so the main thing that we are going to be looking at in this particular video is something called the cardiac cycle the cardiac cycle is just basically a way to describe how the heart uh muscles contract during one heartbeat so before we go through the video there are a few things that I would like to mention first you see the first thing to understand about the heart is in the previous video I did mention that the heart is divided into two sides which are the left and right portions and the left and right side are divided by the septum the good news is the left and right side actually mirror each other what it means by to mirror each other is whatever happens on the left side will also happen on the right side simultaneously so with that being said that is why when I'm focusing on the cardiac cycle I will just merely be looking at the left side of the heart because the left side is usually given more focus in the exam but whatever that is happening like I said again whatever happens on the left side it will also happen on the right I will try to keep the descriptions as generalized as possible I will not use the word left or right so but if you look at the diagram that I'm drawing over here it is representing the left side of the heart isn't it uh you can see the left atrium left ventricle um you can see the altine pulmonary vein but as I've mentioned I'm going to keep the name very general I'm just going to say Atrium ventricle I'm just going to use the word vein and artery so it can be applied to both the left and right side that's the first thing I would like to mention now that that's out of the way the first part as you can see over here is the blood is trickling or the blood is Flowing from the vein into the atrium and from The Atrium if you notice the atrial ventricular valve which I've just basically highlighted in yellow uh is slightly open and because the atrial ventricular valve is slightly open what actually happens here is the blood from the atrium can trickle Into The ventricle and it just starts filling up the ventricles passively passively meaning to say the heart is not doing anything or the muscles are not doing anything at this point they are all totally relaxed the muscles are relaxed so the cardiac muscles are relaxed this is a passive process and the blood just passively trickles into the heart which I have written in red so you can use the word trickles or you can just basically say blood flows into the ventricles that's fine too now the first active process that actually takes place is as you can see here there is still a lot of blood inside the atrial and we want to force the blood into the ventricles so at this point what actually needs to happen is the wall of the atrium which I've highlighted in yellow what will actually happen to this wall the wall will contract and when the Wall contacts as you can see that I'm just drawing it out again it has kind of squeezed inwards and when it is squeezed what happens here is it will force the blood from the atrium to The ventricle this stage of the cardiac cycle is called atrial systole and it will see slowly just basically means the wall of the Atria contract why did it contract well it contracted to basically create a very high pressure in the Atria which is a higher pressure than The ventricle as you can see that I put the word p Atria is more than P vertical you cannot write that in the exam you will have to say the pleasure in the Atria are higher than the pressure in the ventricles and when the pleasure is higher the blood rushes into the ventricles because blood will just basically move from an area of higher pressure to lower pressure that is just the natural way things flow simple stuff that is atrial systole now after it will say slowly what exactly needs to happen now the atrium will just relax again all right now the blood is inside the ventricle now from The ventricle where is it supposed to go if you remember the blood has to go out of the ventricles and it has to move into the artery and for that to happen a process known as ventricular systole has to take place so what does ventricular systole actually mean ventricular systole is what happens when the wall of the ventricles contract now I want you to see here look at the way the walls contact the walls contact a little bit first I'm going to divide ventricular systole into two stages here so the Wall contacts slightly which I've highlighted in yellow that's where the convection is taking place when the wall of The ventricle convex what happens what will happen is the pressure inside The ventricle will start to build up and The Presence at The ventricle will then be higher than the pressure in the Atria I'm just going to write it over the higher and lower now this can be quite bad because if the pressure in The ventricle is higher than the Atria blood might flow back do we want this to happen we don't because that is considered backflow and that is dangerous we do not want that to happen at all so to prevent that from happening what needs to happen then so to prevent that from happening the atrioventricular valves will close now the atrial ventricular valves will close with such a force that it will create the first heart sound Loop okay because remember heartbeat has that two sounds so the first sound look is basically when the atrio ventricular valves close now here's the interesting thing remember I told you whatever happens on the left side will also happen on the right side so even though the left and right atherial ventricular valves close you will only hear one sound the reason why you hear one sound is because they both close simultaneously that is the reason for that therefore the blood cannot flow back into the Atria that's a good thing so that's good but as you can see the blood still cannot flow out of the heart because the pressure is insufficient so in the second part of ventricular systole what happens the walls will further convey it will further squeeze inwards okay and when it further squeezes inwards what happens to the pressure the pressure in the ventricles further increase and this time the pleasure the ventricles are higher than the pressure in the arteries I'm going to put that higher and in the arteries it is lower and when it's lower what will happen is the semilunar valves in this case will open it has to give way and the blood is able to rush out of the heart into the arteries that is basically the second part of ventricular systole do not worry about these first two parts I will summarize it again later for you so it's easier for you to see so after that the blood as you can see while this is going on just a bit of extra information by the way while this is going on during ventricular systole blood is passively moving into the Atria new blood from the veins are coming into the Atria but they cannot go into The ventricle why because the athelial ventricular valve is closed I'm I'm highlighting or I'm coloring that new blood as purple in color just to show you that that is not that blood has nothing to do with the one in the ventricles at all so there is a new batch of blood coming into the heart that's just a passive process just putting it over there you know just as a extra bit of information now the heart muscles cannot contract forever they will then eventually have to relax so when it relaxes what exactly happens here is as you can see this is where the next stage happens which is ventricular diastoli and during ventricular diastole the walls of the ventricles relax it relaxes a little bit not fully yet and when it relaxes a little bit okay the pressure in the ventricles will now be lower than the pressure in the arteries now this can be quite dangerous why is it dangerous because backflow might happen and do we want backflow to happen from the arteries into the ventricles we don't therefore in this case what happens is the semilunar valves will close and when the semilunar valves close it prevents the backflow and that is when you hear the second sound two right so remember there are also two semilunar vowels on the left and right side and the will both close simultaneously too that is why you'll only hear one two sound so the first heart sound happens when the atrial ventricular valves close and the second hot sound happens when the semilunar valves close that is what happens there during ventricular diastolic but we are not done because during ventricular diastole the wall of the ventricles further relaxes and it fully relaxes and when it fully relaxes what happens then is um notice something very interesting the pressure in the ventricles become totally low and the pressure of ventricles it's so low that um it is much lower than the pressure in the Atria and when the pleasure is lower in the uh when the pressure is too low in the ventricles compared to the atrium what will happen is the atrial ventricular valves will open a little bit and blood from the Atria remember that new batch of blood will start trickling into the ventricles and you might go hey wait a second you did mention this earlier because that word trickling sounds familiar right that was the first thing that we talked about when we were covering the cardiac cycle that is the first stage so that is what happens during ventricular diastole new blood from the atrium starts trickling into the ventricles that's basically it okay now so therefore when we are just basically doing the entire process I hope you can see um I hope you can see the whole thing for uh for your sake um that the entire step for the cardiac cycle I have just basically summarized over here now some students will ask if they ask me to describe the cardiac cycle in the exam which one do I start off with you can start off with the atrial systole no harm so during a12 syst only what happens the wall of the Atria convex and pushes the blood into the ventricles during ventricular systole the wall of the ventricles contract and it causes the atrial ventricular valves to close you'll hear the first heart sound and it conflicts for whether to force the blood out of the ventricles into the arteries after that what happens is ventricular diastoli which means the walls of the ventricles relax and when it relaxes the semilunar valves close to prevent backflow and it further closes until the pressure in the Atria is higher than in the ventricles causing the atrioventricular valves to open and blood trickles into The ventricle this is just basically the entirety of the cardiac cycle and then the whole thing starts again okay it will systole ventricular systole ventricular diastole is one cardiac cycle or one heartbeat and then after ventricular diastole it will cystole happens again and the whole thing happens over and over and over and over again until we die so that is what we have to know for the entirety of the cardiac cycle it is very important to know the pressure differences by the way so when the pressure in the Atria are higher than the president of ventricle blood rushes Into The ventricle but with the pressure The ventricle is higher than the Atria we do not want the back flow to happen that's why the valves will close so you must so the reason why the valves are closing is because of the pressure differences the valves don't actively close by themselves the valves will only close due to pressure differences that is an important thing to say that's a that is an important thing to note and another important thing to note is to note is the heart sound the loop to Loop to Loop two happens because the valves are closing that's where the sound comes from because a lot of times my students say think that the heart sound is produced due to the movement of the muscles itself no that's not the case it's the the hot sound happens because of the valves actually so I hope you understand the cardiac cycle