Now let's put together everything we just talked about into one concise package. So, let's begin with diastole, which is, again, the relaxation of the ventricles and atria, which is going to cause them to fill up with blood. Alright. That's what we're seeing here. Right? The ventricles and atria are going to fill with blood. Then, as we can see in our EKG signal, the SA node is going to initiate the action potential, and that's going to cause the atria to contract, or atrial systole, if you will. We have atrial systole, which means that our right atria and left atria are going to empty their blood into the ventricles. We can see that happening there. Now, before our ventricles contract, remember there's a slight delay, and you can actually see that in the signal, it's this little, like, flattening right there. That's the slight delay of the AV node. Right? Delaying the action potential, which is going to allow the atria to completely empty into the ventricles.
Now, in systole, remember, the action potential is going to start at the bottom of the ventricles and move up through the Purkinje fibers, and this causes the ventricles to contract and push blood into the arteries. And in our EKG, that's this big depolarization, a fancy science term for it there. It's the big electrical signal right there, that's that big ventricle contraction. And you can see that happening in these images here. We have our ventricles super full of blood in the left ventricle. These guys are super full of blood in this image right here, and then we're going to get contraction in this image. Right? Those ventricles are getting squeezed and they're going to push the blood into the arteries. Those are, of course, the pulmonary artery and this guy right here, the aorta. So after that, we're going to have relaxation. Right? Go back to diastole, after systole, and that's going to cause the atria and then the ventricles to fill back up with blood, and that relaxation can be seen on the EKG here at the tail end.
Now this whole process is again called the cardiac cycle, and we like to measure it in certain ways. I mean, we look at the electrical signals with EKG, but sometimes we want to know about other facets of the cardiac cycle. One of which is cardiac output, and this is going to be the volume of blood that's pumped per minute by the ventricle. So this is a rate of volume per minute, and it's basically looking at two measurements and putting them together. Those measurements are heart rate, which is heartbeats per minute, or beats per minute. And, you know, this is often written, for example, in music as BPM. Right? If you, you know, like that electronic dance music, you want those high BPMs. You know, obviously, our heart rate we don't want to be too high, but a very interesting thing to note is our heart rate is around 60 beats per minute. Right? A beat every second. And dance music, some of the most popular dance music, is actually at about 120 beats per minute. Double the heart rate. Right? So, kind of an interesting thing to take note of, how, you know, something as obscure as, like, or abstracted from nature as electronic music, adheres to our natural cycles, so to speak.
Now, getting a little distracted, let's get back to cardiac output. The other measure that is involved is stroke volume, which is the volume of blood pumped by a single ventricle contraction. So this is not a rate, this is just a volume. So combining heart rate and stroke volume, you can get cardiac output and see how much blood is being pumped per minute. Let's flip the page.