 Hey, you cute people. Today we're going to continue our treatment of the cardiovascular system. In the last lecture, we talked about how cardiac muscle cells initiate and respond to action potentials in order to create a muscle contraction and a pumping heart. In this section, we're taking a step back and we're going to look at how all of that is coordinated to enable heart function. So we're going to look at the cardiac cycle. And the cardiac cycle includes basically a whole billion different characteristics about heart function all in one place. So we're going to look at one heartbeat from total relaxed chilling heart, which happens 60 times every minute to contraction, pumping blood, and back to relaxed, awesome, chilling heart. While we're doing this, we're going to look at the EKG or the ECG. The EKG is actually, you might look at this little thing and be like, wow, that looks like almost an action potential graph. EKG is a treatment of voltage or electrical activity in the heart. However, it is the sum of all electrical activity in the heart. So all the action potentials that are happening throughout the heart and the directions, the different directions that they're going, if you add them all up together, you can get this shaped wave. With this right here, we can actually correlate the bumps and ridges on the ECG wave with the specific events that are taking place during the cardiac cycle. The cardiac cycle also involves volume. So we want to look at when are chambers full, when are they over full, when are they not very full, and that's something that we'll take into account in this. We're also going to look at pressure. If you can imagine, the heart is almost like a balloon that's filled with water, and you can squish it, you can squeeze and increase the pressure on the fluid in there, or we can open up a little out spout and release the pressure and move fluid along. So there's all sorts of interesting pressure factors that are going to come into consideration as we look at the cardiac cycle. We're going to look at valves and heart sounds. And in anatomy, we definitely talked about the fact that the valves prevent back flow of the blood. It enables the blood to just move in one direction throughout the body, and those valves, when they are preventing back flow, they literally snap shut, and the snapping shut is the sounds, the lubbed up that we hear when we listen to our hearts. I feel like there's something else that we're going to consider. We need two vocabulary words, especially because a heartbeat is contracting parts of the heart. We're going to have two ways, two vocabulary words to describe contraction and relaxation. We have systole, and that is contraction. And so our chambers of our heart actually contract at different times, so you can have ventricular systole, which means the ventricles are contracting, or atrial systole, which means our atria are contracting, and because of our electrical conduction system that we learned about in the last lecture, those happen at different times. The other term is what? Go ahead, take a wild guess. Diastole, and diastole is relaxation. So again, if the ventricles are relaxing, which they do during the cardiac cycle, we call that ventricular diastole. So we are going to break the cardiac cycle into five different stages, and characterized by whether we are having ventricular diastole, atrial diastole, systole, vahana, vahana, vahana, vahana. And the first one we're going to talk about is when both atria and ventricles are in diastole, so everybody is relaxed. So that's where we're going to start in the cardiac cycle, knowing that the whole thing is a big cycle so we could literally start anywhere. Onward.