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So we've gotten kind of used to drawing the heart with four chambers. I'm going to draw it with four chambers here, something like this, with two chambers on top. Those are the atria. And then I'm going to draw some slightly larger chambers down below. These are the ventricles. And just to label it, this would be the RA, this would be the left atrium, the right ventricle, and the left ventricle. And we've always tended to view the heart basically this way, looking at it from the front. But now, what if I wanted to actually change things around? Let's say I actually erase this bit for us. And instead of looking at it from the front, let's say actually I'm going to draw a pair of eyes here. And draw it like this. And let's say you actually could view it instead of from the front, you actually can look down on it, look from the top. What would you see? If you were looking down on, let's say, you cut away the atria, what would it look like? So, I actually did this. I actually took a picture, or drew it rather, and it would look like this. So, you look at this and you think, oh my gosh, this is so different from how we're used to seeing it. So, the first thing you want to do is kind of get oriented. So, let's orient ourselves. And we're going to use little clues. So for example, let's start with this valve right here. We know that it has three cusps-- one, two, three. And it has these little chords that you can see. These little tiny chords right here that it has, right there and there. So think about the fact that it has chords, chordae tendineae. That's what they're called. If it has chordae tendineae, you know there are only a couple of valves that have chordae tendineae. And those are the atrioventricular valves. So it's one of the two. It's got to be one of the two atrioventricular valves. And one of the two atrioventricular valves has three cusps. And in fact, we call it tricuspid. And that's what this is. This is the tricuspid valve. And the other atrioventricular valve, and you can see there are a couple of chordae tendineae here. That clues us in that this is definitely one of the two atrioventricular valves. And this is the other one. This actually only has two cusps, one, two. So this must be the mitral valve, which means that these other two are our left. One's got to be the aortic valve and one's going to be the pulmonary valve. So how do we figure out which is which? Well, let's use another clue. So you can see that there's a little hole here for blood. And actually, blood's flowing this way from there and is flowing that way from there. So, if blood is going into these little vessels, and these vessels that are wrapping around the heart, you remember, they're called the coronary vessels. In this case, they're the arteries, the coronary arteries. These coronary arteries are getting blood and it's coming from just outside of one of the valves. Well, let's think about this. I've drawn it in red, and whenever I use the color red, I'm trying to signify oxygen. So we've got oxygenated blood that's going into these arteries. And where would it be coming from? Well, if it was the pulmonary valve, that would make no sense, because that blood is definitely not oxygenated yet. And we know that the blood here is. So this must be the aortic valve. And the aortic valve has usually three flaps as well. So, this is our aortic valve. And I didn't actually draw the aortic, as we've literally cut the aorta out. But if you were to draw it, if you were to imagine how it would look, it would basically look like this. It would basically go on top like this, and blood would be going out the aorta, right? But I've cut that part away. Now, the last valve also has three flaps, one, two, three. So really, that means that just the mitral valve has two flaps, usually. All the other ones have three. And this is your last valve, and this is your pulmonary valve. So it's pretty cool. We actually were able to orient ourselves just by using some of the clues that are drawn in here. Now if I've talked about the coronary arteries, you're probably also wondering what these blue things are down here. These are the coronary veins. And a very cool thing about coronary veins is that these veins actually drain directly into the right atrium. Most veins in our body, a vein from our, let's say, kidney or a vein from your toe, all those veins drain into where? They drain into one of two large vessels, either your superior or inferior vena cava. I'm going to call them VC, vena cava. And these are the two large veins that drain the whole body. But one unique set of veins is the coronary veins, the veins that actually supply the heart. They, kind of on their own, drain into the right atrium, because this is the right atrium, right? This chamber that we've chopped away. And so they drain directly into that right atrium. They don't actually go into either one of these. So that's actually an interesting feature. And you can see that here as well. So that brings me to my final point about this picture, and that is that in the very, very center, you have a little blue circle. What is that? So, this right here is a very unique bit of tissue. And this is something we've talked about differently, in different settings. And this is our atrioventricular node. Now, the AV node is going to connect the two atria to the ventricles and send a signal. It's going to send a depolarization signal through it. So if your atria contract-- I'm just going to sketch out the atria and your ventricles-- one of the things that always comes up is well, when the signal goes from the SA node, it has to go through this AV node. And then it goes down and splits into the left and right side. And also, of course, it sends a signal to the other atrium. So this is kind of a rough scheme of what it looks like. But just keep in mind now, so we're looking directly at this box right in the middle. Now, if you could actually get a signal to go a different way, let's say you could actually get a electrical connection signal to go, I don't know, maybe through the walls, maybe from the atrial wall to the ventricular wall, or maybe it could go right through the valve. Let's imagine it can go through the valve. Well then, your AV node isn't doing a very good job, because the whole point of the AV node was to create a delay. Remember, that's one of the interesting things that it does is it creates a delay of, it was about 1/10 of a second. Well, this delay wouldn't happen if you could actually just send the signal some other way, if you could just go around or circumvent that node. But the fact is that this entire area, and I'm going to just show you this entire area, all this stuff, is actually unable to send any electrical signal. No charge signal can go through there. You're not going to get any depolarization through any of these spots, not even through the valves. And that's really important, because if you didn't have that, then basically signals would go through any which way. But now, because all this stuff around this AV node is inert, really not going to let any signal through, the AV node is the only passageway from the atria down to the ventricles. And you can see that very clearly when I cut it this way. So the final outstanding question in your mind might be well, what is it made of? This stuff, it's inert, but what is it made of? And most of this stuff is collagen. So, a lot of this kind of inert stuff is collagen. Where should I write that? Maybe I can make a little space right here. This is mostly collagen. So you get collagen and here, the rings of the valves are actually fibrous. So these are fibrous rings. So between the fibrous rings of the valves and the collagen, you basically aren't getting any sort of electrical conduction. It's not actually myocytes, it's not electrical conduction tissue, it's neither of those two types of cells. It's primarily protein. And that's why you can't get a signal through there.