typically on average it takes about 0.8 seconds to complete one cardiac cycle it differs from some from People to People obviously but uh this is 0.8 seconds is the average do you have to memorize 0.8 seconds no okay and within that one heartbeat you will hear two heart sounds which is L upb the love is just basically the left and right AO ventricular valves closing and the dub which is the second sound is the semi lunar valves closing that's it so we are going to be looking at the pressure changes in the left side of the heart more in particular for this video um you might be wondering why aren't we focusing on the right side well sometimes the exam questions can ask about the right side of the heart but um more focus is usually given to the left side and what usually happens is uh there will be a graph that is mentioned in the exam and this graph is just basically uh on the y axis of the graph there is the pressure uh measured in kilopascals and on the x-axis it's time if you notice I've put there three three moments of time 0.1 seconds 0.4 seconds and 0.8 seconds the reason why it ends at 0.8 seconds is if you remember earlier I said that one cardiac cycle typically takes 0.8 seconds now do you have to memorize these times no you don't okay don't have to memorize it but what you do have to what you do have to know for the purpose of the exam is the pattern of the graph that is more important so when you look at this graph usually what is given in the exam is they'll give you three lines you will see a green color line which I have drawn out uh at the bottom and there is a purple color line okay that goes like that and there is also a red color line now these three lines they correspond to pressure changes in three parts of the heart they correspond to the left atrium left ventricle and the aorta now the main question here that we have to know however is which lines correspond to which parts so which line represents the aorta which line represents the left atrium or the left ventricle so how do we try to label each of the lines correctly now most of the times students will just memorize this it's fine to memorize it but there is a way to kind of understand and approach it with a bit of logic you see let's focus on the red line first It generally remains High throughout the cycle yes there is a bit of fluctuation it does go up and then it goes down but compared to the green line and the purple line the red line generally remains high so this tells us that the pressure in this particular area is probably an artery because if you remember an artery is a blood vessel that always transports blood at high pressures so immediately if you look at the diagram on the right which one corresponds to an artery obviously that one is the aort so that's how we recognize it now we have two more choices we have the left atrium and also the left ventricle and these ones will either correspond to the green line or the purple line so the question is which one is which so to see this it's very simple look at the maximum pressure generated for each line so for the purple line the maximum pressure generated is up there which I've represented it's a very high High maximum pressure but for the green line it is only able to generate a maximum pressure of that much down there it's only this much so how do we know which one is so which one is the left atrium and which one is the left ventricle then so if you still have not uh figured it out you just have to compare the muscle wall of the left atrium and the left ventrical because you see I told you before in a previous video that the left trium has a thinner wall and the left ventricle has a thicker wall so if you go back to that previous video as you can see here the left ventricle wall is the thickest and the uh left atrium wall is very thin it's the thinnest the reason why they have that thick and thin wall is because they need to generate a corresponding pressure right so the thicker the wall it's able to generate a higher pressure the thinner the wall it can only generate a lower pressure so in this case by that logic the purple line which is able to go up to an extremely high pressure it has to be the left ventricle and the green color line which is only able to generate that much pressure and uh that has to be the left atrium then so that's how we figure it out so of course then we have to talk about what exactly do these lines actually mean let's focus on the first part I'm going to I'm going to divide the the graph into five sections which I have put like a gray color line a subdiv gray color line for subdivision I hope you can see it quite clearly over that now let's highlight the first part of the graph in yellow so what exactly is happening that so to simplify my diagram what I like to do do is I like to just draw out the heart in a very simplified structure this is how I like to ask my students to do it in class okay because the the heart diagram on the right there it's quite complicated but if you draw the one on the left here it's quite simplified okay you have a room you have a circle that represents the left atrium a larger circle at the bottom that represents the left ventricle and then you have the AA at the top and typically what's supposed to happen is blood is supposed to move into the left atrium and then uh into the left ventricle and then the ort so at this highlighted region what is happening let's compare the graph the first thing you must do is you must compare the left atrium and the left ventricle which I've circled over there so in that Circ in that point where I've have circled compare the pressure between the left atrium and left ventricle the pressure in the left atrium is higher and the pressure in the left ventricle is lower so what will happen to the blood flow in this case very simple the blood will flow or rush into the left ventricle very easily now the next thing that you must also compare is you must also compare the blood pressure or the pressure in the aorta and the pressure in The ventricle next so and we can see that the pressure in the aorta is higher than the pressure in The ventricle and I'm also going to label it in my diagram so in the AA I'm going to write the word higher pressure and in The ventricle lower pressure and we know that this if if this was the case blood will flow back into the heart and we don't want that to happen so in this case the semi lunar valve is closed which I repres presented in that diagram so which stage of the cardiac cycle is this in this is the first stage known as atrial syy that's what's happening there now the second part of the heart which I've also highlighted in yellow let's look at what's happening over here okay I'm just going to change the the graph a little bit I made a slight mistake over there okay right so in this case what exactly is taking place okay now the left ventricle pressure the purple line is suddenly increasing the reason is because the wall of the left ventricles are Contracting so during this time you notice that the left ventricle pressure is sharply increasing so compare the pressure of the left ventricle and the left atrium we notice that the pressure in the left ventricle is higher than the pressure in the left atrium as we can see that in the graph okay and if that's the case we do not want the blood to flow back into the left atrium so what needs to happen in this case you're right the atrio ventricular valve will close which creates the first hot sound so if you were to zoom in on the graph as I'm doing over here okay look at that particular moment when the left ventricle pressure becomes higher than the left atrium pressure and I'm representing it with like a red dot actually the Red Dot can be put at the intersecting point but what I'm going to do is I'm just going to put it slightly beyond that intersecting point at that point the left ventricle pressure is higher than the left atrium pressure and this will signify the moment the left atrio ventricular valve closes the next one that I also want you to see is the left ventricle pressure and the AA pressure we notice that the aeras pressure is still higher than the left ventricle pressure so the semi lunar valve will still remain closed okay so during this moment the blood is not going out of the ventricles at all it is just stuck within the ventricles it's not moving anywhere it's just a small little moment of the cardiac cycle where this happens within like maybe just 0.1 seconds or so so as you can see there I've actually inserted an image that represents the Heart during that moment of the cardiac cycle now what happens in this third stage is the left mentcle pressure further goes up because the wall is still Contracting right and when this happens here what happens is the let's compare The ventricle pressure I'm going to zoom in as you can see here at this point The ventricle pressure surpasses the AA pressure it has gone above the AAS pressure and when it has gone above the AAS pressure let's make the change in our diagram now the pressure in The ventricle is higher the pressure in the aorta is lower so in this case the blood has enough pressure to rush out therefore the semi lunar valves will open so when the semi lunar valves open in this case the blood is able to rush out of the ventricles into the aorta that's how this blood flows out of the heart simple as that and of course that these two diagrams here corespond to the ventricular systo which is after 0.1 seconds now the third stage which I'm going to highlight in yellow in my cardiac cycle what actually happens here is the left ventricle pressure is decreasing sharply and you know that the reason it decreases is because it is going through di stoling so when the left ventricle pressure decreases again let's compare the pressure let's look at the pressure between the left ventricle and the aort now the left ventricle and the aorta the left ventricle has a lower pressure than the aorta right now okay earlier it was higher now it's lower so when it's lower obviously in this case we don't want backf flow to happen so the semi lunar valves will actually close now the next thing that is supposed to happen is the let's compare the next part the final part and in that final part over there what happens is the left ventricle pressure becomes even lower than the pressure of the left Atri and this case again I'm going to change the labels in my diagram if you notice that on the left the the simplified heart and the pressure in the left atrium is higher the pressure in the left ventricle is lower thus the left atrio ventricular valve will open and when the valve opens the blood will passively trickle into the ventricles so in my diagram here the part where I've labeled in green a very mild green color in the cardiac cycle that represents the atrial CYO the cream or Beach color section is the ventricular syy and the light blue section is the ventricular diesto that is what's happening within the heart so in summary what we just have to do here is identify the lines what do all the lines represent so we have to identify the left atrium left ventricle and aota we know that the aota constantly has a high pressure throughout so it's the red line and the left atrium has a lower pressure because it has a thinner wall left ventricle is able to generate a very high pressure because they have thicker walls so that's how we know that the next thing we have to do is we have to point out the intersecting areas so what are the intersecting areas intersecting areas are just where the lines crisscross okay so I'm going to highlight the intersecting areas in a green color highlight I hope you can see that in the diagram the intersecting areas are basically where the valves will either open or close depending on the pressure differences so if we look at this diagram okay to to draw out the simplified heart what I always tell my students to do first this okay so what you do with the pencil draw out two circles one small one large erase the line and connect the two circles together and then just put the connections in so the small circle represents the left atrium big circle left ventricle and the large uh line coming out of the left ventricle is the aorta so normally blood is supposed to go from left atrium to left ventricle and then from left ventricle to aota this is like a single Direction flow that is supposed to happen so let's look at the intersecting points now in the intersecting points we notice let's look at the first intersecting point which is I've I've just been basically pointed it out in um a red arrow so what is happening at that intersecting point in that intersecting point the pressure of the left ventricle is higher than the pressure of the left atrium therefore the left atrio ventricular valve closes now the second intersecting point that we are going to be looking at in this second intersecting point the pressure in the left ventricle is higher than the pressure in the a so therefore in this case the semi lunar valves open to allow blood to rush out of the heart the third intersecting point is where the pressure of the left ventricle is lower than the pressure in the aorta so therefore the semi lunar valves close to prevent back flow and in that fourth intersecting point the left ventricle pressure is lower than the pressure in the left Atria so the atrio ventricular valves open to allow blood to flow in where the left atrio ventricular valve closes you hear the first h s Loop and the semilunar valve closes you hear the second H sound twoo that's how within one cardiac cycle you hear two sounds so as simple as that but sometimes students will be like oh this diagram is still an extremely confusing one is there any way to simplify this let's look at an example of a question that they may ask in so let's say an example of a question they put a point x there and they say what is happening in the Heart during this moment so how would you describe it very simple what you do first is you basically compare the left ventricle and the left atrium you notice that the pressure in the left ventricle is higher than the left atrium so in this case the atrio ventricular valves have to be closed to prevent back flow the second thing is you compare the left ventricle and the aorta and you notice that the left ventricle pressure is higher than the aorta so in this case what happens the blood flows out of the heart because the semi lunar valve is open so the heart is in ventricular systo the pressure in the left ventricle is higher than the pressure in the left atrium atrio ventricular valves are closed and the pressure in the left ventricle is also higher than pressure in the aorta therefore blood rushes out of the heart because the semi lunar valve is opened just a reminder you cannot put the word LV in the exam LV which is left ventricle you have to write out the entire word um and AV as well atrio ventricular valve you I'm just putting it there for the sake of the video because you know sometimes I'm just I'm just too lazy as well and another part of the diagram which I've just basically labeled as why all right and why so what is happening at this part of the heart or what's happening during this part of the cardiac cycle again same concept compare the left ventricle pressure to the left atrium pressure we notice that the left ventricle pressure now is lower than the left atrium pressure so in this case the atrio ventricular valve is opened and blood is passively just flowing into The ventricle compare the left ventricle pressure to the A AA pressure aota's pressure is higher left ventricle pressure is lower so the semi lunar valve is closed so you can use this graph to just determine or explain which valves are open and which valves are closed during a specific moment of the cardiac cycle if we look at Point Z okay at Point Z over here this is a slightly hard one at Point Z left ventricle pressure is higher than the left atrium pressure so the atrio ventricular valve is closed but the left ventricle pressure is lower than the aorta pressure so the semi Luna valve Still Remains close very short moment of time where both valves are also closed as well so this is the main thing that we have to understand for this