Cardiac Cycle

Cardiac Cycle Animation & Overview

Dr. Najeeb is a fantastic instructor and this video covers the cardiac cycle. Below the video is a transcript of the audio in order for you to print this page and have it available with you at all times. Video contents are the property of Dr. Najeeb.


Now, today we will be talking about the cardiac cycle. And cardiac cycle we will be discussing in two fashions. In one way we will be discussing cardiac cycle while I will be doing different diagrams. And in second phase we will discuss the cardiac cycle when I will be presenting the events during the cardiac cycle graphically.
So first of all lets see cardiac cycle, all the events which occur during one cardiac cycle means all the mechanical events which occur during one systole and one diastole. First of all, what is cardiac cycle? One cardiac cycle is equal to: One cardiac cycle = one systole + one diastole.
Systole means contraction and diastole means relaxation. Right? Now first of all we start the events from the left heart from atrial contraction. And the events which I will present in the diagram from the left heart basically similar events occur in the right heart.
Let suppose here is left atrium, here is left ventricle, and aorta. Right? Now, we are to study the events in the left ventricle.
#1 when the atria contract, lets suppose in this diagram I’m presenting the phase of cardiac cycle when left atrium is contracting. Now, listen carefully. When we talk about the cardiac cycle we have to take care of the three chambers. Number one, in a particular phase what is happening in the atrium. Number two, in the same phase, what is happening in the ventricle? Number three, in the same phase what is exactly happening in the aorta.
Again let me repeat it. When we are going to discuss this particular phase of cardiac cycle right, what we are to discuss what is happening in; atrium, what is happening in ventricle, and what is happening in aorta. If I say that this is the phase of atrial contraction then it means that mitral valve should be. . . Closed or open? This should be open. Is that right? But at this very moment, aortic valve will be? Closed. Aortic valve will be closed.
It means that during atrial contraction there is a communication between the left atrium and the left ventricle, but there is no communication between the left ventricle and aorta. It means for the left ventricle input system is working, but for the left ventricle output system is not working. Is that right?
Now, before the atrial contractions start about 80% of ventricular filling is already done. About 80% of ventricular filling is already done before the atrial contractions start. It means that atrial contraction when it occurs it just adds the last 20% of ventricular filling. Now this point is very important, as I told you 80% of ventricular filling is passive it does not require atrial effort. It means that if someone has atria which are mechanically not working still 80% of ventricular filling will be done and patient will not suffer significantly. Atrial filling by the atrial contraction the last 20% of ventricular filling which is done by active atrial contraction that really becomes more valuable to human beings when they are physically active, that is during exercise. Right?

Now, when left atrium will contract naturally the pressure in the left atrium will go, down or up? The pressure in the left atrium will go up. So lets suppose this was the pressure in the left atrium. As soon as left atrium contracted the pressure in left atrium slightly increased. So this wave is produce when there is increased pressure within the left atrium due to atrial contraction. And this wave is called “a” wave. This is a pressure wave. It is produced in which chamber? Left Atrium. Of course all the events which occur in the left side of the heart the same events occur on the right side of the heart. But first we will discuss the events on the left side of the heart.
So what is “A” wave. A wave is pressure wave. Why is it produced? It is produced due to atrial contraction. Where is it produced? It is produced within left atrium. But listen. When atrium is contracting and pressure in atrium is increasing we must notice that mitral valve must be open in a healthy person. So it means all the pressure which is present in the atrium is easily communicated to the ventricle. It means that when pressure in the atrium increases of course blood moves into the ventricle. And when blood moves into ventricle the pressure is transferred to what chamber? Left ventricle.
So we can say that at this particular moment when atrium is contracting before that, just before the contraction of the atrium pressure here was 0. It was very less pressure. It was a relaxed chamber. But as soon as atrium contracts it pushed the last part of the blood the pressure starts increasing. But it is a very mild increase in pressure. Just after the atria completes its contraction the action potential is transferred into the ventricles it goes through the AV node. And now it’s the ventricles turn to contract. So, the next phase. What we’ll see now the ventricular contraction will start. So this phase we will lable this phase as “phase of atrial contraction.
Now we will move into the next phase. I will just brief it. What happens during atrial contraction? Left atria contracts, pressure wave is produced in the left atrium, mitral valve is open and pressure and the volume is transferred to the left ventricle. But you have to remember that the aortic valve is closed because pressure in the aorta is more than the pressure in the left ventricle at this moment. So in this phase you must know what is happening in the atrium, what is happening in the ventricle, what is happening in the aorta. And you must be clear what happens to input valve (which is mitral valve) and what is happening to output valve (which is aortic valve). Are you clear about this? Any questions on this phase?
After that naturally current will come, depolarization will come to the left ventricle. And now is that start of left ventricular contraction. Of course, by that time it is in the next phase the atrial contraction has been completed and we are entering into the phase of atrial relaxation, but onset of ventricular contraction. Now, in the next phase again I will show all three chambers:
This is the input chamber which is the atrium. Here the ventricle the major pump. And this is the output chamber which is the aorta. Now in the next phase it is properly filled. As you know already at the onset of this phase, aortic valve is closed. At the onset of ventricular contraction, aortic valve is already closed because pressure here is about 80mmHg. Now, what really happens at this stage is when the ventricles start contracting pressure in the ventricles start building. Pressure in the ventricle is going up. When pressure in the ventricle is going up lets suppose pressure in the ventricle becomes around 5mmHg. This is the pressure where intraventricular pressure becomes more than atrial pressure. When ventricles start contracting at the very beginning of contraction pressure goes up around 5-6mmHg and now pressure in the ventricle is slightly above the pressure in atrium. As soon as, you can say pressure in the ventricle becomes more that atrium blood will try to go back and mitral valve will be closed.