052 The Cardiac Cycle
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.
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.