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Cardiac Cycle and Cardiac Conduction System

Cardiac Cycle and Cardiac Conduction System

Author: Amanda Soderlind

Determine the components of the cardiac cycle.

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Cardiac Cycle

Source: Heart Image from Adobe Illustrator; Public Domain

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Welcome to this lesson today on the cardiac cycle overview.

Today we will be discussing the processes that occur in the cardiac cycle.

The cardiac cycle is a sequence of events that occurs when the chambers of our heart contract and relax. We have four chambers in our heart. We have our right atrium, our left atrium, our right ventricle, and our left ventricle.

When these chambers contract and relax, it acts to help pump blood throughout our body, and the contraction and relaxation-- I can't speak today-- contraction and relaxation occur at the same time on each side of the heart.

For example, when the right atrium relaxes, the left atrium relaxes. When the right atrium contracts, the left atrium contracts.

The two phases of the cardiac cycle, then, are the contraction phase and the relaxation phase.

The contraction phase is called systole, and the relaxation phase is called diastole.

We will take a look at what happens in this cardiac cycle, this sequence of relaxation and contraction of the different chambers of the heart.

Let's start with our atria. As I mentioned, we have a right and left atrium. When our atria contract, it's going to push blood into the ventricles. We have something that separates our atrium from our ventricles, and it's called an atrial ventricular valve. Here's our atrial ventricular valves, here and here

So our atrial ventricular valves will open, the atria will contract on both the right and left side at the same time, and that will push blood into the ventricles. From there, the ventricles are then going to contract and, depending on which side of the heart it is, it's either going to pump blood into the pulmonary artery or the aorta.

On this side of our heart, this is going to be our left ventricle. From here, it's going to pump blood up through our aorta. So when the ventricle contracts, it's going to contract and push blood up into the aorta.

This is our right ventricle. And what is going to happen here is that this ventricle will contract, and it's going to push the blood up through this pulmonary valve, and then the blood will make its way to the lungs to collect more oxygen. Then the ventricles will relax, and then the atria will begin to fill again for another cycle.

So basically, we have blood moving from our atria to our ventricles, and then the process will happen again.

Basically, the valves that I mentioned, the atrial ventricular valves and then also the valves that we have-- our aortic valve and our pulmonary valves-- they act to prevent back flow.

If you listen to your heart, it makes this lub-dub sound. And this lub-dub sound is the sound that your heart makes because of the closing of the heart's valves. That first part-- the lub sound-- is made by your AV valves closing simultaneously. And the dub sound is caused by the closing of the aortic and pulmonary valves simultaneously.

This lesson has been a brief overview on the cardiac cycle.

Cardiac Conduction System


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Welcome to this lesson today on the cardiac conduction system. Today, we will be discussing how heart contractions occur by certain cells located within the heart. So about 1%, a very small group of cells within the heart are a part of this cardiac conduction system. So this group of cells are self-exciting, and basically, what they do is produce electrical signals or electrical impulses that stimulate heart contractions.

So these cells that are part of the cardiac conduction system act like a pacemaker, and they drive our heart contractions. And you know our heart contractions are very important, because our heart contractions are what pump blood throughout our body. And these cells that are part of the cardiac conduction system that drive these heart contractions are independent of the nervous system.

So this is a really interesting fact, because even if all nerves that were leading to the heart were cut, our heart would still be able to beat. So these cells that are part of the cardiac conduction system are independent of the nervous system. So we're going to take a look at our diagram right here and label a few of the important parts that we need to know to understand the cardiac conduction system.

So the first part that we're going to label right here is a sinoatrial node. And then we're also going to label here is our atrioventricular node. Now, these two nodes are made up of these self-exciting heart cells. We have our sinoatrial and our atrioventricular nodes. So these two nodes play a huge role in the cardiac conduction system.

And then connected to those we have conducting muscle fibers. So they're connected to these conducting muscle fibers that spread throughout the atria and the ventricles. And I'll explain the importance of these in just a moment.

OK, so we're going to discuss how these structures allow for stimulation of heart contractions. OK, so basically, what happens is the sinoatrial nodes, which I have labeled right here, those will produce an excitation wave, and that wave will spread over both of the atria. So our atria are the upper chambers of our heart. We have one here and one here.

So it's going to spread from our left atria to our right atria. And these signals for contractions spread so fast that the cardiac muscle cells will contract together almost as one unit. So as this excitation wave spreads through the atria, it causes all of the cells to contract simultaneously. And so we have our atria then contracting, and when our atria contracts, it allows blood to flow into our ventricles.

OK, so then what happens is that wave will start to slow down as it reaches our atrioventricular node, which is the second node. So it starts to slow down, and we'll move along those conducting muscle fibers down towards our ventricles. So the slower conduction in the atrial ventricular node is important because it allows for the atria to have time to finish contracting before the ventricles contract.

OK, so we have the wave of excitation spreading over the atria, and the atria contracting, pushing blood down into the ventricles. While that's happening, that excitation wave is moving back to the atrial ventricular node and slowing down, and then it moves along those conducting muscle fibers slowly until it gets to the ventricles. And then once the atria are done contracting and the blood has filled up the ventricles, the ventricles can then contract.

So that signal will allow those ventricles then to contract and pump the blood either through the pulmonary valve or the aortic valve. So the SA node or this sinoatrial node is what generates our normal heartbeat, and it provides the stimulus for these important heart contractions. So if you've known somebody who's had an artificial pacemaker before, it's because there was some sort of malfunction in their own sinoatrial node. So an artificial pacemakers then implanted so that they still have the stimulus providing these heart contractions, providing the signals for these heart contractions. So this lesson has been an overview on the cardiac conduction system.

Video Transcription

Terms to Know
Atrioventricular Node (AV Node)

Known as the “gateway” between the atria and ventricles; the AV node slows down the rate at which the action potential travels from the atria to the ventricles allowing for more filling time.

Cardiac Conduction System

A system of specialized cells and nodes in the heart that rhythmically pace the heart and time when the chambers contract. It includes the sinoatrial node (SA node) and the atrioventricular node (AV node).

Cardiac Cycle

All of the events that occur during one heart beat that drive blood flow into and out of the heart; a series of well-timed pressure and volume changes within the chambers of the heart that drive blood flow throughout its chambers.


A term used to describe a decrease in pressure and work; measured in millimeters of mercury (mmHg).

Sinoatrial node (SA node)

The SA node is known as the pacemaker of the heart due to its ability to depolarize and repolarize faster than the rest of the cardiac conduction system. It is located in the right atrium of the heart.


A term used to describe an increase in pressure and work; measured in millimeters of mercury (mmHg).