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Two circulations in the body

Learn the difference between the pulmonary and systemic circulatory systems in the body. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

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Video transcript

So what you're looking at is basically kind of a mechanistic way of thinking about the heart, almost as if it's a couple of pumps with pipes attached to the pumps. And in a way, that's not a bad way to think about the heart. In fact, we're going to kind of move through this diagram. And I realize it looks a little bit scary. But once I start labeling stuff, you'll start seeing that it's actually not as bad as it seems. So let's get started in the upper part of the heart. The right atrium is right there, and then blood goes down into the right ventricle. And then on the other side, I'm going to label the last two chambers, the left atrium and the left ventricle. And we're going to actually follow the path of blood after it leaves the right ventricle. We're going to start our journey here at the right ventricle. So what's the first thing that kind of comes across? Well, blood leaves the right ventricle, goes through a valve. And after, on the other side, you've got this area right here. And I've drawn it as one tube with no split, and this is the pulmonary trunk. So blood is headed towards the lungs, going first through the trunk. And of course, after the trunk there's a left and right. So I'll write that up here. The pulmonary arteries are next. And remember, I call them arteries because they're going away from the heart. And there are two arteries. So pulmonary arteries, left and right pulmonary arteries. And that's why you see two things here. One here, and you see one here. And those are basically tubes. So it's going from the pulmonary trunk. It's splitting into one of the two tubes. And now I'm drawing the left and right lung up top. And of course, you know that the lungs are actually not going to look this way. So this is not anatomically correct. But this is not, as I said in the beginning, a bad way of thinking about it. So blood goes through the lungs and then comes out on the other side. And there we're going to talk about left and right pulmonary veins. And actually, here I'm going to make the point that there aren't just two pulmonary veins, one for each lung. Actually, there are usually more than that. And so when I draw this, too, I really want you to just think of the two sides, veins coming from both sides. But the actual number could be a few pulmonary veins in total. The pulmonary veins drain blood, then, into the left atrium, right? So now we're on the other side of the heart. And for the moment, I'm going to kind of pause the journey here. So we've gone from the right ventricle, around to the lungs, back again in the left atrium. And this is kind of the first part of our journey. And this part of the journey is called the pulmonary circulation. I'm actually just going to write that here. Pulmonary circulation. So the fact that this part is going-- starting from the heart and going back to the heart is one circuit, and our heart actually has two circuits. But I want to point out the circuits one at a time, and we've already kind of completed one circuit. So I'm going to take some blue paint to indicate deoxygenated blood, or blood without oxygen. I'm going to paint in how it would look. And actually, it stopped there because my arrows. But you can actually see now the deoxygenated blood kind of goes from the right ventricle, through the pulmonary trunk, through the arteries, and into the left and right lung. And once it's there, it's going to kind of mix in with the capillaries. And it's going to start getting oxygen, and then it's going to come out on the other side in the pulmonary veins. And it's again stopped a little bit there, but I can fill it in. And the pulmonary veins are going to deliver that oxygenated blood to the left atrium. So what you see kind of colored in now is the pulmonary circulation. That's the first part of our circuit. But let's now keep going and now talk about the second part of the circulatory system, which is the systemic circulation. So now the journey starts with the left ventricle, so let me start there. So for the systemic circulation, I'm going to start the left ventricle. And it's going to go around to the body, right? The body is kind of the thing that's going to be receiving all of the blood. And when I say body, I really mean lots of things. I mean things like the brain, so it could be an organ like the brain or the liver. It could also be maybe things like bones, if you have bones in your fingers. Could be the toes. Could be any part of your body that you can think of, right? So all these different organs and tissues are going to be getting blood from the left ventricle. It's going to be going initially through a giant vessel. This vessel I'm going to label it say, here, as the aorta. So this large vessel is the aorta. And of course, it branches and splits. And I haven't shown all the branches that come off the aorta. But there are many, many of them. It goes to the various tissues and organs, and then it comes back out on the other side. And at the end, kind of somewhere up here, it's going to go back to the right atrium. And it generally travels through two major kind of vessels. One is the inferior, and the other is the superior, meaning lower and upper. And they're both called vena cava. So superior vena cava and inferior vena cava. I'll just write that here. So these are the two major veins that are bringing back blood from all the different parts of the body. And so now you've actually seen the second circulation, because it all ends at the right atrium. And this is the systemic circulation. I'm going to write systemic circulation here. So now you can see that the heart is really two different systems-- or let's say two different circuits, rather. And the first one, the pulmonary circulation, is really kind of relying on the right ventricle as the pump. And the second one is relying on the left ventricle as the pump. And actually, now that you've seen it, I can now color it in. There it would kind of deliver all the blood to the various organs, and then the organs would use up the oxygen. So let me show it kind of now going back to blue just to indicate deoxygenated blood, and it goes back to the right atrium as deoxygenated blood. So this is kind of how we sometimes see it. And again, this doesn't show you or give you an appreciation for the anatomy exactly, where things branch off and where the different names make sense. But you get a kind of overall feel for the fact that we have two different circulations, and you can see where the blood is going for the two different circuits. Now, when you look at this picture, you could say, well, OK, I guess you can see where all the different tissues are getting the blood. Basically sounds like everything is coming from the oxygenated blood coming out of the left ventricle. But some tissues always kind of trip people up or kind of spark a question, and I'm just going to kind of try to target a couple of the tissues that I think people sometimes might have questions about. And one of them-- wouldn't really call it a tissue, but you could definitely call it a cell type. It's the RBC, and RBC just stands for red blood cell. So people sometimes wonder if the red blood cells are carrying blood to other parts of the body, then how do they themselves get oxygen? Do they just kind of use up some of the oxygen that they're carrying, or what exactly? And to answer this question, I would have to remind you that a red blood cell basically looks a little bit like this in cross-section. And it doesn't have any mitochondria. No mitochondria. So it has no mitochondria. And remember, mitochondria are these tiny little organelles inside of cells that are using oxygen. So if it has no mitochondria, then it is not really using oxygen. So it's not using oxygen. And really, these cells-- we call them cells, and they are in many senses of the word. But they don't really have mitochondria. They also don't have a nucleus. I mean, these are, literally, these amazingly designed cells that are made for the purpose of carrying around oxygen to the body, because they literally are bags of hemoglobin. So just remember, red blood cells don't really need oxygen. They simply carry it around. Another tissue or organ that makes people kind of wonder is the heart. The heart is pumping all the blood around. But does it actually get oxygen from vessels that are in the pulmonary circulation or from the systemic circulation or what? And here, the short answer is systemic circulation. That's kind of the quick answer. And let me actually show you where the blood vessels come from. These are called coronary vessels. I'll just write the word coronary here, coronary vessels. So coronary artery and vein. And these coronary vessels, they actually come right off the aorta here and here. So they kind of come off of the aorta right away. They're the first branches off the aorta, actually. So the first branches go and serve the heart, so it's kind of the first to get systemic circulation blood. And the veins actually drain into a spot directly into the right atrium, so there's actually a little spot right there that they drain into. So the blood from the arterial side is coming from the aorta. And on the venous side, it's actually not even dumping into the inferior or superior vena cava. Kind of a little known fact, it's going directly into the right atrium. And finally, kind of a tricky one, but the lungs. Where do the lungs get their oxygen from? And this is, I say, tricky. And I kind of saved this for last, because there are actually blood vessels. And again, don't worry so much about the anatomy in terms of where is it coming off of exactly? But it's coming off of the systemic circulation, and you've got a vessel going to the right lung. Let's say something like that. And you've got another vessel going-- I'll just draw it kind of going-- to the left lung like that. So you've got a couple of arteries that are branching off, and these are also part of the systemic circulation. And these are called the bronchial arteries. And these bronchial arteries bring then really wonderful oxygenated blood, right? These are the bronchial arteries. And you might say, well, that wasn't so difficult. I'll say bronchial vessels, because there are also some veins coming off. Actually, let me draw the veins now for you, so you can see how those kind of end up. And these veins, they come from, of course, the right lung. And they actually end up dumping in here. You can't really show blue on blue, but just trust they go there. And you've got another vein from the other lung, kind of following a parallel path and going into the veins as well. And so they don't dump necessarily directly into the inferior or superior vena cava, but I just want to show that they go into the venous side on the systemic circulation. So if that was it, that would be really simple. But actually, lungs are kind of interesting in the sense that there is a lot of mixing going on. So you've got pulmonary arteries carrying blood. You've got bronchial arteries carrying blood, and that blood mixes. And then you've got-- at the capillaries, you've got some blood kind of from both places, the pulmonary circulation and blood from the systemic circulation again mixing. And then on the other side, where the veins are bringing blood back to the heart, most of the blood-- as it turns out, most of the blood actually goes this way, into the pulmonary veins. So even though you have bronchial arteries bringing blood in-- that would be right here-- you only have bronchial arteries bringing blood in, a lot of that blood ends up going into the pulmonary circulation. So that's kind of an interesting fact, that not as much goes this way directly into that systemic circulation. So it's kind of a tricky thing to keep in mind. But the lungs, then, technically really are getting blood both from the systemic circulation, but also they're kind of mixing blood. And they're mixing blood with the pulmonary circulation. We'll talk a little bit more about this in another video. I don't want you to feel like this kind of got too confusing. But I just want you to be aware that there is mixing going on in the lungs with the systemic and pulmonary circulation, kind of a neat organ in that sense.