052 The Cardiac Cycle

This post was updated on March 24, 2022

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Ever wonder what happens in a heartbeat? What happens inside our heart when we hear the ‘lub-dub’ sound? When I say cardiac cycle, I’m talking about everything that happens from the beginning of one heartbeat to the beginning of the next. So we’re dealing with an entire heartbeat.

Introduction

Let’s look at the cardiac cycle diagram. I know, it looks hard, but it isn’t really.  Here’s why it seems so difficult. Because when your professor teaches you about the cardiac cycle, you see this complicated diagram right here.

The Cardiac Cycle Diagram
The Cardiac Cycle Diagram

It has a TON of details and it looks kinda scary. Let’s not do it like that. Let’s break it apart, and build it together.

What happens in a Cardiac Cycle?

Let’s start with blood coming back from the body. Blood enters the heart first through the atria. On the left side, we have blood coming back from the lungs and on the right side, we have blood coming back from the rest of the body. 

Heart blood flow.
Heart blood flow.

When the atria contract, they push blood into the ventricles. And then when the ventricles contract, they push blood out of the heart. Those are the two contractions that cause blood flow.

If this seems unfamiliar or unclear for you, head on to my article on How Blood Flows Through the Heart and you’ll get a more detailed explanation.

The Cardiac Cycle Diagram Explained

The first place we’re going to look at is the electrocardiogram. We’re going to start here because this shows the electrical signals that are responsible for the heart beating.

Electrocardiogram highlighted in the diagram.
Electrocardiogram

Atrial Systole

The first thing we see is the P wave. This shows the depolarization of the atria. That’s the electrical signal traveling through the atria. When that happens, that causes the atria to contract. 

To see that, we’re going to look at a different line on this graph, the one that shows you the atrial pressure.

Diagram highlighting the relationship between the P wave and atrial pressure during atrial systole.
The P wave and atrial pressure during atrial systole.

What do you expect to see right after the depolarization of the atria? Well, that’s the signal. It tells the atria to contract, and if the atria contracts, you would expect to see an increase in atrial pressure. Right? Of course. If you squeeze something, the pressure increases. And that’s exactly what we see here – right after the P wave. Makes sense. I love it.

Now, there’s a fancy word that goes along with this and that’s systole. When you see systole, think contraction. And here, the atria are contracting, so we have Atrial Systole. Simple. 

What happens to the blood when the atria contract?

Well, it can only go one place. It’s gonna go through the atrioventricular valves – the one-way valves between the atria and the ventricles, and it’s gonna go into the ventricles. So what would you expect to see happen to the ventricular volume? You’d expect an increase in ventricular volume because you’re filling it with blood. And that’s exactly what we see right here.

During atrial systole, ventricular volume increases as blood is pumped down to the ventricles during atrial contraction.
During atrial systole, ventricular volume increases as blood are pumped down to the ventricles during atrial contraction.

Right there, that bump in the volume of the ventricle. So this entire section is what happens during atrial systole. And we are ready for the next step. 

Isovolumetric Contraction

Looking again at the ECG, we see that the next thing that happens is that we get the QRS complex. The QRS complex shows the depolarization of the ventricles. You can see, it’s much larger than the P wave and that’s because it’s a much larger structure – the signal is going to be larger. And just like what happened with the atria, with the ventricular depolarization, you expect to have ventricular contraction.

This is the beginning of the phase of systole, where the ventricles are contracting.

When they contract, what will that do to the pressure in the ventricles?

Well, during atrial contraction we did get a little increase in ventricular pressure of course – because blood was rushing into the ventricles, but now that the ventricles are contracting, you’re gonna see a much greater increase in ventricular pressure. 

Diagram shows ventricular pressure increases during the QRS phase of the electrocardiogram.
The diagram shows ventricular pressure increases (blue graph) during the QRS phase of the electrocardiogram.

And it makes sense. You have a container, you squeeze that container, the pressure inside the container increases. In this case, the container is the ventricles, it’s made up of pretty strong muscle, so we get that huge increase in pressure. 

Now, there’s a key thing that happens when the ventricles contract. As you see here, there’s a short phase called isovolumetric contraction. What exactly is that?

During isovolumetric contraction, the volume inside the ventricles (red graph) remains the same as both the AV and semilunar valves are closed.
During isovolumetric contraction, the volume inside the ventricles (red graph) remains the same as both the AV and semilunar valves are closed.

Well, the word isovolumetric means – the volume stays the same. The amount of blood in the ventricles remains the same. Look at the ventricular volume – it’s pretty much a straight line. And that’s because when the ventricles start contracting, that actually closes all the valves. 

For example, if we’re looking at the left side of the heart, this atrioventricular valve gets shut and the semilunar valve is also closed.

Isovolumetric volume is achieved when both semilunar and AV valves are closed. This happens for a very short period in the cardiac cycle.
Isovolumetric volume is achieved when both semilunar and AV valves are closed. This happens for a very short period in the cardiac cycle.

If they are both closed, we have a sealed container that is contracting, so we get this huge increase in pressure, but that isovolumetric stage only lasts a short period and that’s until the semilunar valve opens. That’s the valve between the left ventricle and the aorta. It’s going to be closed until a certain point.

Ejection Stage

Now, what point would that be? Well, let’s think about it.

On the other side of that semilunar valve is the aorta. And at this point right here, the aortic pressure is somewhere around 80 millimeters of mercury. So if you want to push blood in there, you have enough pressure in the ventricle to overcome that 80 millimeters of mercury. 

And right at that point, the semilunar valve will finally open and the blood can be sent into the aorta so that it can go to the rest of the body.

The ventricles need enough pressure (around 80mmHg) to push blood out into the aorta and to the rest of the body.
The ventricles need enough pressure (around 80mmHg) to push blood out into the aorta and to the rest of the body.

So, blood is leaving the ventricle. What will happen to the ventricular volume? It’s going to go down because we have blood getting out. And that’s exactly what you want. You want the blood to leave the ventricles and go out to the body.

This stage here is the Ejection Stage. That’s when blood is being ejected from the heart and specifically the ventricles.

During the ejection stage, ventricular volume decreases (red graph) as blood is ejected out to the aorta and to the different parts of the body.
During the ejection stage, ventricular volume decreases (red graph) as blood is ejected out to the aorta and to the different parts of the body.

Isovolumetric Relaxation

Let’s look back at the ECG, we then have the T wave. What does the T wave show?

Well, that’s Ventricular Repolarization – the opposite of depolarization. So the ventricles are going to relax now. What happens when the ventricles relax?

The pressure in the ventricles will come back down. At a certain point, the valves are going to close again and we get Isovolumetric Relaxation.

In Isovolumetric Relaxation, ventricular pressure decreases as the ventricles start to relax and the valves close again.
In Isovolumetric Relaxation, ventricular pressure decreases as the ventricles start to relax and the valves close again.

Valves are closed, ventricles are relaxing so the pressure in the ventricles drops significantly. And that’s exactly what you’d expect.

Now, once that ventricular pressure gets below the atrial pressure, what’s going to happen to the atrioventricular valve?

It’s going to open up again. And at that point, the valves are open and the blood that’s coming back from the body will just start passively filling the ventricles. And that continues up until the point where we get our next P wave to start the entire process again. That’s pretty much the cardiac cycle.

Once ventricular pressure gets below the atrial pressure, the atrioventricular valves open up again. And at that point, the valves are open and the blood that’s coming back from the body will just start passively filling the ventricles, continuing up until the point where we get our next P wave to start the entire process again.
Once ventricular pressure gets below the atrial pressure, the atrioventricular valves open up again. And at that point, the valves are open and the blood that’s coming back from the body will just start passively filling the ventricles, continuing up until the point where we get our next P wave to start the entire process again.

Phonocardiogram – Heart Sounds

Now, there’s one more thing that we didn’t cover and that is the phonocardiogram. That shows the sounds of the heartbeat. When you listen to the heartbeat, like with a stethoscope, you hear a sound that goes like this – lub dub. Lub dub. Lub dub. That’s what you’re seeing here. 

What’s causing the Heart Sounds?

They are actually the sounds of the valves in the heart closing. Let’s look at when they happen. The first one happens right by the QRS complex.

The first heart sound, the "Lub" sound happens at the QRS complex, as the atrioventricular valve closes.
The first heart sound, the “Lub” sound happens at the QRS complex, as the atrioventricular valve closes.

Remember, that shows ventricular depolarization, which causes the ventricles to contract. When the ventricles contract, that pushed the atrioventricular valve close. That’s why you get the first sound.

The second sound happens after the T wave – the ventricles relax, and the semilunar valves close. That causes the dub sound. That’s why we hear lub dub, lub dub, lub dub.

The second heart sound, the "Dub" sound, happens after the T wave when the ventricles are relaxing and the semilunar valves close.
The second heart sound, the “Dub” sound, happens after the T wave when the ventricles are relaxing and the semilunar valves close.

And now THAT is the entire cardiac cycle. Does it make sense?

If it doesn’t, watch it again, pause it where you need to, and get a good understanding of what this entire graph is trying to show.



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Leave a Reply

  1. this vid make cardiac cycle very clear to me and can you post vid talking about cardiac arrhythmia and i will be grateful

  2. Unfortunately, I can’t. I’m focusing on making videos as I need them for my classes, so I can’t take anymore requests at the moment.

  3. never mind,i appreciate that and all your vid are useful to me because i am in college of medicine

  4. Great video!
    Question: Aortic valve opens when contracting vent. pressure exceeds the resting diastolic pressure in the aorta (80mmHg). Pressure waves cross there. At the end of vent. syst., relaxing vent. pressure drops below chock-full-O-blood aortic pressure and the valve shuts as aortic blood presses against the cusps. Why are the pressure waves different there? Why is aortic pressure 100mmHg and vent. pressure 60mmHg? Why doesn’t the aortic valve close at 80mmHg?

  5. All questions are answered in the Interactive Biology forums from now on. Go to the website in the description and then visit the forum. This is to make it as efficient as possible as we have multiple people over there to help answer questions.

    All the best

  6. @Neopluto We’ll see what we can do. 🙂 Thanks for watching though! There are more videos you might want to watch from our Biology website. More will be uploaded so, please stay tuned!

  7. We’ll see what we can do. 🙂 Thanks for watching though! There are more videos you might want to watch from our Biology website. More will be uploaded so, please stay tuned!

  8. This video gets to the point and is described clearly; did not over exaggerate or over complicate things either. I appreciate this video (and many other students) very much, thank you!

  9. This video gets to the point and is described clearly; did not over exaggerate or over complicate things either. I appreciate this video (and many other students) very much, thank you!

  10. Just saying,, the girls is HOT,, btw Thank You Very Much,,it helps,,, *whos the girl again?? Nvm

  11. Hi, Leslie Samuel! i really really like your Episodes. it’s very helpful for me. actually i’m a Tibetan monk who is learing homan physiology at Emory university. and my english is not good. so your videos with lecture article help me to good understand what’s going on in the classes. your speech is very clear and more easier to follow. thank you very much. i will be your student who really like teacher as you.

  12. I wish it was you I was paying thousands of dollars to teach me this, instead of an overeducated old man who has tenure and can’t be fired from his university. I learned more here than I have in the passed 3 lectures. Thanks!

  13. I wish it was you I was paying thousands of dollars to teach me this, instead of an overeducated old man who has tenure and can’t be fired from his university. I learned more here than I have in the passed 3 lectures. Thanks!

  14. Thank you very much! Your explanation of cardiac cycle is much better than both Guyten&Hall and Ganong. Helps me very much in my preparations for the physiology exam in medical school in Norway.

  15. Thank you very much! Your explanation of cardiac cycle is much better than both Guyten&Hall and Ganong. Helps me very much in my preparations for the physiology exam in medical school in Norway.

  16. im watching all your muscle and heart videos one day before my finals and i must say iv learnt more in these past two hours than i have in the past 4months!!!!! THANK YOU

  17. im watching all your muscle and heart videos one day before my finals and i must say iv learnt more in these past two hours than i have in the past 4months!!!!! THANK YOU

  18. im watching all your muscle and heart videos one day before my finals and i must say iv learnt more in these past two hours than i have in the past 4months!!!!! THANK YOU

  19. Someone may already have made this point, but I notice that in the diagram here the pressure in the ventricle drops below that in the aorta well before the aortic valve closes. I think a corrected image is on wikipedia now.

  20. Someone may already have made this point, but I notice that in the diagram here the pressure in the ventricle drops below that in the aorta well before the aortic valve closes. I think a corrected image is on wikipedia now.

  21. Hi Leslie congratulations and thank you, I just discovered your videos, I was thinking that another way that you can help you to afford this effort is become a partner with google and youtube, and allow ads in your videos, with any click to the ads that you got you will receive some money. I know you are doing this for share and help but also you have to be compensate for your time because is the only way to make this nice effort sustainable.
    Thanks again and congratulations, i wish you success.

  22. Hi Leslie congratulations and thank you, I just discovered your videos, I was thinking that another way that you can help you to afford this effort is become a partner with google and youtube, and allow ads in your videos, with any click to the ads that you got you will receive some money. I know you are doing this for share and help but also you have to be compensate for your time because is the only way to make this nice effort sustainable.
    Thanks again and congratulations, i wish you success.

  23. totally awesome and your voice is so smooth and easy to listen too, no jumping around , concise all the way through. WOW!!! is right , so glad I saw this and your other videos. Give yourself a you tube nobel prize.

  24. I seriously love your videos! Thank you for doing this! You are such a great help to students and anyone who just wants to learn everywhere!

  25. Hey, your website and your video are awesome. I m a visual learner, and your videos help me perfectly with A&P class. Thanks a lot!

  26. I needed a quick refresher and this really helped. Best video I’ve seen on the subject.

  27. thank you for the video!! I understood so much more from this than a two hour lesson from my teacher ;w;

  28. Are you Jamaican?
    I hear a hint of that in your accent.
    Anyway great videos and greetings from Jamaica!

  29. No other video or college professor has been able to make this any more clear for me…and you did it in less than 14 minutes! I wish I would have found your videos earlier! This video makes the whole process so easy to understand. Your videos make an excellent reference. Thank you!

  30. Congratulations and thank you very much for sharing all these videos. You make something complex very simple and clear !!! Some people said that it is a little bit slow, but for me it has the right speed to make it comprehensible. Thanks again !!!

  31. Thanks for the video 🙂 5* I’ve got an exam on the 9th of January 2013 and though a video could help! Will be visiting your website and may tell my friends about it! Thanks again

  32. c wave is due to rise in the atrial pressure produced by sudden closure and bulging of the tricuspid valve into the atria during isometric ventricular contraction , while v wave is due to rise in the atrial pressure as well but here due to accumulation of the venous return while the AV valves r closed , that’s all i know 😀

  33. My heart has gone through so many rhythms since watching this video. Makes you think a little!

  34. thank u indeed , an easy and simple way of teaching , but wishing a richer scientific subject .

  35. Thanks, a Lot Samuel. Kui very Nice. I had neuer unterstand that diagram but now ist cristall clear. Thanks

  36. I have a similar diagram in my book as was beyond confused in trying to make sense of it and understand how all the different parts correlate with each other. This video was absolutely amazing at describing each little part, and it put everything together like a story. Very thorough and helpful!!

  37. Im in nursing school and i couldnt understand this and I was soo stressed. But now i get it finally…I want to thank you!!!!!!!!! YOU HAVE NOOO IDEA HOW MUCH THIS HELPED ! KEEP IT UP :))))

  38. I just discovered your channel. Great presentation with easy to absorb descriptions and instructional materials. I shall explore your other videos. Thank You!

  39. I just got a decent grade in my last anaotmy exam! Thanks soo much to this channel !!! I have another exam comming up and this vid is awsome!

  40. Thank you for such informative, and easy to understand videos. You really do a wonderful job. Keep it up.

  41. Excellent teaching of the Cardiac cycle, may God continue to bless you with your great generosity

  42. Universities should replace all professors with these youtube personnel (people who actually enjoy teaching and speak proper English).

  43. Comer pan negro, cereales integrales, frutas y lechuga. La leche de vaca es peligrosa. Tener alegria, habitos positivos y menos estres, enfrentar los problemas con esperanza y optimismo. No consumir grasa animal como la crema, helado de crema, etc. Recomiendo no fumar no tomar alcohol. La depresion hace mal, debemos buscar buen apoyo. Si se puede, realizar ejercicio aerobico. Llevar una vida espiritual y tener un caracter manso. Tomar mas agua conservando la moderacion en todo:.

  44. Awesome, very clear and understandable language. It will helpful for me for my tomorrow physiology test. I learn much more in this 13 min than my hoursssssssss study.
    Thank you, love it.

  45. Thank you SO MUCH for these videos, I have a particularly inconsistent and confusing teacher for physiology who is unable to organize the material in a way that makes sense and is cohesive. I wish I could have paid you the tuition instead as these videos are how I am really learning. I wish all teachers could teach the way you do. you are amazing! Truly, from the bottom of my heart, thank you!

  46. I would to thanks you for contributing time and efforts to make this video! A+++ Awesome! You are much better than my professor, who should learn more of how to teach! Not just speak!!

  47. I would to thanks you for contributing time and efforts to make this video! A+++ Awesome! You are much better than my professor, who should learn more of how to teach! Not just speak!!

  48. because it contracts thus increasing the pressure within it,,and of course it will not contract as much as the ventricle contraction ( the blood will pass from the atrium to the ventricle below it,,so we don’t need a huge pressure to do that, bearing in mind that the flow of blood will pass passively at first depending on the pressure gradient only),,if you are asking about why does it increase in the ventricle systole,that is because the AV valves bulge into the atria…good luck 🙂

  49. i had never understand the cardiac cycle and how it works but now its diffrent i really wanna thank u

  50. I’ve found my new bestie!!!! Hello A. Thank you Sam. So clear, so concise, so simple.

  51. I’m just wondering, when the ventricles are contracting and the aortic value is open, aka when the ventricular volume is dropping, are the atria filling up with blood?

  52. I’m just wondering, when the ventricles are contracting and the aortic value is open, aka when the ventricular volume is dropping, are the atria filling up with blood?

  53. Hi mr.samuel
    these 3 questions have baffled me for a while,
    Q1:when exactly dose the sodium-potassium pump’s job start?
    1 right after the hyperpolarization 2 as soon as the cell reaches the resting state
    if the assumption #2 is correct then in that case the hyperpolarization is sending K+ out while the pump is pumping K+ in?? is’nt it contradictory or maybe the assumption #1 is correct.
    (plese check out my next comment because youtube doesnt let me to post it all in one comment)

  54. Q2:IS refactory period the same as the activity of sodium-potassium pump? if the assumption 1 is correct then it can not be said that all that part of the graph which is under the line of resting state is all”refractory state”
    Q3:why the Q and S in ORS complex are under hre isoelectric line?and which one causes the U wave, the depolarization of the purkinje fibers or repolarization of them
    I would mean a lot if you take your time answering my question,thanks….

  55. Why does the QRS wave go below the baseline? I am confused about what the axes of the graph are representing. What is on the y axis? Is going above the baseline becoming more positive? Or is it measuring volts or something of the like? Please help this poor confused student!

  56. Hi, why is the volume in the ventricles increasing before you reach the Pwave? Is there blood entering the ventricles BEFORE the atria contract?

  57. Yes you are correct. The ventricle AV valves are open and at ventricular low pressure (after relaxation) so the blood will pour in from the atria. Remember you have all that blood coming into the atria from the Vena Cava which makes atria have more pressure than vent. In young people ventricles actually get over 80% of the blood before the atria contract. When you get older however the ventricles get stiffer so that atrial pumping mechanism becomes more important to squeeze that blood in.

  58. I can’t say anything more than: “Respect.” . I do respect this man for what he is doing and I think we all should.

  59. Hey, from what I know, after the atria filling (isovolumetric relaxation), the pressure rises in atria causing the AV valves to open and blood starts filling the ventricles. THEN the atria depolarizes and contracts finish filling the ventricle (sort of like topping it off)

  60. What word that was used is not considered english. Maybe you should teach people. Judging those who help, why?

  61. I’ve learned more from you in just one video than I have from a human physiology class in a four year university. Thank you soo much! 😀

  62. Hey, Leslie. I’m a senior in college and am in Animal Physiology this
    semester and it’s killing me! Thank you so much for taking the time to post
    all these videos. They are so helpful!! I just wish I had discovered these
    earlier and I’d probably have an A and not a B! 🙂

  63. I would like to add extra clarity to one commonly tested point: the heart sounds (seen on the phonocardiogram) are caused by blood hitting a closed valve. It is easy to think that the sounds are due to the valves themselves closing, but it is not so. Immediately after the valves close, blood smashes into the newly closed valves causing turbulent flow. It is this hitting of the valves and subsequent turbulence that results in the heart sounds. Mr. Samuel was not incorrect in what he said- I just wanted to emphasize that point. Thanks.

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