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.
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.
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.
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.
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.
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.
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.
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.
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?
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.
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.
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.
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.
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.
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.
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.
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.
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.