You're almost certainly very familiar with this sound, that sound of the heartbeat. Very familiar. Lub dub, lub dub, lub dub. It is the heart. Now we want to talk about what causes those sounds in some detail here, and we're doing it now because it directly relates to the cardiac cycle and how pressure changes in the ventricle affect specifically the valves. So we're going to start out by saying that the heartbeat sounds are created when valves close, and that's important. Some people have a misconception that the blood going through the heart that you hear, or it's the valves opening and closing that you hear. Really, when you listen to a heartbeat, you're really listening to those valves closing. Now to understand why that is, I really like that analogy of a door. These valves are very much the doors to the ventricle. Well, as I walk through a door, if I open the door, it doesn't make a lot of noise. As I walk through it, not much noise there. As I slam it behind me, that makes a lot of noise. The same thing with these valves. As blood pushes them open, that's pretty quiet. As blood flows through them, that's typically quiet. But as the change in pressure causes these valves to slam shut, that you can hear.
Alright. So that means there's going to be 2 basic heart sounds because there's 2 times in the cardiac cycle when valves close. Now to remind us of this, we have these stages of a cardiac cycle, written out here sort of in this circle. And for each one, we have an image of the heart to remind you what's going on during that stage. So the first heart sound, let's start talking about ventricular filling. Remember, in ventricular filling, the pressure in the ventricle is going to be lower than the pressure in the atria. That means that that atrioventricular valve, that AV valve is open and blood is moving from the atria into the ventricle. Well, as we go from ventricular filling over to isovolumetric contraction over here, well, that involves the ventricles starting to contract. They're going to go into systole, that systole squeeze. Now that is a really rapid and fast change in pressure, where the pressure in the ventricle shoots up. So if the pressure in the ventricle shoots up, that's going to cause that AV valve to slam shut. So that first heart sound, what we often call the lub in that lub dub, we're going to say here is the atrioventricular valves, that mitral valve and the tricuspid valve, it's when those valves close. And the atrioventricular valves, when especially when compared to the semilunar valves, they're kind of bigger, heavier valves. And this pressure change that happens here from the time you're going in ventricular filling when the, when the ventricle's in diastole and then it goes into systole, that's a really fast and dramatic change in pressure. So you want to think about this. These are kind of big, heavy doors that you're slamming shut with a lot of force. So that means that this sound is going to be louder and a little longer lasting than the second heart sound.
Alright. With that in mind, when we go from isovolumetric contraction to ventricular ejection here, well, that does involve valves. But here, the pressure in the ventricle is going to raise enough that it's going to push open those semilunar valves. Well, remember, as you push open a valve, you don't really hear anything. As you get into ventricular ejection, that means that blood is flowing out of the heart through those semilunar valves. Again, going through a door doesn't make a lot of sound. But then as you go from ventricular ejection and you go over to isovolumetric relaxation, well, that's going to involve the closing of those semilunar valves. So that second sound, what we often call the dub, not lub dub, this is going to be the semilunar valves closing. And these semilunar valves close when the pressure in the ventricles drops down below the pressure in the arteries. Now that change in pressure in the ventricles as it goes from systole into diastole, it's not as fast and as dramatic. It just sort of starts relaxing. And so it's not as big a pressure change or as rapid a pressure change. And those valves, again, they're kinda smaller, so that's why that dub is usually heard as sort of a softer, shorter sound than the first heart sound, that lub.
Just to finish out our cardiac cycle here, we go from isovolumetric relaxation to ventricular filling. To go from one to the other there, well, now the pressure in the ventricle just falls enough that it falls below the pressure in the atrium. That means that that AV valve gets pushed open. Again, you can't really hear a valve open. And then during ventricular filling, blood is flowing from the atrium into the ventricle. You can't really hear that again. So now I'm going to play these heart sounds again, and as you listen to it, really you think we are hearing the atrioventricular valve close, then the semilunar valve. That's the lub followed by the dub. Lub dub, lub dub, lub dub. All right. Of course, people don't listen to a heart, though, to hear a perfectly healthy heart. It's always nice when you do. But the reason your doctor takes out a stethoscope and listens is to see if there's something wrong. So let's talk about the basic things you can hear when you're listening to a heart. So the regularity and the timing of those heart sounds can really help indicate different heart valve pathologies. Now just to be clear, a doctor is listening to more than just for the valves, but if we're listening to the valves closing, that's the main sound, if there's a problem with those heart sounds, it usually indicates something that is wrong with the valves. Now when there is something wrong, we usually call that a murmur, and a murmur is just a sound created by a turbulent flow of blood. Now typically, again, we said that the blood going through, you can't really hear it because it's, you know, going through in the direction it's supposed to go. No problems. But sometimes you'll hear something like a, Well, that whoosh at the end might indicate something like those semilunar valves aren't closing all the way. You get a turbulent flow of blood as some blood flows backwards through the semilunar valves. Similarly, you might hear something like a, That push after the lub, well, that would indicate something wrong with the AV valves. Now it is important to note heart murmurs can be serious, but often they are harmless. And it takes someone who is trained listening to the heart and even other tests sometimes to know the difference. We call a harmless heart murmur an innocent murmur, and it can be caused by lots of things. Even just becoming dehydrated can temporarily cause your blood flow to be more turbulent and can cause a heart murmur. It's especially true though in children. Now, children can have very serious heart valve problems. I don't want to give you the idea that they can't, but because children's hearts are smaller, they're not as tough, they're not as robust, there tends to be just more vibration and turbulence going on. So oftentimes, children will have minor heart murmurs that we call innocent murmurs that eventually they grow out of.
Alright. Those are the heart sounds. We're going to one more time go through all of this cardiac cycle, trying to tie together everything that we've learned about it. We'll do that coming up. It's going to be fun. I'll see you there.