If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:14:59

Video transcript

so you're pretty familiar with this image of the heart we have the four chambers and I'm just going to start by labeling them the right atrium and right ventricle on this side and the left atrium and left ventricle on this side and the question is this what happens if we choose two spots I'm going to choose this one right here with a little X and left ventricle and some other spot over here in the aorta let's say with the purple X and this is of course our aorta up here what happens if I follow the pressure at those two spots so of course you know I was gonna say maybe if I'm the red blood cell sitting there but we know red blood cells move around but let's say I'm just kind of following the pressure at those two spots of those locations over time so this will be time over here what do the pressure what does the pressure look like at that location and let's say I'm following for let's say one second so if my heart rate was somewhere like sixty beats a minute that would basically mean one second would be one beat right this is 60 beats a minute and just keep in mind 60 beats a minute is actually pretty pretty low so that would be like if I'm reading a book or sitting around relaxing and on this side let's do pressure and I'll do zero to let's say a hundred up here and remember the units of pressure millimeters of mercury right now actually before I start let me even jump into some naming so just so we don't have to stop later this is the these are the valves right this is the mitral valve mitral and below it I've drawn the tricuspid valve and these are the put together we would call these the atrioventricular valves right this is the atrial ventricular valve and atrial ventricular capitalize a and V because sometimes you'll see the word AV valve atrioventricular valves are also known as AV valves and the other two valves here this is the pulmonary valve down here pulmonary and on the other side of it right here is the aortic valve right these are the put together we would call these the semilunar valves a or Tec semilunar valves and actually semilunar you might think it was like a half moon but there's no shorthand for that usually so people just call them the semilunar valves alright so what does the pressure tracing look like at my yellow X let's go back to that question so I'll start out here on the axis so it starts out really low left ventricular pressure is surprisingly low most of the time and you'll actually see that when I draw it out you always think of it kind of as this chamber that's cranking and pushing and high pressures and that's true there are some points along the way when the high pressure gets very high but for most of the time it's actually pretty low and it kind of creeps along goes up very slowly well why is it going up at all that's the first question why doesn't it just stay steady whatever pressure it is why is it going up well it's going up because there's actually flow here through the mitral valve so blood is going into the left ventricle initially and when there's more blood in that chamber over time the pressure kind of slowly builds up just like if I'm pouring water into a water balloon over time every little bit more water I put in there the pressure in the balloon goes up so that's why it's creeping up and then you get to a point where all of a sudden there's a muscle contraction so you have a depolarization wave that comes through and all of this heart muscle is cranking just come pushing in and of course simultaneously the left and the right ventricle is doing the exact same thing right so all the things that I'm saying for the left ventricle for the most part apply also on the other side so they're kind of contracting right and the left ventricle contracts hard and the pressure begins to rise now just right there just right where it begins to rise you might say well what did you even do at that moment where I kind of drew a slight increase tiny little increase you you can see it if you squint your eyes at that moment this valve closes why because at that moment at that very moment there's a little bit of push back here right and the slightest bit of push back and the valve shut so one once the pressure on the left ventricle side is greater than the pressure on the left atrium side the valve shuts and so the valve closes at that point and then you have a rise a rapid rise in the left ventricular pressure and it goes up up up like that so let me just write that down since I just said it the AV valves AV valves closed and that's a new thing right because they were open so me box that AV valves closed and the semilunar valves the other ones they stay open semilunar stays open nothing new there so Ashley sorry I said open and I'm even writing open but I mean closed sorry so let me let me not confuse you and just change it right now semilunar valve stays shut and to drive home that message I'll even kind of put a block there so they're still shut so that all the valves at this point are now closed right and that might be news to you might have thought while I thought at some point some valves are always open and that's not true at this point both valves are closed and the left ventricle is basically squeezing but but the blood has really nowhere to go is just sitting in a trapped room and the pressure is rising and rising fast right and then at some point it gets to a spot where an interesting thing occurs so let me actually draw that here let's let me erase this slightly let's draw the aortic pressure say your duct pressure let's say is something like this and it's kind of drifting down well why is a or t'k pressure drifting down well it's because blood is actually kind of in the order blood is rushing away remember from the previous squeeze you've got blood kind of rushing away out all these vessels right it's kind of going away and as blood rushes away of course pressure is gonna fall remember there is a relationship between having more blood volume and pressure so as the the blood volume they order kind of drifts away the pressure drifts down and this a or duct pressure is kind of rising and at that spot it's hard to see again I'm gonna just circle it to draw your attention to it but at that spot you see the aortic pressure is slightly below left ventricular pressure slightly below and because the pressure is slightly below all of a sudden this is no longer blocked now you've got a free pass here this valve opens up and blood can come in so new blood can come in from the left ventricle so blood starts rushing in and of course the left ventricle is still contracting so it still continues to rise and it gets to about there and the aortic pressure is going to rise and so let me draw that actually I might have switch colors but you'll you'll forgive me it starts to rise and it follows the course of the left ventricle so basically it's rising with the left ventricular pressure because there's you know continuous space there's no valve between the two right so what happened to this spot exactly let's just recap it well the AV valve AV valve is still closed nothing has changed there but the semilunar valve opened right semilunar valve opened and that's the kind of interesting new thing right that's the cool new thing that allowed the blood to go from the left ventricle out into the aorta and now what happens well at some point all this contraction I've drawn relaxes right finally kind of goes away kind of goes away on both sides and all these black arrows I'm gonna erase right and this muscle now instead of depolarizing begins to repolarize repolarize now why didn't I erase that last black arrow because again there is some pressure in the left ventricle in fact looking at my graph you can see there's not just some but there's a lot of pressure still in there so all that's changed is that the muscle is now relaxing and if the muscle is relaxing then this yellow line begins to drift down right begins to drift down and if the left ventricular pressure drifts down so does the aortic pressure that drifts down to well at some point what will happen the aortic pressure is still high and think about this is actually kind of a tricky point it's still high because there's resistance remember there's resistance from all the blood vessels there's resistance here all these blood vessels are offering lots of resistance so with all this resistance from the blood vessels the order pressure stays high stays high and the left ventricular pressure is relaxing it's kind of going drifting back down and so this arrow instead of blood kind of just going one way there's a little bit of pressure coming back the other way right there's a little bit of pressure kind of going this way this way and this way and there's some pressure to this way still because the left ventricle still has some pressure so they're kind of matched right there's pressure coming from the aorta and also pressure coming from the left ventricle initially the left ventricle kind of just overwhelmed the amount of pressure in the aorta but as it's drifting back down now that aortic pressure is matching the left ventricular pressure and add a particular moment that left ventricular pressure is going to be lower lower than the aortic pressure something like that an interesting kind of crossover right and that's again because the aortic pressure stays high because of all that resistance but the left ventricular pressure continues to drift down because it's relaxing and the moment that the pressure in the aorta is greater than the pressure in the left ventricle this shuts down again so that valve slams shut and so at that point what would we say well we say well the semilunar valve closed semilunar valve closed and the AV valve opened or sorry I keep saying that I apologize AV valve is still closed I didn't mention anything about the AV valve that's still closed as it has been for the last two points on our graph so this is in fact let me just label in this 0.1 0.2 0.3 so at this point what's the next thing that's going to happen well the left ventricle continues to relax or it continues to relax and it goes all the way down through my word semilunar closed all the way down almost to zero almost to zero and before it gets there the four gets all the way to zero the pressure is so darn low in here so low that now left atrial pressure is actually higher than left ventricular pressure and blood can flow back through so if blood can flow back through then we we know that as blood kind of fills up a ventricle the pressure continues to slowly rise slowly rise over time right and I get strove out there which is the same spot that we kind of began it right and meanwhile the aorta continues to drift down because just as in the beginning we said well when you know time passes blood kind of drifts down through to all the vessels so blood is now drifting away as it did before into all the vessels and as it drifts away the pressure and they order kind of drifts back down again and it goes something like that actually I guess further because my second is not that or something like that and you have to you have to assume based on my drawing that these two points are the same and if they're not then I haven't drawn it correctly so that's kind of what the aorta does at the end so let me just label this now this last point here is 0.4 and sorry before I get to point four point three the thing that was interesting and new I should just box it I'm just trying to box the things I want to draw your attention to is that the semilunar valve closed so what is happening at point four now point four a couple of things the AV valve now finally opens I said that prematurely before and that's the new thing right the AV valve opened and they semilunar valve is still closed so that did not change so closed so this is exactly what happens at the four stages right the four important points so just to recap it at point one the AV valve and actually as I recap we mentioned something new that's kind of interesting as well remember that when valves close the they usually make noise so it's kind of like a slamming of the door right so when a valve shuts it makes noise and so you have to look on these four points when does a valve closed for the first time well Navy valve closes right here right this is the point where it first closed so when the AV valve closes it makes a loud swak loud noise right in fact sometimes people call it a lub you hear that love noise if you listen to your heart well that's the first heart sound because that door just shut and you can see based on the pressure differences it's it's really closing because the left ventricular pressure is going up so fast so when you hear that first lub dub you know and they hear that lub that's also the beginning of the high pressure the contraction of the left ventricle now point to does any door closes any valve shut not really I mean the semilunar valve opens but opening them a valve doesn't really cause any noise at point three the semilunar valve closes right so that's where the aortic valve and pulmonary valve slams shut so again you get some noise up here so right this causes noise so a point three if I was to draw a straight line down here you'd get some noise I'm gonna cover up my number four but it's okay you know what where it is and this is a second noise and it's coming from the closing of this emulator valve and then point four they V valve opening while opening doesn't create noise and the semilunar valve is still shut so really the two sounds the heart sounds s1 s2 come from the closing of the AV valves initially the mitral and tricuspid and then the dub becomes from the closing of the aortic and pulmonary valves