, semilunar, you might think of it as like a half moon. But there's no shorthand for that, usually. So people just call them the semilunar valves. So what does the pressure tracing look like at my yellow x? Let's go back to that question. So I'll start out here on the axis. So it starts out really low. Left ventricular pressure is surprisingly low most of the time. And you'll actually see that now when I draw it out. You know, I always think of it as this chamber that's cranking and pushing and high pressure. And that's true. There is some of the points along the way when the pressure gets very high. But for most of the time, it's actually pretty low. And it creeps along, goes up very slowly. Well, why is it going up at all? That's the first question. Why doesn't it just stay steady? Whatever pressure it is, why is it going up? Well, it's going up because there's actually flow here through the mitral valve. So blood is going into the left ventricle initially, and when there's more blood in that chamber, over time, the pressure slowly builds up. Just like if I'm pouring water into a water balloon, over time, every little bit more water I put in there, the pressure in the balloon goes up. So, that's why it's creeping up. And then you get to a point where all of the sudden, there's a muscle contraction. So you have a depolarization wave that comes through, and all of this heart muscle is cranking, just pushing in. And of course, simultaneously, the right ventricle is doing the exact same thing. So all of the things that I'm saying for the left ventricle, for the most part apply also on the other side. So they're contracting. And the left ventricle contracts hard, and the pressure begins to rise. Now just right there, just right where it begins to rise, you might say, well, what did you even do? And at that moment where I drew a slight increase, a tiny little increase-- you can see it if you squint your eyes-- at that moment, this valve closes. Why? Because at that moment, at that very moment, there is a little bit of push back here. And the slightest bit of push back makes the valve shut. So once the pressure on the left ventricle side is greater than the pressure on the left atrium side, the valve shuts. And so the valve closes at that point. And then you have a rise, a rapid rise, in the left ventricular pressure. And it goes up, up, up, like that. So let me just write that down since I just said it. The AV valves close. And that's a new thing, because they were open. So let me box that. AV valves close. And the semilunar valves, the other ones, they stay open. Semilunar stays open-- nothing new there. Actually, sorry, I said open, and I'm even writing open. But I mean close. Sorry. So let me not confuse you and just change that right now. Semilunar valves stay shut. And to drive home that message, I'll even put a block there. So they're still shut. So all the valves at this point are now closed. And that might be news to you. You might have thought, well, I thought at some point, some valves were always open. And that's not true. At this point, both valves are closed. And the left ventricle is basically squeezing. But the blood has really nowhere to go. It's just sitting in a trapped room, and the pressure is rising and rising fast. And then at some point, it gets to a spot where an interesting thing occurs. So let me actually draw that here. Let me erase this slightly. Let's draw the aortic pressure. So the aortic pressure, let's say, is something like this. And it's drifting down. Well, why is aortic pressure drifting down? Well, it's because in the aorta, blood is rushing away. Remember, from the previous squeeze, you've got blood rushing away, out all these vessels. It's going away. And as blood rushes away, of course, pressure is going to fall. Remember, there is a relationship between having more blood volume and pressure. So as the blood volume in the aorta drifts away, the pressure drifts down. And this aortic pressure is rising. And at that spot-- it's hard to see, again. I'm going to just circle it to draw your attention to it. But at that spot, you'll see the aortic pressure is slightly below left ventricular pressure. And because the pressure is slightly below, all of the sudden, this is no longer blocked. Now you've got a free path here. This valve opens up, and blood can come in. So new blood can come in from the left ventricle. So blood starts rushing in. And of course, the left ventricle is still contracting, so it still continues to rise. And it gets to about there. And the aortic pressure is going to rise. And so let me draw that. Actually, I might have switched colors, but you'll forgive me. It starts to rise. And it follows the course of the left ventricle. So basically, it's rising with the left ventricular pressure because there's a continuous space there. There's no valve between the two. So what happened at this spot exactly? Let's just recap it. Well, the AV valve is still closed. Nothing has changed there. But the semilunar valve opened. And that's the interesting new thing. That's the cool, new thing that allowed the blood to go from the left ventricle out into the aorta. And now what happens? Well at some point, all this contraction I've drawn relaxes. It finally goes away. It goes away on both sides. And all these black arrows, I'm going to erase. And this muscle now, instead of depolarizing, begins to re-polarize. Now, why didn't I erase that last black arrow? Because again, there is some pressure in the left ventricle. And in fact, looking at my graph, you can see there's not just some, but there's a lot of pressure still in there. So all that's changed is that the muscle is now relaxing. And if the muscle is relaxing, then this yellow line begins to drift down. And if the left ventricular pressure drifts down, so does the aortic pressure. That drifts down too. Well at some point, what will happen? Well, the aortic pressure is still high. And think about this. This is actually a tricky point. It's still high because there's resistance. Remember, there's resistance from all of the blood vessels. There's resistance here. All these blood vessels are offering lots of resistance. So, with all this resistance from the blood vessels, the aortic pressure stays high. And the left ventricular pressure is relaxing. It's drifting back down. And so this arrow, instead of blood just going one way, there's a little bit of pressure coming back the other way. There's a little bit of pressure going this way, this way, and this way. And there is some pressure this way still because the left ventricle still has some pressure. So they're matched. There's pressure coming from the aorta and also pressure coming from the left ventricle. Initially, the left ventricle just overwhelmed the amount of pressure in the aorta. But as it's drifting back down, now that aortic pressure is matching the left ventricular pressure. And at this particular moment, that left ventricular pressure is going to be lower than the aortic pressure, something like that-- an interesting cross-over. And that's, again, because the aortic pressure stays high because of all that resistance, but the left ventricular pressure continues to drift down because it's relaxing. And the moment that the pressure in the aorta is greater than the pressure in the left ventricle, this shuts down again. So that valve slams shut. And so at that point, what would we say? Well we say, well, the semilunar valve closed, and the AV valve opened. Sorry, I keep saying that, and I apologize. The AV valve is still closed. I didn't mention anything with the AV valve. That's still closed, as it has been for the last two points on our graph. In fact, let me just label this point one, point two, point three. So at this point, what's the next thing that's going to happen? Well, the left ventricle continues to relax, and it goes all the way down through my word, semilunar closed, all the way down almost to 0. And before it gets there, before it gets all the way to 0, the pressure's so darn low in here that now, left atrial pressure is actually higher than left ventricular pressure. And blood can flow back through. So if blood can flow back through, then we know that as blood fills up a ventricle, the pressure continues to slowly rise over time. And it gets to about there, which is the same spot that we began at. And meanwhile, the aorta continues to drift down because, just as in the beginning, we said well, when time passes, blood drifts down through to all the vessels. So blood is now drifting away, as it did before, into all the vessels. And as it drifts away, the pressure in the aorta drifts back down again. And it goes something like that. Actually I guess further, because my second is not there-- something like that. And you have to assume, based on my drawing, that these two points are the same. And if they're not, then I haven't drawn it correctly. So that's what the aorta does at the end. So let me just label this now, this last point here, this point four. Sorry, before I get to point four, point three, the thing that was interesting and new-- I should just box it. I'm just trying to box the things I want to draw your attention to-- is that the semilunar valve closed. So what is happening at point four? Now at point four-- a couple things. The AV valve now finally opens. I said that prematurely before. And that's the new thing. The AV valve opened. And the semilunar valve is still closed. So that did not change. So this is exactly what happens at the four stages, the four important points. So, just to recap-- and actually, as I recap, let me mention something new that's interesting as well. Remember that when valves close, they usually make noise. So it's like a slamming of the door. So when a valve shuts, it makes noise. And so you have to look on these four points, when does a valve close for the first time? Well, the AV valve closes right here. This is the point where it first closed. So when the AV valve closes, it makes a loud thwack, a loud noise. In fact, sometimes people call it a lub. You hear that lub noise if you listen to your heart. And that's the first heart sound because that door just shut. And you can see, based on the pressure differences, it's really closing because the left ventricular pressure is going up so fast. So when you hear that first lub-dub, when you hear that lub, that's also the beginning of the high pressure, the contraction of the left ventricle. Now point two, does any door close? Does any valve shut? Not really. The semilunar valve opens, but opening of a valve doesn't really cause any noise. At point three, the semilunar valve closes. So that's where the aortic valve and pulmonary valve slam shut. So again, you get some noise up here. This causes noise. So at point three, if I was to draw a straight line down here, you'd get some noise. I'm going to cover up my number four, but that's OK. You know where it is. And this is the second noise. And it's coming from the closing of the semilunar valve. And then, point four, the AV valve opening, well, opening doesn't create noise, and the semilunar valve is still shut. So really, the two sounds, the heart sounds, S1, S2, come from the closing of the AV valves initially, the mitral and tricuspid. And then the dub comes from the closing of the aortic and pulmonary valves.