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I really like this picture that I found. It actually shows you really neatly where the heart sits in our body so you can see the heart is surrounded, on both sides, by ribs, right? And in fact, I didn't draw it in yet, but let me show you where the lungs would be. This is the right lung and on this side you'd have the left lung. So this is where your heart sits: between two lungs. And I'm saying left and right from the perspective of the the person who owns this heart. So this is their left and right, which is the opposite of us if we're looking at it. The heart is actually sitting between the two lungs within this protective casing that the ribs are basically there to keep all these important organs safe. And then below them, so if you draw this here. Or if I draw it, you can see now that below all this stuff is a really really important muscle. So this muscle people don't talk about this muscle, or this is not the kind of muscle that you usually see people working on at the gym, but this muscle is called the diaphragm. So your diaphragm muscle and your ribs are enclosing a space, right? The diaphragm becomes the floor, and the ribs are kind of the ceiling and the walls of this space. And if you look at the contents of this space, you'd have your lung and you'd have your heart. So, this entire space then is called your thorax. So what exactly does the heart do? Let's actually make a little bit of space now, and bring up a zoomed in version of the heart. Let me start by orienting you to the heart. This is our right lung and on the other side we have our left lung. And all this would be inside of the rib cage, but I'm not going to draw that now, because that would make it harder to see the heart itself. So to think about exactly what the heart does, I think one, kind of neat way to do it is to actually imagine that you're a cell. So put yourself in the perspective of a cell, and let's say you're a cell hanging out over here. This is you. And you can think about any part of the body that you could be. Let's say a little toe cell. So let's say you're a toe cell and your job, of course, is to live and be happy,and you've got near by, a little blood vessel. And in fact, every cell in our body has a little blood vessel that's near by. And this toe cell is just trying to make a living. And toe cells need certain things, right? They need, for example, let's say oxygen. I'll write it in white so it's very clear. They need oxygen and they need nutrients, right? So cells need certain things to live and be happy. And on the flip-side, they also make waste. They're in a sense just like us, they make waste. And that waste could be all sorts of things, and one that kind of jumps to mind is carbon dioxide (CO2). So carbon dioxide is waste for this cell. So it's making some waste and for the moment let's imagine that there's no blood flow. So, even though there's a blood vessel near by, really, no flow is happening, so I'll just write "no flow". So as the little cell makes waste. That waste, let's draw a little ball right here, it's going to start accumulating, you're going to start collecting more and more of it since the blood is not really flowing. And it might kind of end up getting all the way around our toe cell. So our toe cell is getting swamped, literally getting kind of covered by its own waste. And on the flip-side, is it getting oxygen or nutrients? No. It's not getting either of these things. So, before very long, I would say within minutes, our toe cell is thinking, "Well this is not a very happy way to live!" this is actually really very sad, this is awful. And if this continues the toe cell would die. So, what a toe cell needs, and what every cell needs, and that could be a finger cell or a skin cell, or really any cell that's living, needs flow. Right? It needs this blood to be flowing nicely and smoothly. And if there is flow then you get a very different picture, right? If there's flow then all the sudden all the waste product is actually now lifted and taken away. It's flowing away, and it's a little bit like having someone come by and pick up the trash, then you don't have trash all over the house. So then you have nice flow, and in return, oxygen and nutrients are delivered. So this stuff gets delivered as well. So, all of the sudden the cell is going to be very, very happy, and is going to be living just fine. So, really if you want all of the cells in your body to be living just fine like this cell here, you really want good flow throughout the body. And so this is really point number one. Is that you really need, somehow, to have blood flow moving and pushing blood constantly throught the body. So, to do this for billions and billions of cells you would need a pretty powerful pump, right? Something that's going to be able to pull in all the blood from the body, and then push it back out. And that's what the heart is. I mean at its core, that's exactly what the heart is doing. It's an amazing pump, pushing blood, so that you have good blood flow. And so I'm going to write that on the side as kind of job number one. These are the jobs of the heart. So jobs, and number one, would be blood flow. And I'll write systemic flow. Systemic flow. And all that systemic means is that I'm refering to the entire body. So systemic when I say that word, I just mean the entire body. All the cells in the body. Now, exactly how that happens actually you can see on this picture. So, here you have a giant vein, this is a vein, and you have an artery. This is an artery. And blood is actually going through the artery, that way. And it's actually coming into two veins, the one at the top, this is called the superior, superior just kind of means at the top. Superior vena cava. That's the name of the vein. And at the bottom here, you can't see it because it's on the other side of the heart, but there's another vein called the inferior vena cava. And these two veins, this is also a vein, these two veins are actually dragging blood in from all over the body, into the heart. And then, when the heart is ready to pump it back out, it goes into this artery, and the name of it is the aorta. So if you've heard of the aorta, this is the artery that people are talking about. So this is how blood comes and gets pumped around. But this isn't actually the only job of the heart. The job, the second job of the heart, is actually also in this picture, and it's called pulmonary flow. Pulmonary flow. So, what does that mean? Well, we know that cells are expecting oxygen, right? We know this. And that they have a lot of carbon dioxide waste. Well, it's good to move things around. It's good to move blood around. But if you actually never got rid of that carbon dioxide or brought in new oxygen, then a cell is not going to be very happy either. I mean, you can have blood flow, but at some point it's also going to want some oxygen. And it's going to want to get rid of that carbon dioxide. So, that's where the lungs come in. So what happens is that the heart, before sending blood out the aorta, before just dishing it out back into the body, it actually sends the blood over to the lungs. And it goes over to the left lung, and over to the right lung. And the blood comes back from the right lung and the left lung, and gets pushed back into the heart, and then gets squeezed through the aorta. So there's this actual extra little step here, where blood is going to and from the lungs, and that's the pulmonary flow. So the final thing you'll notice, if you look at this picture it's hard not to notice, is that there are these, kind of wriggly looking little blood vessels all over the heart. And what are these exactly? I mean, you've got red ones, and blue ones, and the blue ones are the veins, and the red ones are the arteries but are they part of the systemic flow, or pulmonary flow, or something else? Well, these vessels, all of them, together are called coronary vessels. And so specifically you might hear about a coronary artery, or a coronary vein, but together you can call them coronary blood vessels. I'll add the word blood here. So these coronary blood vessels are actually serving the heart muscle itself. I mean remember, the heart is made up of thousands and thousands, actually tens of thousands of cells and those cells, just like our toe cell that we drew, they also need oxygen, nutrients, and have waste. So, those cells are going to need blood vessels supplying them as well. So, that's what the coronary blood vessels are. They're literally the blood vessels that go to and serve the heart. So these are the ones that serve the heart. Now, if they're serving the heart muscles and the heart cells, then, think about it, would they fit under the systemic flow, or pulmonary flow? Well if the main job is to serve the needs of cells, then the coronary vessels fall under the systemic flow.