 With the help of this diagram, let us try to understand cardiac cycle. Cardiac cycle is basically the sequence of electrical and mechanical events which happen in heart in one beat. So, we have to study when we are talking about the cardiac cycle, the electrical activity and the mechanical activity which is happening. So, if we talk about electrical activity. So, just let me draw a schematic diagram of heart. If we talk about the electrical activity, say this is SA node. What happens that the SA node generates a signal which travels in the conducting pathways to the AV node and from there via the bundle of his and Purkinje fibres, it bundle of his the left and right branch and then the Purkinje fibres, it spreads to the heart. So, electrical activity starts at SA node, spreads in atria, for that we get P wave and after this electrical activity there is contraction of atria and then as the electrical impulse spreads in the ventricle, we get the ventricular contraction and then ventricular relaxation. In the meantime, when the ventricles are contracting the atria also relax. So, first thing is not to confuse between two things that the electrical activity and the mechanical activity. Mechanical activity that is the contraction occurs due to the electrical activity. So, let us see that what are the these mechanical activity and what is the timeline in which they are occurring. So, first is as I said that atrial contraction because of the P wave. So, one heartbeat if we say that the heart rate is 75 beats per minute. If the heart rate is 75 beats per minute, the duration of one heartbeat will be how much? That will be we can calculate by in 60 seconds, 75 beats are happening. So, in one second or a cell in one beat will happen in how many seconds? That will be 60 divided by 75, it will come to 0.8 seconds. So, taking that the heart rate is 75 beats per minute, then the duration of single cardiac cycle will be 0.8 seconds. So, we will take it as an example to see that what will be the duration of various events in cardiac cycle. So, initially as I said that there is atrial contraction. So, that lasts for 0.1 seconds. Then in the rest 0.7 seconds of these 0.8 seconds the atria relax. Now, in the mean time there is overlap basically with the mechanical actions of the atria and the ventricles there is overlap. So, when this atria is contracting that is 0.1 seconds the ventricles are relaxing. So, here it will be ventricles are relaxing during this time that is the diaz tool. So, that is ventricular diaz tool and just after the atria contraction is over ventricles start contracting. So, here we will see ventricular contraction which lasts for 0.3 seconds and then again the ventricular diaz tool starts. So, this ventricular diaz tool you see it is overlapping the atria diaz tool as well as atrial contraction right and this lasts for 0.5 seconds. So, with this let us see what are the various phases of cardiac cycle. Now, here broadly we have said the phases of cardiac cycle that is the atrial systole and atrial diaz tool which is overlapped by ventricular systole and ventricular diaz tool. So, these are parallel events which are occurring they are not in series not one after the other. But to understand actually what is happening within the heart in these phases we divide this cardiac cycle phases into sub phases. So, what are these sub phases? These sub phases we divide it in two whether they are occurring during ventricular systole or ventricular diaz tool and remember that if I only use the words systole and diaz tool and do not attach atria and ventricular as the prefix to that then by default it means ventricular systole and ventricular diaz tool. So, ventricular systole it includes isovolumetric contraction then there is phase of rapid ejection then there is phase of slow ejection. So, during systole ejection of blood from the ventricular is going to take place and during diaz tool filling of ventricular relaxor and the phases in ventricular diaz tool is proto diaz tool very small phase okay proto diaz tool then there is isovolumetric relaxation then the phase of rapid filling then there is phase of slow filling and then there is second phase of rapid filling which is due to the atrial contraction we saw that how ventricular diastole is overlapped with atrial contraction. So, these are the phases of the cardiac cycle and from where to start the cycle? So, let us see in this graph you see the ECG because the electrical activity precedes the mechanical activity we are starting with the P wave okay. So, here the P wave is happening. So, first what we are seeing is atrial contraction right and just after atrial contraction ventricular systole is going to start. So, let us see one by one what is happening in these phases just let me run this and we will start with the atrial systole or atrial contraction and again come back through atrial contraction at the end of the cardiac cycle. So, initially there is P wave okay. So, this is the P wave right and this leads to atrial contraction. Now, during atrial contraction what will happen is let me draw the atrial and ventricular as two boxes okay which have valves in between and what are these valves actually between the atrial and ventricular there is a bicaspit or tricuspid valve right bicaspit valve on the left side and tricuspid valve on the right side and between the ventricular and the vessel which is coming out of the ventricular there is senile lumbar valve and on the left side it is aortic valve and on the right side it is pulmonary valve. So, this is the region of valve and here there is valve. Now, when atrial contracts what happened that this valve is open and because of the contraction the blood flows very fast it is like a push to the blood which is present in atria into the ventricular okay. So, that is the atrial contraction or the rapid filling phase the second rapid filling phase of the cardiac cycle. Now, by this time the electrical activity actually has deep hold rise to the ventricle and so the ventricle contraction starts. Now, as the ventricle starts contracting what happens that these valves shut okay. So, this valve which are present in the atria and ventricle close and this produces first heart sound okay. So, before the start of the ventricular contraction or the end of the atrial contraction there is first heart sound and if we see in this diagram you see here this is the atrial contraction right and this is the start of the ventricular system here we have drawn first heart sound fine. Now, these semilunar valves are already closed that they see before that in the cardiac cycle when they become closed but at the beginning of the ventricular contraction you see now the both valves are closed the bicuspid tricuspid valves or the av valves and the semilunar valves. So, with the contraction of the ventricles what happens that suddenly the pressure in the ventricles rises to a very high level and because the valves are closed volume doesn't move out that is the reason that this phase is known as isobolumetric contraction. So, let us see it in the graph for the pressure changes which occur in ventricle and the volume changes which are occurring in ventricle in this phase. So, this phase is the isobolumetric contraction you see this is the ventricular pressure in the diastole ventricular pressure is 0 millimeter mercury but in the phase of isobolumetric contraction it rises very steeply from 0 to 80 millimeter mercury okay. So, the change in ventricular pressure is from 0 to 80 millimeter mercury what about the volume change in the ventricle will there be any volume change no there will be no volume change. So, you see that right now there is no volume change in isobolumetric contraction this this plant and the amount of blood which is coming through the ventricle by the end of ventricular relaxation is 130 ml approximately. So, that is known as the end diastolic volume that is the volume present in the ventricle by the end of the diastole. So, that is 130 ml and in isobolumetric contraction it is you see not changing. So, name itself is statistic isobolumetric volume is same fine. Now, with the rise of pressure of ventricles to 80 millimeter mercury what happens at these semilunar valves which are there and when we are talking about 80 millimeter mercury we are talking about the left side of the heart left ventricle right. So, at 80 millimeter mercury these semilunar valves open why so because at this point the ventricular pressure becomes greater than that of the pressure in the aorta what is the diastolic pressure in the aorta it is at 80 millimeter mercury right. So, as soon as the ventricular pressure becomes little bit more than that it pushes open the semilunar valves and because of this extreme rise in pressure there is so much push to the flow of the blood that it flows very fast out of the ventricles into the aorta. So, that is why this phase is known as rapid ejection phase right rapid ejection phase and you see that the pressure is still rising not so much as that in case of isobolumetric contraction but it is still rising and reaches up to 120 millimeter mercury. What about the volume change? Yes as I told that it is a rapid ejection phase volume you see is falling very fast it is a steep fall in volume in the ventricle. Then what happens? The ventricle keeps on contracting but because there is ejection of the blood this pressure you see starts falling and because of this volume pressure the volume amount of volume which moves out also decreases. So, this is the phase of slow ejection okay slow ejection and by the end of both phases of rapid ejection and slow ejection how much amount of ventricular blood has moved out of ventricles approximately 70 ml and this is the stroke volume right. Then how much amount of blood is still present in the ventricles not entire 130 ml which is present in the ventricles is thrown out some amount is still present in the ventricle and it is basically 130 minus 70 and that is 50 ml and because this amount is present at the end of the systole. So, it is known as this 50 ml is known as end systole volume fine. So, that was about the phases of systole before we end this let us see that how much is the duration of each phase in systole. So, we saw that there are three phases iso volumetric contraction and there is a rapid ejection and slow ejection and the total ventricular systole duration is 0.3 seconds so simple to remember how I do is that one iso volumetric contraction is 0.05 second just keep on adding 0.05 to this okay. So, this is 0.1 second and slow ejection so this is a longest duration so it is 0.15 seconds. So, these are the duration of various phases of ventricular systole coming to ventricular diastole. So, what we said is that during ventricular systole 70 ml of blood has moved out of the ventricles and these valves are closed that is the ventricular valves are closed and these senolone valves are open. Now, after contraction the ventricles start relaxing okay. So, with relaxation the pressure in ventricles is going to fall right. So, you see here here right so in this diagram I have not shown proto diastole but as soon as it starts relaxing the pressure falls such that it becomes immediately less than that of the aorta and what happens that the aortic valves or the senolone valves actually close. So, here you see what we saw that in rapid ejection phase there was rise in pressure and the pressure started falling in slow ejection itself and then just as the ventricular start relaxing these senolone valves close. So, again that is happening at what pressure that is again happening at 80 millimeter mercury little less than 80 millimeter mercury they will close. Then start proper phases of ventricular relaxation that is isovolumetric relaxation rapid filling slow filling which is also known as diastasis and the phase of second rapid filling or what is caused by ventricular systole okay. So, isovolumetric relaxation as the name suggests again that volume is constant iso volume is constant and while it is constant you see both valves are closed 80 valves as well as the senolone valves by the way the closure of the senolone valves produces second hard sound. So, you see at the end of the ventricular systole we get second hard sound at the beginning of ventricular systole first hard sound at the end of ventricular systole second hard sound. So, both the valves are closed hence there is no change in the volume within the ventricles but what about pressure as there was rise in pressure steeply in isovolumetric contraction there is a steep fall in pressure in isovolumetric relaxation. So, you see here this is the steep fall in isovolumetric relaxation and again the pressure comes to zero. So, the minimum pressure in ventricles is zero the pressure in ventricles ranges from zero to 120 millimeter mercury it is not like that in a hotel where it ranges from 80 minimum and 120 maximum okay. So, with isovolumetric relaxation pressure comes back almost to zero and as that happens these a b valves are going to open right. So, how it will become these valves are going to open and you see that during this whole time when the ventricle was contracting what was happening in atria it was relaxed right. So, all the blood which was returning from various parts of the body it was entering into the atria. So, this atria was full of blood. So, as soon as these a b valves open there is sudden gushing of blood into the ventricles from the atria and because of this sudden gushing there is production of third heart sound. So, during this rapid filling phase there is third heart sound production but you see that despite filling of the ventricles. So, you see here what will get a ventricular volume change it is going to rise is steeply ventricle the volume is rising is steeply but the pressure change will not be much why because the ventricles are still relaxing and physiologically there should not be much pressure change because if that happens then these valves are going to close and the filling of the ventricles will be compromised. So, actually that is a disorder known as gastrolyph dysfunction. So, pressure should not rise and you see here in the entire ventricular install pressure is almost up to zero millimeter mercury fine. So, that was about rapid filling then what happens then there is a phase of slow filling in which whatever blood is coming from the veins enter into atria and because the valves are open they keep on entering into the ventricles. So, it is like a direct pathway from vein to atria to the ventricles. So, that is known as diastasis or slow filling phase and you see that in this phase because slow filling is nothing what is happening through the ventricular volume it is rising but not that much. And finally there is a phase of atrial contraction. So, you see here there is a slow filling finally there is the phase of atrial contraction because the new impulse has started it will lead to a little bounce in the ventricular volume. So, that is a second rapid filling and as in the first rapid filling I told you that sudden crushing of blood creates third arc sound this atrial contraction again there is a small crushing of blood from the atria to the ventricles it leads to fourth heart sound but physiologically remember we hear only first heart sound and second heart sound fine. And then again what will happen the next beat will start at the end of the atrial system again the AB valves are going to close. So, before we end just to note on the durations of the phases of the ventricular diastole what are the durations again they are simple to remember as we saw in ventricular systole ventricular diastole has phases iso volumetric relaxation proto diastole is there but for the sake of simplicity I am just omitting it here iso volumetric relaxation is there then there is phase of rapid filling then slow filling and again there is phase of atrial systole or second rapid fillings. So, we said that ventricular diastole duration is 0.5 seconds right. So, what I will do is except this slow filling this phase is slow for other phases I will just write 0.1 second 0.1 second 0.1 second right and this slow phase it is just 0.2 second the remaining 0.2 second. So, those are the durations of various phases of the cardiac cycle. So, that was all about the cardiac cycle. 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