052 The Cardiac Cycle

052 The Cardiac Cycle

Leslie Samuel IBTV, Physiology, The Circulatory System 229 Comments

Ever wonder what happens in a heartbeat? What happens inside our heart when we hear the ‘lub-dub’ sound?

Watch and see as Leslie describes in full detail what a cardiac cycle is and how this is reflected in one heartbeat.

Have fun!

Transcript of Today’s Episode

Hello and welcome to another episode of Interactive-Biology TV where we’re making Biology fun! My name is Leslie Samuel, and in this episode, Episode 52, I’m going to be talking about the ‘Cardiac Cycle.’ When I say the cardiac cycle, I’m talking about all of the events that happen with one complete heartbeat. So, we’re going to go into the details of this. There are a lot of details but, we’re going to try to break it down, one step at a time to make it as easy and as fun as possible. Let’s get right into it.

Here, we are looking at the entire cardiac cycle. We have this graph here that shows a number of details and, to make this as easy as possible, what we’re going to do is we’re going to take everything and break it down one section at a time. I want you to follow me on this. Like I said, we’re talking about one complete heartbeat and with one complete heartbeat, I’m talking about the contraction and the relaxation of the atria and the contraction and the relaxation of the ventricles. Anytime I say contraction, I’m referring to systole which is the CMS contraction; and, when I say relaxation, I am referring to diastole.

What we’re going to do is first, I’m going to describe what all these things show, and then, we’re going to take it one stage at a time.

Here, we’re looking at the phonocardiogram. In other words, we’re looking at the sounds that we hear when the heart beats, when we’re looking through the different stages of the heartbeat. Then, here we’re looking at the electrocardiogram, and we’ve looked at that in a previous episode. You can always revisit that to get a good understanding of how the electrocardiogram or the ECG or the EKG works.

Then, we’re looking at the ventricular volume. In other words, we’re looking at the amount of blood, the volume of blood in the ventricle, specifically we’re looking at the left ventricle. Then, here in blue, we are looking at ventricular pressure. So, that’s the pressure in the left ventricle. Here in gray, we’re looking at the atrial pressure, so, the pressure in the atria. Then, last but not least, we’re looking at the aortic pressure, and that’s the pressure in the aorta which is this structure right here, which sends the blood from the heart to the rest of the body.

That’s an overview of what we’re going to be looking at. Now, we’re going to take it one section at a time. This has a lot of details in it. It summarizes the entire cardiac cycle so, we’re going to take it one step at a time and get a good understanding of what is going on. Actually, I am going to start right in this section here. The reason I’m going to start here is because here we have showing the P, Q, R, S complex and the T wave. This is one full cycle but, this is labeled differently so, we’re going to look at that.

I’m going to start by looking at the electrocardiogram. First thing we’re going to see is we have the P wave, and if you remember from a previous episode, the P wave represents atrial depolarization. So, we’re talking about the depolarization of the atria. Once this happens, that is going to cause the atria to contract. So, let’s jump up here and look at the atrial pressure. You can see here, right after the P wave we get this increase in pressure in the atria. That is when the atria are contracting and that is why we see that increase in pressure as a result of the depolarization of the atria. Once again, the P wave represents atrial depolarization that is going to cause atrial contraction or, as you can see here, atrial systole. What that’s going to do is that’s going to push the last bit of blood from the atria, which is this part over here (I’ll show it with a blue pointer), from the atria into the ventricle. So, we’re going from the left atria, left atrium to the left ventricle when that contracts. That is why we see this quick increase in the amount of blood in the left ventricle. So, we get an increase in ventricular volume right after the atria contracts.

We’ve described this section here which is the atrial systole. And then, after the P wave, we then get the QRS complex. Once again, we’ve covered this in a previous episode, I think it’s Episode 48 where we talked about the EKG. The QRS complex represents the depolarization of the ventricles. And of course, once the ventricles depolarize, that’s going to cause the ventricles to start to contract just like after atrial depolarization, we get atrial contraction.

Now, after ventricular depolarization, we also get ventricular contraction. So, that is where we’re starting this phase of systole where the ventricles are contracting. However, there is a short period here, and you can see here the same period. It’s called isovolumic or isovolumetric contraction. That is when these valves, the atrioventricular valves and the semilunar valves are closed as there’s a buildup in pressure. You can see there’s a significant increase in pressure here during the stage of isovolumetric contraction. Of course, the volume of blood in the ventricles is staying the same. So, here, you see the left ventricular volume is staying the same. At a certain point, we will have enough pressure in the ventricle to cause the semi-lunar valve to open so that you can pump the blood into the aorta. Now, at what point is that? Well, you can see here the aortic pressure is somewhere around 80 mm Hg as you can see here. So, once we have enough pressure to overcome the pressure in the aorta, that’s going to cause the valve to open, and then the blood can be sent to the aorta which can then go to the rest of the body.

In the case of the ventricular volume, when those valves open, and the blood leaves, that’s going to cause the ventricular volume, the amount of blood in the ventricles, to decrease significantly. Why? Because the ventricles are contracting, we’re sending the blood out to the body so, that it can go to the muscles, the organs that need blood and oxygen, and all that good stuff. That is the result of the ventricles contracting, so you can see the increase in pressure, the blood being ejected so, you can see the decrease in ventricular volume, and that happens right after the QRS complex which is the depolarization of the ventricles.

So, thus far, we’ve looked at the P wave, atrial depolarization causing atrial systole or atrial contraction. Then, we’ve looked at the QRS complex which then causes ventricular contraction and increase in pressure. Here you have isovolumetric contraction where the valves are close and no blood is exiting the ventricles and then, you have the semilunar valves opening when the pressure in the ventricles can overcome the amount of pressure in the aorta, and that causes ventricular ejection. You can see the term here, ‘ejection’ that means that the blood is being ejected from the ventricles. You can see there’s a significant decrease in ventricular volume.

Then, we have this T wave that represents the repolarization of the ventricles. When the ventricles repolarize, that’s also going to cause the ventricles to relax so, you can see now the amount of pressure in the ventricle is going down significantly. Then, at a certain point, we didn’t talk about this in Episode 51 where we spoke about isovolumetric contraction, the same thing is going to happen on the opposite end where we have isovolumetric relaxation. And, that’s because the valves are closed, the pressure is decreasing, so, you’re going to get a significant drop in pressure while the volume of blood in the ventricle is staying constant. Then, once that’s done, the ventricles are relaxed, the atria are also relaxed the, we can get ventricular filling where blood is being sent back. You can see here during relaxation, this is where the filling happens, blood is coming back from the body, blood is coming back from the lungs and entering the ventricle, so the ventricular volume is going to start going up again, it’s going to increase as you see here, the full thing, increases until we get that P wave again which causes atrial depolarization, and the cycle continues.

Now, there’s one thing we didn’t look at as yet, and that’s the phonocardiogram. You can see here, we have a signal here, a signal here, a third signal here, and there’s actually even a fourth signal. What these represent would be the sounds of the heartbeat. When you listen to the heartbeat, you usually hear a ‘lub-dub’ sound: ‘lub-dub,’ ‘lub-dub,’ ‘lub-dub.’ What that refers to would be the first and the second sound that you see here. Yes, there’s a third sound and there is a fourth sound but, you don’t hear those because they’re not strong sounds. The two main sounds that you hear are the first and the second. What that refers to is the closing of these valves. So, at this point after the QRS complex where the ventricles depolarize and the ventricles start to contract, when they start to contract, that’s going to cause an increase in force closing the valve, closing this atrioventricular valve, and when that valve shuts, you’re going to hear that first sound, the ‘lub’ sound. Then, here, where the ventricles relax after sending all of that blood out, you’re going to get a closing of the semilunar valves and you’re going to hear the second sound which is the ‘dub’ sound. So, you get ‘lub-dub,’ ‘lub-dub,’ ‘lub-dub.’ That is the heartbeat that you hear. This third sound here is when the ventricles are being filled, and as the ventricles are being filled, there’s going to be some turbulence in there and that’s going to cause a third sound but, you don’t hear that as much because there are no valves that are closing or anything of that sort. It’s just blood that’s flowing. The fourth sound that there is represents the filling of the atria with blood. Of course, since the atria are significantly smaller than the ventricles, that sound is going to be even softer. So, you don’t even see it in this phonocardiogram.

This shows the entire cardiac cycle. I hope that now that we’ve gone through all of the details, this entire graph does not look as intimidating as it might have looked when we first started. You can always revisit some of the previous episodes. In Episode 48, we deal with the electrocardiogram, we talk about isovolumetric contraction in Episode 51, and there are a few other episodes covering other details involved in this process.

That’s all for this video. Of course, you can visit the website at Interactive-Biology.com for more Biology videos and other resources. That’s it for now, and I’ll see you on the next one.

Comments 229

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      Author
  1. rmcdaniel423

    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?

  2. InteractiveBiology

    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

  3. Aku899

    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!

  4. Aku899

    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!

  5. sherab

    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.

  6. mww7889

    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!

  7. mww7889

    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!

  8. cicinh0

    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.

  9. cicinh0

    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.

  10. 786slizza

    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

  11. juman islam

    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

  12. juman islam

    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

  13. unloopy1

    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.

  14. unloopy1

    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.

  15. YcDoc

    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.

  16. YcDoc

    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.

  17. zorro vaughn

    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.

  18. magicalair

    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!

  19. tuanngo0990

    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!

  20. hesc2139

    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!

  21. elgonzobonzo

    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 !!!

  22. Chris D'Almeida

    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

  23. zeinab galal

    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 :D

  24. mimi mj

    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!!

  25. 703411

    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 :))))

  26. MMM2345DDD

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

  27. Trang Cao

    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!

  28. Patrick Wilkinson

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

  29. volumelow

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

  30. BibliaInspirada

    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:.

  31. Saima Zaki

    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.

  32. Burnin' Helen

    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!

  33. Praew Chinsomboon

    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!!

  34. Praew Chinsomboon

    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!!

  35. Areen Ghazi

    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 :)

  36. Sartorius988

    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?

  37. Sartorius988

    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?

  38. sam aryan

    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)

  39. sam aryan

    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….

  40. Jaroli

    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!

  41. Ilhan Ali

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

  42. thegarbageman21

    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.

  43. Canpean Ionut

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

  44. Jennifer Chen

    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)

  45. Debster675

    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! :D

  46. Courtney Westbrook

    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! :)

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