Cardiac Cycle - Online Tutor, Practice Problems & Exam Prep

The cardiac cycle consists of contraction (systole) and relaxation (diastole) phases that regulate blood flow through the heart. During systole, ventricular pressure rises, closing the atrioventricular (AV) valves and opening the semilunar valves, allowing blood ejection. Diastole sees pressure drop, closing semilunar valves and reopening AV valves for filling. Heart sounds, "lub" and "dub," arise from valve closures. Understanding these dynamics is crucial for grasping cardiovascular physiology and diagnosing conditions like heart murmurs, which indicate turbulent blood flow.

Systole, Diastole, Pressure, & Valves

Now I want to talk some about the cardiac cycle, and I'm just going to start by defining that. The cardiac cycle is this cycle of contraction and relaxation that moves blood through the heart. Now, at this point, you should know your structures of the heart. You should know the basic pathway that blood takes through the heart. What we want to talk about now is why blood takes that very specific one way path, and how the contraction and relaxation change the pressure in the heart in a way that makes those one way valves open and close. Now, here we're going to talk about it sort of very generally and just talk about pressure and valves in more general terms, and then we'll take what we learn and apply that to the 4 chambers of the heart and really see what's going on in that beating thing in your chest. Alright.

So to start, remember, we're talking about contraction and relaxation, but this is anatomy and physiology, so we have fancy words for things. We're going to say systole is contraction. And the way you use that word in a sentence, you would say that a particular chamber is in systole. Now the way I remember what systole means is I have a little alliteration memory tool here. I say the systole squeeze. Alright. So systole equals squeeze. When the chamber is contracting, it's in systole, and that squeezes inwards. Now systole, we also have diastole. Diastole, therefore, is relaxation. And, again, how you use diastole in a sentence, you would say a particular chamber is in diastole. Now to remember this, we also have a little alliteration memory tool here. I say the diastole drop. During diastole, the pressure drops because the chamber's relaxing. Alright. We're talking about squeezing and relaxing, and that's going to change the pressure in the heart, and it's going to be the changes in this pressure that's going to force valves open and close. Whenever you're thinking about a valve, whether it's open and closed, you want to think what is the pressure on one side of the valve compared to the pressure on the other side of the valve, and that's going to push the valve in one direction or the other. Now, when you're thinking about that, we want to say that the ventricles cause most of the pressure change in the heart. So, sure, the atria go through systole and diastole, but it's the ventricles whose pressure changes are really important. Those are the big, heavy parts of the heart, and when they contract, when they go into systole, that pressure changes dramatically. So while the atria and the ventricles both go into systole and diastole and they go through them at different times, we can even just say that the heart goes into systole or the heart goes into diastole. When we say that, we're talking about the ventricles, because for pressure changes, that's really what matters.

Okay. Now before we go on, we just want to remind ourselves of the valves in the heart. For every ventricle you have 2 ventricles, and for each ventricle, there is an atrioventricular valve or an AV valve, and I think of that as the in valve to that ventricle. Things go into the ventricle through that valve, and that valve opens inwards into the ventricle. Now we also have a semilunar valve, which we can abbreviate as an SL valve sometimes. Now the semilunar valves for the ventricle, that's going to be the out valve, and importantly, it opens outwards. Now to remember those, we have this fun little made-up word here. You can just shout out Avanslout. Avanslout, AV in. Things go into the ventricle through the AV valve, and it opens into the ventricle. SL out, things go out of the ventricle through that semilunar valve, and it opens outwards.

All right. Now to think this through, we have a sort of simplified pump here representing a ventricle and our AV valves and semilunar valves. So if you're following along on your printable PDF, you'll see that this is more in a comic book form. Here, we're going to look at an animation. As we look at this, we have the AV valves over here on the left. Those are those valves that are pointing into the ventricle, and we have the semilunar valves here. Those are our out valves on the right. Those are the valves that open outwards away from the ventricle. Instead of showing something the whole ventricle squeezing here, we have a simplified, again, pump system where we're going to have a piston that goes up and down to change the pressure.

Alright. So let's think through what's going to happen during systole and diastole of the ventricles. So during ventricular systole well, during systole, we have the systole squeeze, so you can probably guess what's going to happen to the pressure. As it contracts, that pressure is going to go up. Now let's think what that will do to the different valves. Well, we can watch our animation to see. So we're going to see this piston push upwards. I just want to stop it right there. We can see at this point, we've increased the pressure enough that those indoors, the AV valves, are now shut. The pressure in the ventricle here on this side of the valve is greater than on the other side of the valve, so the doors are closed. So the first thing as that ventricular pressure rises, we're going to see that the AV valve closes. Alright. As we let this contract even more, it continues through systole, and we will see that that piston is going to push up and up. And we can see now that that semilunar valve well, the semilunar valve was pushed open. Semilunar valve opens because the pressure in this ventricle was higher than on the other side of that valve. And as the out valve, it opened outwards. Well, as we keep it going here well, now we've finished systole, and now we go into diastole. And diastole, the pressure drops, so this piston is going to come back down again. Well, as this goes, you can see the pressure drops. We're going to put the arrow down there, and I'll start this up. But we see, right away, the first thing that we see, as that pressure drops, it's no longer putting pressure on that semilunar valve. Now there's some blood in this artery here that's pushing backwards, so now the pressure up here on this side of the valve is greater than the pressure in the ventricle. So that out valve, that semilunar valve closes. So the semilunar valve closes, And as we keep going here, this pressure continues to drop, and we can see now the AV valve opens. The pressure on the top side of the AV valve there is now greater than the pressure on the bottom. And I'll just keep this playing here, but here, I'm going to say the AV valve opens, and we can see it going. Alright. Pressure rises, AV valve closed, semilunar valve opens. As the pressure falls in diastole, the semilunar valve closes and then the AV valve opens. Again, as you follow this through, always remember in which way does the valve open and what is the pressure on either side of that valve? If you know those things, you know whether the valve is going to be open and closed, and you know which way blood can move through that valve. Alright. We're going to practice this more in practice problems, and, again, then we will look at actually the 4 chambers of the heart and lay all this information on there. Looking forward to it. I'll see you there.

Cardiac Cycle Example 1

Our example tells us that the opening and closing of heart valves is due to the relative pressure in the ventricles compared to the atria and the arteries. We want to use our knowledge of heart anatomy to complete the following table. In this table, we see the relative pressure between two different places in the heart. Then, we want to know which valve is going to be affected by that relative pressure. Is the valve open or closed, and is the ventricle in systole or diastole? Alright.

So far, we've just talked about sort of pressure changes generally. Now we're going to go through all this and compare it to the heart. But even right now, you should know your heart anatomy decently well, and just knowing these pressure changes, you should be able to figure out what's going on here. So let's give it a try. We're going to start with the aorta. It says the pressure in the aorta is greater than the pressure in the left ventricle.

Well, here's our aorta, and here's our left ventricle. So the first question is, what's the valve that's affected? What valve is between the aorta and the left ventricle? Well, I'm going to say that that's the left semilunar valve. I'm just going to write abbreviations there just to save space.

Or slash. We can also call that the aortic valve or the aortic semilunar valve. Alright. So the left semilunar valve or the aortic semilunar valve. And so if the pressure is greater in the aorta than the left ventricle, is that valve going to be open or closed?

Well, if the pressure is greater up here, that means that it's sort of pushing backwards on there, and that's the opposite way that we want blood to flow. Right? Those semilunar valves, those are the outdoors for that ventricle. So if the blood's trying to sort of go in the opposite direction than it's supposed to go, that's going to push that valve closed. So this means that the valve is going to be closed.

And next, we want to know is the ventricle going to be in systole or diastole? Remember, we have the systole squeeze, that's a contraction. Diastole, the pressure drops during relaxation. Now just to be really clear, for all of these, technically it could be in either one, and it will be in either one during some times during the heart cycle. However, for most of the time, generally, we can say systole or diastole for this.

So if the pressure in the left ventricle is lower than the pressure in the aorta, would you expect that that heart is in systole or diastole? Well, if the pressure is low in the ventricle, that means that it is likely in diastole. That pressure is dropped. The pressure has fallen because the ventricle is relaxing. Alright.

Next up, we have the pressure in the right atrium is greater than the pressure in the right ventricle. So here's our right atrium. Here's our right ventricle. So first off, what i

When the pressure inside the ventricle causes the atrioventricular valve to close and the semilunar valve to open, which statement about the heart is true?

• A. The ventricles are in systole.
• B. The ventricles are in diastole.
• C. The pressure in the left ventricle is greater than in the aorta.
• D. Both A & C are correct.

For the atrioventricular valves to be open, the pressure in the ventricles must be:

• A. Greater than the pressure in the pulmonary artery.
• B. Greater than the pressure in the aorta.
• C. Greater to the pressure in the atria.
• D. Less than the pressure in the atria.

Events in the Cardiac Cycle

We previously looked at how pressure changes in the ventricles, whether they're in systole or diastole, whether they're contracting or relaxing, is really going to dictate the movement of blood and the opening and closing of valves in the heart. Well, now we want to take those ideas and apply them to the actual heart and look at how this works through one heartbeat. The events of one heartbeat we call the cardiac cycle, the filling of blood into the heart and then the squeezing of the heart to push that blood into circulation. So as we start looking at the cardiac cycle, we're going to say that there are actually 4 events that we want to name and describe in this cardiac cycle, and we're going to break these events up based on sort of two factors. First, whether the ventricles are in systole or diastole. Remember, the ventricles, that's the main mass of the heart, and they contract with a lot of pressure. So whether they are in systole, that systole squeeze contracting, or whether they're in diastole, that diastole drops, the pressure drops during relaxation. Whether they're squeezing or relaxing, that's going to dictate these pressure changes more than anything else. So that's number 1. And number 2, we want to look at whether blood is moving in the heart. The pressure changes, we said, are going to open and close valves. Now you may think, isn't the blood always moving? But there's actually going to be some periods when all the valves are closed. All the doors in the heart are shut, so the blood actually doesn't move for some short periods, and that's going to break up this cardiac cycle some as well. Alright. So let’s take a look. You can see we have this image here of the heart. We have blood on it. Now this is going to be actually an animation. I’m going to play through this, so we can watch through the cardiac cycle. Now if you're following along on your student you'll see that we have you already have these phases of the cardiac cycle named, and you have freeze frames from this animation there. Now I like to show this animation because I think it's important to remember that this is a process. It doesn’t jump from one to the other. These are changes that flow from one to the other. So to get that idea, let’s watch one cardiac cycle first before we name things, And just know here we’re going in very slow motion. Alright. So first up, we have the blood flowing into the ventricles. The ventricles fill up. Now the ventricles squeeze. That closes the AV valve, pushes open the semilunar valve. The blood flows into the arteries. Now the ventricles start relaxing, that semilunar valve closes, then that AV valve opens again so it can fill. That was 1 heartbeat, 1 cardiac cycle. Alright. So let’s go through what just happened. So the first thing that happened, we’re gonna call ventricular filling. Alright. From that name, it should be pretty obvious what's going on here. The blood flows through the atria and into the ventricles. Now, as we go through this, for each one of these stages we're going to try and name: are the ventricles in systole or diastole? Is the ventricular pressure high or low? And are the AV valves and the semilunar valves open or closed? So, for ventricular filling, if blood is flowing into the ventricles, well, that means that the ventricles must be in diastole. Remember, diastole means that they are relaxed. That means that there's very low pressure, so the ventricular pressure is going to be low. And we can see here on the image I’ll actually just advance it just a little bit here. We had that blood flowing into the heart. That means that the AV valves those are open those valves between the atria and the ventricles because the pressure in the ventricle is lower than the pressure in the atria. The semilunar valves, you’ll see those are closed because the pressure in those arteries is going to also be greater. Well, this time, it's going to be greater than the pressure in the ventricles. Alright. So we’re going to fill up, and then those ventricles are going to start contracting. So I'm going to advance our video here. They start contracting. Those AV valves close, and I'm going to stop it right there. Alright. Here, we've started contracting. The AV valves have been pushed closed because the pressure has risen, but you can see the semilunar valves, those valves from the ventricles into the arteries, they haven’t opened yet. So we're going to call this stage isovolumetric contraction. And again, if we break down these words here, we can see what's happening. ISO, well, ISO means the same, and volumetric it refers to the volume. The volume isn't changing, but the ventricle is contracting. So we’re gonna say here the blood is contained in the ventricles as they contract. Alright. So what's happening in these things we want to fill in? Well, the ventricles, they're contracting, so that means that they are in systole. We have that systole squeeze going on. That means that well, if they're contracting, that means that the pressure in the ventricles well, I’m gonna say here it's rising. It’s not as high as it's gonna get, but it is going up. We can see the pressure has risen enough that the pressure in the ventricle is now greater than the atria. That has pushed that AV valve closed, But the pressure in the ventricle has not gotten enough to be greater than in the arteries, so that semilunar valve is also closed. Alright. All the valves, all the doors to the ventricle are shut. So it means even though it's contracting, the pressure hasn’t changed enough to actually move the blood, so we’re in an isovolumetric stage. The volume is staying the same. Alright. Pressure’s gonna keep rising though, and we'll follow this along. It's

Cardiac Cycle Example 2

In this example, it says that the steps of the cardiac cycle are described below, match each description below with the phase of the cardiac cycle it describes. And we see 1, 2, 3, 4 descriptions. For each of them, it tells us what's happening with both sets of valves, and it tells us whether the atria and the ventricles are in systole or diastole. And then we see our 1, 2, 3, 4 stages of the cardiac cycle here, and so we need to match those things together. Now just note, you could do it either way. We could start at the bottom and think what's happening in each one and find the description that matches. I'm going to do it the other way around. I'm going to start with the a b c d and match it down to the stage that that describes. The other thing that I want to note here, though, is just that it's giving us, for the systole and diastole, information on both the atria and the ventricles. But remember, what really just matters here is what's happening with the ventricles. So this information of whether the atria are in systole or in diastole, I actually don't really care. It's not important. So I'm just going to cross that part out because it's not relevant for the answer. Alright. So now a says both sets of valves are closed and the ventricles are in diastole. Well, knowing that, which stage of the cardiac cycle do you think that represents? Well, both sets of valves are closed. That means that blood cannot move through the heart because all the valves are closed. That must mean that it must be an isovolumetric stage. So we have 2 isovolumetric stages, but then it says that the ventricles are in diastole. Diastole drop, Diastole drop. The ventricles are relaxing, so that has to be isovolumetric relaxation. So I'm going to put an a here, and I'll cross out that one and move down to b. It says that the AV valves are open, the semilunar valves are closed, and the ventricles are in diastole. Alright. Which stage do you think that represents? Well, if the AV valves are open and the semilunar valves are closed remember, we had that word, AV into the ventricle, semilunar SL out of the ventricles. So, the indoors are open. Well, that means that blood can flow into the ventricle because it is in diastole, the diastole drop, it's relaxing. So here the ventricle is filling with blood, and that's the name of a stage right here. Ventricular filling. So I'm going to put b on a line there and did that one. Alright. Next, we have c. Both sets of valves are closed. The ventricles are in systole. Which one do you think that one represents? Well, again, because both sets of valves are closed, that means that no blood is moving through the heart, which means it must be an isovolumetric state. The ventricle is in systole. We get the systole squeeze, so it is contracting. So that means that I can put c next to isovolumetric contraction, and we've answered that one. N

What event immediately follows the closure of the atrioventricular valves and coincides with the onset of ventricular systole?

• A. Isovolumetric contraction.
• B. Atrial systole.
• C. Ventricular ejection.
• D. Isovolumetric relaxation.

During which following phase (or phases) of the cardiac cycle is the blood volume in the ventricles constant?

• A. Isovolumetric contraction.
• B. Isovolumetric relaxation.
• C. Ventricular ejection.
• D. A & B are correct.

If the pressure in the ventricles is higher than in the atria, which of the following statements must be true?

I. The atrioventricular valves are closed.

II. The semilunar valves are open. 

III. The ventricles are in systole.

• A. I only.
• B. I and II.
• C. I and III.
• D. II and III.

True or False: if false, choose the answer that best corrects the statement.

Ventricular filling is defined as the period that the atria are in systole.

• A. True.
• B. False; ventricular filling is defined as the period that the semilunar valves are open.
• C. False; ventricular filling is defined as the period when the AV valves are open, and the SL valves are closed.
• D. False; ventricular filling is defined as the period when the pressure in the ventricles is greater than in the atria.

Sounds in the Cardiac Cycle

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.

Cardiac Cycle Example 3

Here it says that heart sounds are made at the transition from one stage of the cardiac cycle to the next. We want to identify which heart sound would be heard at the start of each phase of the cardiac cycle. And if there is no distinct heart sound at the beginning of the phase, we can write none. Alright. So we have our 4 phases here. And for each phase, we see a diagram of the heart, reminding us what's going on. So let's go through them one by one. Remember, these phases start with either the opening or the closing of a valve, and so let's think which ones we would be able to hear. So we'll start with ventricular filling. Do you think you could hear the start of ventricular filling and why? Well, ventricular filling is really sort of defined as these AV valves being open. When those AV valves are open, blood from the atria can flow through the AV valves and fill up the ventricles. The ventricles are in diastole. During that time, they are relaxed. Well, the opening of those AV valves is what starts ventricular filling. Can you hear the opening of a valve? No, you can't. So there's going to be no heart sound, or I'm going to write none, that you can hear at the start of ventricular filling.

That brings us to isovolumetric contraction. Will you be able to hear the start of that phase, and if so, why? Well, isovolumetric contraction. Contraction, this means that the ventricle goes into systole, that systole squeeze. The pressure shoots up, and that pressure shooting up is what causes these AV valves to snap shut. When they snap shut, you can hear that. That's that first heart sound. We can write that first heart sound as S_1, or we can write it's the lub of the heart, if we're being a little less technical.

All right. Now, isovolumetric. Remember, it's snapped these AV valves shut, but the pressure hasn't risen enough to push these semilunar valves open yet, so the blood can't go anywhere because all the doors, all the valves to the ventricles are closed. Alright. That brings us to ventricular ejection. What starts that and will you be able to hear it? Well, in ventricular ejection, your heart your ventricle is still in systole, and it is now squeezing enough. The pressure goes enough that it pushes open these semilunar valves, and blood is ejected or pushed out of the heart. So could you hear that? No. You cannot hear the opening of a valve, so you can't hear that, so we're gonna write none.

Alright. That brings us to isovolumetric relaxation. There you go. Relaxation behind my head. Alright. Isovolumetric relaxation. Can you hear that? And if so, why? Well, relaxation. That means that the heart is going into diastole. The ventricles are relaxing. That's gonna lower pressure. As that pressure lowers, well, you're gonna have some back pressure, from the arteries here, and that's gonna snap the semilunar valves closed. The snapping of those semilunar valves closed, that's gonna be heart sound. We're gonna call that our second heart sound or what we can sometimes just write as S_2. Or if we want to be a little less technical about it, we can call it the dub in that lub dub of the heart. Alright. Remember, isovolumetric. That's because now all 4 valves are closed. All 4 valves are closed, but relaxation, it's still in diastole, but because the valves are closed, there's no change in volume in that ventricle.

Okay. Again, remember, the heart sounds are the valves closing, and that's, those valves opening and closing are the beginning and the ends of these phases, so those heart sounds should line up with the starts of both of those isovolumetric phases. Alright. More practice problems to follow. You should give them a try.

Using a stethoscope, you listen to the heart of a patient in their early forties. You hear a clear 1st heart sound (lub), but the second heart sound is muffled and followed by a whooshing noise. What structure could be dysfunctional in this patient?

• A. Mitral valve.
• B. Right atrioventricular valve.
• C. Aortic valve.
• D. Both A & B could be the cause.

Cardiac Cycle: Putting It All Together

We now want to spend some time trying to tie together all these different concepts that we've talked about that relate to the cardiac cycle, and we're going to do it by making one big graph here. Now I'm going to fill in this graph step by step as we go. If you're following along in your student PDF, you'll see it's already filled in for you. A graph like this, you are very likely to see, either because your professor is going to teach from it or almost certainly it's in your textbook. A graph like this is great because it shows all the different things we've talked about and how they relate to each other. But I also understand it can be kind of confusing and a little bit overwhelming the first time you see it. So that's why we're going to try and put it together piece by piece. So I'm going to start at the very top. We're going to start with this electrocardiogram, or the ECG. So we're going to draw in our ECG here, and you should be familiar with this shape. Now we're drawing it on a slightly different scale here, so it might look a little bit more stretched out, but we can still see all the parts we're familiar with. We see here the P wave. The P wave, remember, that's that atria depolarizing. We see the QRS complex, that's the ventricles depolarizing and also the atria repolarizing. We see the T wave, remember, the T wave is the ventricles repolarizing. And then if we follow it all the way down, we see the P wave starting again for the next heart contraction. Alright. Remember, those depolarizations, that's those contractile cells in the heart. That's the action potentials in those contractile cells, and that's the stimulation for them to contract. So we can relate this ECG to our systole and diastole of the heart. Remember, systole squeeze. That's when the heart is contracting, and diastole drops, the pressure drops when the heart relaxes. So we can do that for both the atria and the ventricles. So let's start by looking at the atria. We can draw this in, and we can see that systole occurs. Well, it starts at that P wave. That's when the atria depolarize, and it ends basically at that QRS complex when the atria repolarize, and the rest of the time it's in diastole. The ventricles, well, you can predict what's going to happen there. The ventricles go into systole during that QRS complex when the ventricles depolarize, and they go back into diastole around that T wave when the ventricles repolarize. Now before we go on, I'm just going to put some sort of gray bars here on the end where we want to look at 1 cardiac cycle. And so we have a little bit more than 1 cardiac cycle drawn here. For 1 cardiac cycle, we just want to focus on the white part, and outside of that, that's sort of just drawn in so we can see the beginnings and ends leading in and out. Now here, we're defining this cardiac cycle as starting with systole of the atria, or the P wave of that atria, and ending when the atria goes into systole again. Now I just want to note that's different than how we broke it up when we talked about the events of the cardiac cycle. Now we're going to look at that and compare that later on here, but just remember, this is a cycle. It's kind of like a circle. There is no strict beginning and end, so it's alright if we start or end in a different place because, really, hopefully, your heart doesn't start and end beating. Hopefully, it just keeps on going from one heartbeat to the next.

The next graph we're going to draw here is pressure. And you can see here it goes from 0 up to 120 millimeters of mercury. Now when we draw this in, we're just going to be looking at the left side of the heart. We're going to be looking at the left atrium, the left ventricle, and the aorta. If you want to think what's happening on the right side of the heart, the shapes of the curves will basically be the same, but the values are going to be different because it's a lower pressure system. Okay. So first, remember, it's the systole and the diastole of the ventricles that really drives this process, and that's when I drew in systole and diastole up there. I made that sort of a bolder color. That's going to be what's really driving things. So let's look at this left ventricle first. Well, we can follow it along. We say, well, over there, there's a little bump, and that happens when the atrium goes into systole. That's the blood being pushed into that ventricle. You get a little raise in pressure. But really, the main thing that's happening here, right, when the ventricle goes into systole, you get this major change in pressure. The pressure stays really high basically until it goes into diastole again. And diastole, the pressure drops. Alright. We can compare that to the pressure in the aorta. Now remember the aorta, it's part of that systemic circulation. It's a high pressure system. So the pressure here is always going to be high. It starts out sort of level, going down a little bit, and then it gets that push from the systole of the ventricles. That blood gets pushed into the aorta, and you can see the pressure rises. It basically follows that shape of the curve of the ventricle, until the ventricle goes back into diastole, and then it sort of levels out again. You can see there is one sort of funny blip up there. We'll talk about that in a second. What makes that happen? But we can also compare that to the pressure change in the atrium. So you can see here the atrium, it's low pressure the entire time. You can see one little blip when the, atria go into systole. So that's it, just sort of squeezing down, pushing the blood into the ventricle. You can see another little bump when the, ventricles actually contract. That's just because the ventricles are s

Our example says on the graph below, identify the following areas, and we have 4 things that we need to look for on this graph. Before we look for 1, though, let's just orient ourselves to the graph here. This is that pressure graph that we looked at previously. On the y-axis, we have pressure in millimeters of mercury, and on the x-axis, we have time. There are three lines on this graph. Here in black, we see this is the pressure changes in the aorta. The red line is the left ventricle, and the purple line represents the pressure changes in the left atrium. Alright. Now that we've seen that, let's see what it wants us to do. It says a, circle the area or areas of the plot where the atrioventricular valves will be open. Alright. So take a look at the graph. When do you think those AV valves will be open? Well, remember the AV valves will be open whenever the pressure in the ventricle is lower than the pressure in the atrium. That allows blood from the atrium to push that AV valve open and flow into the ventricle. Now on this graph, that means that it's going to be the times when the red line is lower than the purple line. So I see that here and I see that here. Now you'll see that on the graph here, those areas have that sort of bluish background that represents that this is the period of the cardiac cycle that we call ventricular filling. Alright. So I've done a. I'm going to cross that out. Next, we have b. Put a box around the area or areas where the semilunar valves will be open. Alright. Again, take a look at this graph. Where do you think the semilunar valves will be open? Well, the semilunar valves are going to be open whenever the pressure in the ventricle is greater than the pressure in the artery. That means that the blood's going to be pushing out of the ventricle and will push that semilunar valve open. So in this graph, it's when this red line representing the pressure in the left ventricle is greater than the pressure in this black line representing the aorta, and that's this area right there. On this graph, it has that sort of beige background representing the stage of the cardiac cycle we call ventricular ejection, when blood is being pushed out of the heart. Alright. I've done b. I'll cross that out. Next it says, c, place a star where you would hear the first heart sound, the lub of a heartbeat. Alright? So where would you put that star? Well, the lub, right, that's the first heart sound. Remember, the heart sounds come from the valves closing, and we said that whenever the lines cross on this graph, that represents a valve either opening or closing. So where would that first heart sound come? That's when the AV valve closes and that's when that pressure in the ventricle shoots up and crosses that line that's the pressure in the atrium. So that happens right here. Alright. Next, we have place an x where you would hear the second heart sound, the dub of a heartbeat. Alright? So where would you put that x? Well, I would put that x, well, again, where these lines cross. And here, where the valve is going to close, is where the pressure in the ventricle falls down again below the pressure in the artery or the pressure in the ureter. That happens right here. So I will put my x right here on the graph. And with that, we've marked up the graph how we were asked. Again, remember, always think what is the relative pressure in these two different places, and that's going to tell you what's happening in the heart. We'll practice it some more. I'll see you there.

During which phase or phases of the cardiac cycle does the ventricular pressure change the least?

• A. Ventricular filling.
• B. Isometric contraction.
• C. Isometric relaxation.
• D. Both B & C are correct.

During which phase of the cardiac cycle do you hear the second heart sound?

• A. Ventricular filling.
• B. Isometric contraction.
• C. Isovolumetric relaxation.
• D. Ventricular ejection.

You are listening to the heart through a stethoscope. When you hear the first heart sound, which of the following options correctly matches the structure to the pressure change that would be most responsible for producing that sound?

• A. Ventricles: pressure increasing.
• B. Aorta: pressure decreasing.
• C. Ventricles: pressure decreasing.
• D. Atria: pressure increasing.

Here’s what students ask on this topic:

What are the phases of the cardiac cycle?

The cardiac cycle consists of four main phases: ventricular filling, isovolumetric contraction, ventricular ejection, and isovolumetric relaxation. During ventricular filling, blood flows from the atria into the ventricles, which are in diastole. Isovolumetric contraction occurs when the ventricles begin to contract (systole), but all valves are closed, so the volume remains constant. Ventricular ejection follows, where the pressure in the ventricles exceeds that in the arteries, opening the semilunar valves and allowing blood to be ejected. Finally, isovolumetric relaxation happens when the ventricles relax (diastole), and all valves are closed again, leading to a drop in pressure without a change in volume.

What causes the heart sounds 'lub' and 'dub'?

The heart sounds 'lub' and 'dub' are caused by the closing of the heart valves. The 'lub' sound, or the first heart sound (S1), occurs when the atrioventricular (AV) valves (mitral and tricuspid) close at the beginning of ventricular systole. This is due to the rapid increase in ventricular pressure. The 'dub' sound, or the second heart sound (S2), happens when the semilunar valves (aortic and pulmonary) close at the beginning of ventricular diastole, as the pressure in the ventricles falls below that in the arteries. These sounds are crucial for diagnosing heart conditions.

How do pressure changes in the heart affect valve function?

Pressure changes in the heart are critical for valve function. During ventricular systole, the pressure in the ventricles rises, causing the AV valves to close and the semilunar valves to open, allowing blood to be ejected into the arteries. During diastole, the pressure in the ventricles falls, leading to the closure of the semilunar valves and the opening of the AV valves, permitting blood to flow from the atria into the ventricles. These pressure-driven valve movements ensure unidirectional blood flow through the heart.

What is the significance of the isovolumetric phases in the cardiac cycle?

The isovolumetric phases are crucial for maintaining efficient cardiac function. During isovolumetric contraction, the ventricles contract with all valves closed, leading to a rapid increase in pressure without a change in volume. This phase ensures that the pressure is high enough to open the semilunar valves for blood ejection. During isovolumetric relaxation, the ventricles relax with all valves closed, leading to a rapid decrease in pressure without a change in volume. This phase ensures that the pressure falls enough to open the AV valves for ventricular filling. These phases are essential for the proper timing and coordination of blood flow through the heart.

What are heart murmurs and what do they indicate?

Heart murmurs are sounds created by turbulent blood flow within the heart, often heard as whooshing or swishing noises. They can indicate various heart valve pathologies. For example, a murmur following the 'lub' sound may suggest an issue with the AV valves, while a murmur after the 'dub' sound may indicate a problem with the semilunar valves. Murmurs can be caused by valve stenosis (narrowing) or regurgitation (leakage). While some murmurs are harmless (innocent murmurs), others may require medical evaluation to determine the underlying cause and appropriate treatment.