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Lub dub

Ever wonder why the heart sounds the way that it does? Opening and closing of heart valves makes the heart rhythm come alive with its lub dub beats... Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

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Video transcript

If you take a good long listen to your heart, you'll actually notice that it makes sounds. And those sounds are usually described as lub dub, lub dub, lub dub. And if you actually try to figure out what that would spell out like, usually it's something L- U- B, D- U- B. And it just repeats over and over and over. And to sort of figure out where those sounds come from, what I did is I took that diagram of the heart that we've been using and actually exaggerated the valves, made them really, really clear to see in this picture. And we'll use those valves to kind of talk through where those sounds are coming from. So let's start by labeling our heart. So we've got at the top, blood is coming into the right atrium and going to the right ventricle. It goes off to the lungs, comes back into the left atrium and then the left ventricle. So these are the chambers of our heart. Now, keep your eye on the valves. And we'll actually talk about them as the blood moves through. So let's start with blood going from the right atrium this way into the right ventricle. Now, at the same moment that blood is actually going from the right atrium to the right ventricle, blood is actually also going from the left atrium to the left ventricle. Now, you might think, well, how's that possible? How can blood be in two places at one time? But now remember that blood is constantly moving through the heart. So in a previous cycle, you actually had some blood that was coming back from the lungs, and that's what's dumping into the left ventricle. And in a new cycle, you have a bit of blood that's going from the right atrium to the right ventricle. So you have simultaneously two chambers that are full of blood-- the right and left ventricle. Now, to get the blood into those ventricles, the valves had to open. And specifically, let's label all the valves now. So here you have our tricuspid valve, and I'm going to label that as just a T. And then up here, you have the pulmonary valve, and this'll be just a P. And on the other side, you've got the mitral valve, which separates the left atrium from the left ventricle. And you've got the aortic valve. So these are the four valves of the heart. And as the blood is now in the ventricles, you can see that the tricuspid and the mitral valve are open. So far, so good. Now, I've actually drawn the pulmonary valve as being open. But is that really the case? And the answer is no, because what happens is that as blood is moving down from the right atrium to the right ventricle, let's say that-- and I'm going to draw it in black. Black arrows represent the bad or the wrong direction of flow. So let's say some blood is actually trying to go that way, which is not the way it should be going. What happens is that these two valves, they, based on their shape, are actually not-- they're going to jam up. They're going to basically just jam up like this, and they're not going to let the blood pass through. So this is what happens as that valve closes down. And the same thing happens on this side. Let's imagine you have some backwards flow of blood by accident, meaning that it's going in the wrong direction. Well, then these valves are going to close down. So the white arrows represent the correct flow of blood, and the black arrows represent the incorrect flow of blood. So these valves shut down like that. So now you can see how the valves, the aortic and pulmonary valve, are actually closed when the mitral and tricuspid valve are open. So what happens after this? So now our ventricles are full of blood, right? They're full of blood. And let's say they squeeze down, and they jettison all the blood into those arteries. Well, now you're going to have-- this is actually going to close down. Let's say this arrow flips around. These arrows become white, because the direction of flow is going to be in the direction we want it. It's going to go this way and this way And to allow that, of course, I need to show you that these open up. And they allow the blood to go the way that we want it to go, so now blood is going to flow through those two valves. But similar to before, you could have some backflow here. You could have backflow here. And you can have backflow here. So you can imagine now, let's say you have a little bit of backflow that wants to go this way, which is the wrong direction. Right? Well, then these valves are going to close up. They're going to say, no, you can't go that way. They're going to close right up, and they're going to not allow blood to go that way. So this is going to happen on both sides, both ventricles. And the valves shut. And so basically the backflow of blood is not allowed, because the valves keep shutting. And when the valves snap shut-- so for example, right now the tricuspid valve and the mitral valve snapped shut. Well, that makes a noise. So when T and M snap shut, that makes a noise that we call lub. That's that first noise, that first heart sound. In fact, sometimes people don't even call it lub dub. They say, well, it's the first heart sound. And to make that even shorter, sometimes people call that S1. So if you hear S1, you know they're talking about that same exact thing. And this dub is called the second heart sound. And, no surprise, just as before, if that's S1, this is S2. So you'll hear S1 when the tricuspid and mitral valve snap shut. So far, so good. But you also know that if that's what's making noise, you can kind of guess-- and it's a very smart guess-- that at the same time, the pulmonic valve and the aortic valve just opened. So if the other valves snap shut, these just opened. Right? You can kind of assume that, although the noise you're hearing is actually from here. So what's happening with dub? Well, the opposite. And what I mean by that is-- let me now show you what happens a moment later. Well, after the ventricles are done squeezing, then we get to a point where you might have a little bit of flow that way and that way, just as I drew before. And these valves snap shut as well. So now these snap shut. And as these snap shut-- because they don't want to allow backflow, right? They're going to snap shut like that. They make noise. And so when you have dub, you actually have noise coming from the pulmonic and aortic valve snapping shut. And that must mean that then the other two valves just opened up-- the tricuspid and mitral just opened. You can assume that, right? And I didn't draw that in the picture. Let me update my picture now to show that. So now these two have opened up, and blood is coming into the ventricles again. So it's actually a nice little rhythm that you get going. And every time these valves go open and shut, you hear noise. So you can kind of figure out what's happening based on-- and these actually-- let me erase that. And now you have white arrows going this way. And we've returned to where we started from. So you basically have a full cycle, and between these two-- so let's say from lub to dub, because there's a little bit of space there. If you were to follow it over time, over time, this is what it might look like if this is a little timeline. You might hear lub here, or the first heart sound. I'll just call it S1. And you might hear S2 here, the second heart sound. And then you'll hear S1 again over here and S2. And what's happening between the two-- so between these two, this time lag here-- is that blood is actually squeezing out, because the pulmonary and aortic valves just opened. It is squeezing out and going out to the whole body. So this is when blood is going to the body, and sometimes we call that systole. And between dub and the next lub-- so in this area right here-- well, at that point, blood is kind of refilling from the atriums into the ventricles, and we call that diastole. So now you can actually listen to your heart. And you can actually figure out, well, if you're listening to the sound between lub and dub or the space in time between lub and dub, that's when you're having systole. And if you're listening to or waiting for the sound to start up again-- so you just heard dub, and you're waiting for lub again-- then that space in time is diastole.