 Don't forget, we could start anywhere. We could start anywhere in this process. Where I'm choosing to have a start is this moment when everything is relaxed. And it just gives us kind of a nice marker. There's no action potentials running through. Everything, all the muscles are just coming back and repolarizing. And then the myofibers are just stretching apart, the sarcomeres are lengthening. The whole thing is just relaxed. It's ventricular and atrial diastole. And you can see that here in this visual. This is from your text. You can see that there aren't any colors and there isn't anything indicating that we've got a contraction taking place. During this time, now keep in mind, it's a cycle. Something had to happen down here in order to create this situation here. But until we get down here, we might just have to accept that, okay, these are the things that are happening at stage one. First of all, the atria are filling. They're filling from the veins. The left atrium is filling from the pulmonary veins. The right atrium is filling from superior to the inferior vena cava. The AV valves are open. AV valves are open. And remember, AV valves are between the atria and the ventricles. The AV valves are open. And so as the atria fill, so do the ventricles. The myocardium is relaxed. And what that means is that the atria, we're filling passively. So essentially, the volume in the atria and the ventricles is increasing, but my pressure isn't changing. I don't know how to say there's no change. That means change, no change. The pressure isn't changing yet, mostly because we aren't having any kind of contraction. So during this first stage, this diastole stage, now think about this for a second, we're actually going to fire the SA node. Does that freak you out? Does that... If we fire the SA node, what should happen? What should follow? Remember, here's my SA node. And we're going to begin depolarizing the atria. We're going to fire the SA node. And if we depolarize the atria, what's going to happen next? We're going to expect the atria to contract. Do they contract during this stage? No, it takes time. It takes time for the SA node to send the message to everybody else. It takes time for the message from the auto-rhythmic cells to reach the contractile cells. So the SA node fires, and we don't actually end up with a contraction yet. Let's see if anything else is important. No. Can you anticipate what the next stage is going to be? We just fired the SA node in this stage. I'll let you guess what's coming next.