[BLANK_AUDIO] In an earlier class we talked about the route that blood follows as it flows through the heart. I hope you've all been studying that. >> Dr.Scanga, I've been studying it and I notice it always follows the same path. And I was wondering what keeps it from going in a different direction? >> You mean, that blood always goes from the right atrium to the right ventricle and then into the pulmonary trunk? >> Correct. >> Yeah. It's the heart valves that are, that ensure that blood flows in one direction only as it goes through the heart. And I'm glad you asked that question because I wanted to talk about the heart valves today. So we actually have four heart valves. And they're organized as two pairs of heart valves. We have what are called atrioventricular valves or AV valves, and then we have semilunar valves. I'm going to talk first about the atrioventricular valves. Why do you think they have that name, atrioventricular? >> [INAUDIBLE]. >> Yeah. Because they're between the atria and ven, ventricles. >> Exactly, yes. So, we have, over here on the, whoops. Over here, on the right side of the heart, we have the right AV valve, which is positioned between the right atrium and the right ventricle. And then, on the left side of the heart, we have the left AV valve, which is positioned between the left atrium and left ventricle. All of the heart valves are composed of a dense, fibrous connective tissue, which means that they are kind of sturdy. And they have to be sturdy because they move around a lot inside the heart as the heart is beating. And that dense connective tissue is covered on both sides with a very smooth, thin layer called endocardium. The smooth, thin layer is important because it allows blood to flow through and past the heart valves with less friction. So when we look specifically over here at the right AV valve, what we see is that there are three cusps of connective tissue which create that heart valve. And so we refer to the valve as the tricuspid valve because of those three cusps of tissue. On the left side of the heart, the left AV valve is made up of two cusps of tissue. And so it could be referred to as the? >> Bicuspid. >> Bicuspid valve. But, it's more often, in fact I almost want to say it's always called the mitral valve, not the bicuspid valve, okay? So, those two valves are what ensures that blood does not flow from the ventricle into the atrium. They prevent blood from flowing backward from the ventricle into the atrium. How do they do that? What happens let's take a look at what these valves look like in your textbook. And you can see the valves in both the open and the closed position. [BLANK_AUDIO]. Notice that when the valves are open, the cusps are in the position that you see here in this model. And they just create an open channel from the atria into the ventricles. So blood can flow easily from the right atrium through the tricuspid valve, into the right ventricle. Or it can flow easily from the left atrium through the mitral valve into the left ventricle. What happens though, is when the ventricles contract, [SOUND] if you would just think about this, you have to imagine it, because we're looking at our model. But if you would just think of the ventricles contracting and squeezing in the blood that's present in the ventricles, what do you think is going to happen to pressure in the ventricles when that squeezing action happens? >> It will increase. >> Pressure is going to increase in the ventricles, right? And at the point that pressure in the ventricles gets higher than pressure in the atria, the cusps of the AV valves swing closed and that prevents blood from flowing from the ventricle into the atrium. >> I was going to ask, but why wouldn't the the cuspid just go back into the atria, like swing-. >> Like this. [LAUGH]. >> Yeah exactly. >> And then per, and then blood would flow into the atria if they swung like that, right? Well structurally, we have some structures that prevent that from happening. Now we can't see them very well in this model, but you can see them pictured in your textbook. And I can see these relevant structures on this model. So specifically, what we have attached to the free edges of the valves cusps are little thready structures that are made up of very tough collagen. Those fibers are called chordae tendineae and they anchor into a special part of the miacardium called papillary muscles. So the papillary muscles just extend like little fingers from the walls of the myocardium. And chordae tendineae attach on the ends of the little fingers at one end. And then the chordae tendineae attach to the valve cusps at the other end. So what happens is that when the ventricles contract, the papillary muscles contract too. And they create tension in the chordae tendineae and that tension prevents the free edges of the valves from swinging upward. Okay? So that tension that's created when the chordae tendineae ensures that the valves will close and then stay in the closed position. Okay. So why did the AV valves close. >> Because of the increased pressure when the ventricles contract. >> And they will close when pressure in the ventricles? >> Is higher than pressure in the atrium. >> Is higher than pressure in the atrium, right. Now, why do you think the AV valves open? >> When the pressure decreases so when the blood goes into the pulmonary trunk, there's less pressure in the ventricle, and there's more pressure in the atrium, so then they need to open to release that blood. >> That's, that's the general idea. So you remember, we have the heart contracting like this, atria contract, ventricles contract. And then they relax. Right? When the ventricles relax, pressure in the ventricles falls. And when pressure in the ventricle becomes lower than pressure in the atria the AV valve is open, right? And then in the next heart beat, when the ventricles contract, pressure in the ventricles will get higher than pressure in the atria, and the AV valves will swing closed, right? So it's all about pressure that's being generated by contraction of the ventricles, right? Pressure is high in the ventricles, AV valves close. Pressure falls, AV valves open.
