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Flow through the heart

Learn how blood flows through the heart, and understand the difference between systemic and pulmonary blood flow. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

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  • purple pi purple style avatar for user Gabby
    what color is the heart?
    (192 votes)
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  • piceratops ultimate style avatar for user ∫∫ Greg Boyle  dG dB
    Do the tricuspid, mitrial and aortic valves have the same basic physical structure? If they don't, why is there a difference?
    (97 votes)
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    • piceratops seed style avatar for user Andrew Thomas
      Hey there Greg. This is a good question and in short the answer is no. The Tricuspid valves (also called the atrioventricular valves) have a three flap design. Imagine three flaps of tissue that come together to form a seal so that there is no back flow of blood. Since they have three flaps of skin, they are called TRIcuspid (easy to remember). The other valves, the Pulmonary and Aortic valves are semi-lunar valves. These two valves are called this because they have two flaps of skin that can only open in one direction, again to prevent backflow. To help envision this, imagine that you have a hinged door on the wall. With the turn of a knob, you are able to open that door and walk in. However, you are not able to open the door in the opposite direction because the hinge will not allow the door to swing that way. This is the same principle as the semi-lunar valves. They have fibrous "strings" attached to the back so they can only open in one way. The way these valves open is because the blood is literally being forced out of the Ventricle, and this is the main reason why they are different in structure. So when the blood initially flows into the heart, it needs to be stopped in the Atrium and the Ventricle before being pushed out of the heart. This means that there needs to be a reliable barrier between the Atrium and Ventricle which is why there are three flaps. The other valves are meant to be more easily opened so that blood can easily be pushed out of the heart. I hope that helps!
      (194 votes)
  • marcimus pink style avatar for user Aakriti
    Why are there three 'flaps', hence the name tricuspid valve, between the right chambers of the heart as compared to two flaps, because of bicuspid and bi meaning two, between the left chambers of the heart? (even though the right side has blood flowing at a lower pressure) Thank you.
    (36 votes)
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    • hopper jumping style avatar for user Jacob Anderson
      I thought this was a really interesting question, and I did a little bit of research. I only found one theory that makes some sense.

      The theory is as follows (paraphrased):
      -During embryonic development pressures are higher in the right side of the heart than the left
      -Blood joins the left atrium from the regular sources (pulmonary veins) and also from the right atrium.
      -The excess blood in the left atrium forces it to bulge to make room for excess blood
      -This bulging forces two leaflets of the proliferating cell to join and form one leaflet
      http://www.madsci.org/posts/archives/1997-03/855255174.An.r.html#conclusion

      I think there were two problems with this. It seems that an assumed premise is both the tricuspid valve and mitral valve start out with three leaflets. I think the assumption is not grounded in any facts, but speculation.

      My second problem is that the ductus arteriosis and the high resistance in the lungs limits the blood returning to the atrium via the pulmonary vein. These two factors take away from the "bulge" argument.

      As of right now, I do not believe there is an answer to your question, but it definitely has to do with fetal development and early fetal circulation.
      (34 votes)
  • leaf blue style avatar for user Brenson1212
    if heart pumps blood throughout the body, what supplies blood to the heart then?
    (11 votes)
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    • orange juice squid orange style avatar for user Vinícius Rosa Correia
      The heart has a thick layer of muscle that needs to be supplied with blood to live and maintain our life. Some of this blood comes from blood diffusion from inside, but that's not enought no fully penetrate heart's thick layer of muscle. Coronary arteries exist for that, those are the first branches of arteries that come out from aorta to supply all heart's extension.
      (19 votes)
  • male robot hal style avatar for user Corey Piper
    Why exactly does the heart beat twice? Is it like because pumps once to go through the first two left chambers and then the lungs and then pumps another time to get out of the lungs and into the right chambers and arteries?
    (5 votes)
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  • aqualine tree style avatar for user Trina Stanley
    when you cut an artery a lot of blood comes out, but when you cut a vein its just like any other cut. why is that?
    (5 votes)
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    • piceratops ultimate style avatar for user Ivan Occam
      Blood in the arteries is under a lot higher pressure, whereas the veins are very low pressure vessels.

      The higher pressure causes the arteries to leak violently when they are cut, but the blood just seeps out of veins because they are low pressure.
      (18 votes)
  • blobby green style avatar for user iverson_crossova
    Which of the following veins carries blood with relatively HIGH levels of oxygen? this question that was asked in the video I don't get... how is the pulmonary vein the one that carries high levels of O2? Doesn't it carry deoxygenated blood to the lungs to get oxygenated? so wouldn't the deoxygenated blood contain LOW O2 and not HIGH O2 ? I'm confused
    (3 votes)
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    • spunky sam blue style avatar for user Raliflowz
      Vien- carries blood towards the heart (Note: O2 concentration does not matter)
      Artery- carries blood away from the heart (Note O2 concentration does not matter)

      So in the case of the lungs-
      1. After blood comes to the heart from the body it goes into the right side of the heart. The right side of the heart pumps blood out into the Pulmonary artery going away from the heart and towards the lungs, this blood still has not been oxygenated!

      2. Next the blood enters the lungs and performs gas exchange, the blood now has high O2 and goes to the pulmonary vein back towards the heart.

      3. This newly oxygenated blood goes through the left side and out the aorta, the main artery going from the heart to the body.

      I hope that helps you see where the O2 is at better.
      (9 votes)
  • starky sapling style avatar for user Baxton_Official
    Is the heart a type of muscle or organ?
    (3 votes)
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  • orange juice squid orange style avatar for user bohl.grace
    is there any difference between the Superior Vena Cava and the Inferior Vena Cava?
    (3 votes)
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  • aqualine ultimate style avatar for user georgy varghese
    what makes the heart pump?
    (4 votes)
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    • spunky sam green style avatar for user Mark Bell
      The heart has a natural pacemaker called the 'Sinoatrial Node' (SA Node) which polarises (contracts the heart muscle) and depolarises (relaxes the heart muscle) using automaticity. It is this continuous cycle that causes the heart to 'pump' the blood through the Pulmonary artery into the lungs and through the Aorta and around the body's cells. The pumping is involuntary and doesn't actually need your brain to command the motion. The heart will beat at an 'intrinsic rate' (set frequency) even without a brain on the network
      (4 votes)

Video transcript

So what you're looking at is one of the most amazing organs in your body. This is the human heart. And it's shown with all the vessels on it. And you can see the vessels coming into it and out of it. But the heart, at its core, is a pump. And this pump is why we call it the hardest working organ in our body. Because it starts pumping blood from the point where you're a little fetus, maybe about eight weeks old, all the way until the point where you die. And so this organ, I think, would be really cool to look at in a little bit more detail. But it's hard to do that looking just at the outside. So what I did is I actually drew what it might look like on the inside. So let me actually just show you that now. And we'll follow the path of blood through the heart using this diagram. Let me start with a little picture in the corner. So let's say we have a person here. And this is their face, and this is their neck. I'm going to draw their arms. And they have, in the middle of their chest, their heart. And so the whole goal is to make sure that blood from all parts of their body, including their legs, can make its way back to the heart, first of all, and then get pumped back out to the body. So blood is going to come up from this arm, let's say, and dump into there. And the same on this side. And it's going to come from their head. And all three sources, the two arms and the head, are going to come together into one big vein. And that's going to be dumping into the top of the heart. And then separately, you've got veins from the legs meeting up with veins from the belly, coming into another opening into the heart. So that's how the blood gets back to the heart. And any time I mention the word vein, I just want you to make sure you think of blood going towards the heart. Now if blood is going towards the heart, then after the blood is pumped by the heart, it's going to have to go out to the heart. It's going to have to go away from the heart. So that's the aorta. And the aorta actually has a little arch, like that. We call it the aortic arch. And it sends off one vessel to the arm, one vessel up this way, a vessel over this way. And then this arch kind of goes down, down, down and splits like that. So this is kind of a simplified version of it. But you can see how there are definitely some parallels between how the veins and the arteries are set up. And arteries, anytime I mention the word artery, I want you to think of blood going away from the heart. And an easy way to remember that is that they both start with the letter A. So going to our big diagram now. We can see that blood coming in this way and blood coming in this way is ending up at the same spot. It's going to end up at the-- actually, maybe I'll draw it here-- is ending up at the right atrium. That's just the name of the chamber that the blood ends up in. And it came into the right atrium from a giant vessel up top called the superior vena cava. And this is a vein, of course, because it's bringing blood towards the heart. And down here, the inferior vena cava. So these are the two directions that blood is going to be flowing. And once blood is in the right atrium, it's going to head down into the right ventricle. So this is the right ventricle, down here. This is the second chamber of the heart. And it gets there by passing through a valve. And this valve, and all valves in the heart, are basically there to keep blood moving in the right direction. So it doesn't go in the backwards direction. So this valve is called the tricuspid valve. And it's called that because it's basically got three little flaps. That's why they call it tri. And I know you can only see two in my drawing, and that's just because my drawing is not perfect. And it's hard to show a flap coming out at you, but you can imagine it. So blood goes into the right ventricle. And where does it go next? Well after that, it's going to go this way. It's going to go into this vessel, and it's going to split. But before it goes there, it has to pass through another valve. So this is a valve, right here, called the pulmonary valve. And it gives you a clue as to where things are going to go next. Right? Because the word pulmonary means lungs. And so, if this is my lung, on this side, this is my left lung. And this is my right lung, on this side. Then these vessels-- and I'll let you try to guess what they would be called-- these vessels. This would be my-- I want to make sure I get my right and left straight. This is my left pulmonary artery. And I hesitated there just to make sure you got that because it's taking blood away from the heart. And this is my right pulmonary artery. So this is my right and left pulmonary artery. And so blood goes, now, into my lungs. These are the lungs that are kind of nestled into my thorax, where my heart is sitting. It goes into my lungs. And remember, this blood is blue. Why is it blue? Well, it's blue because it doesn't have very much oxygen. And so one thing that I need to pick up is oxygen. And so that's one thing that the lungs are going to help me pick up. And I'm going to write O2 for oxygen. And it's also blue. And that reminds us that it's full of carbon dioxide. It's full of waste because it's coming from the body. And the body's made a lot of carbon dioxide that it's trying to get rid of. So in the lungs, you get rid of your carbon dioxide and you pick up oxygen. So that's why I switch, at this point, from a blue-colored vessel to a red-colored vessel. So now blood comes back in this way and this way and dumps into this chamber. So what is that? This is our left atrium. So just like our right atrium, we have one on the left. And it goes down into-- and you can probably guess what this one is called-- it's our left ventricle. So just like before, where it went from the right atrium to the right ventricle, now we're going from the left atrium to the left ventricle. And it passes through a valve here. So this valve is called the mitral valve. And its job is, of course, to make sure that blood does not go from the left ventricle back to the left atrium by accident. It wants to make sure that there's forward flow. And then the final valve-- I have to find a nice spot to write it, maybe right here. This final valve that it passes through is called the aortic valve. And the aortic valve is going to be what divides the left ventricle from this giant vessel that we talked about earlier. And this is, of course, the aorta. This is my aorta. So now blood is going to go through the aorta to the rest of the body. So you can see how blood now flows from the body into the four chambers. First into the right atrium-- this is chamber number one. And then it goes into the right ventricle. This is chamber number two. It goes to the lungs and then back out to the left atrium. So this is chamber number three. And then the left ventricle. And this happens every moment of every day. Every time you hear your heart beating, this process is going on.