 pressure volume loop explains the relationship between left ventricular volume which we write in x-axis and left ventricular pressure and the changes which take place in this left ventricular pressure and left ventricular volume in one cardiac cycle in one beat basically. So when we are understanding pressure volume loop that is the relationship between ventricular pressure and ventricular volume then there are certain key points which we should remember. First is how much volume and how much pressure we are talking that is at how much volume what will be the ventricular pressure. So for that we have to start with the phases of the cardiac cycle. So let's start with a point when the ventricle is completely filled that is at the end of diastole. So at that point the volume present in the left ventricle is the end diastolic volume and it is approximately 120 ml. 120 ml is the end diastolic volume and the pressure which is present in ventricles at this time is actually very very less right close to zero we can say. So around two to three millimeter mercury is the left ventricular pressure. Now at the end of the diastole left ventricle starts contracting okay and because of this contraction there is tremendous increase in pressure in the left ventricle but actually the volume doesn't change and this phase which we are talking is the iso volumetric contraction phase where basically both the valves that is the AV valves and the semilunar valves are closed. So volume cannot go out of the ventricle or enter into the ventricles but since it is contracting there is a buildup of the pressure in the ventricles that is why the pressure is rising that much and what happens that at approximately 80 millimeter mercury that is the diastolic blood pressure and it is the same pressure which is present in the aorta. So when the ventricular pressure little bit exceeds the diastolic blood pressure here the semilunar valves open. So at this point the semilunar valves are going to open and with this starts the ejection phase when the blood starts moving out of the ventricle into the aorta. So what we see is that the volume starts decreasing but the pressure actually continues to rise because the ventricles are still contracting and they are contracting with much force such that despite the ejection of the blood into the aorta the pressure continues to rise and it kind of reaches to a maximum point of 120 millimeter mercury. So that is the systolic blood pressure that is the maximum pressure attained during the ventricular systole. Then even though the contraction continues the ventricular pressure starts falling and at some point midway between the systolic and diastolic blood pressure again this aortic valves close. So remember that as the ejection is started the pressure in the ventricle and the aorta is almost same. So this rise in ventricular pressure to 120 millimeter mercury is accompanied by the rise in aortic pressure also to 120 millimeter mercury and after that despite contraction because so much blood is leaving the ventricle the pressure begins to fall and somewhere in between the aortic valves close and this produces a hard sound that is the S2 hard sound due to the closure of the aortic valves. Now the volume at this point is how much it is the end systolic volume and this end systolic volume is approximately 50 ml. Then starts the diastole that is the ventricle start relaxing but you see here again the aortic valves have closed. So again both the valves that is the AB valves and aortic valves are closed. So with the relaxation the pressure starts falling down tremendously. So here the pressure starts decreasing and this is the phase of iso volumetric relaxation. Now at the end of iso volumetric relaxation these valves the AB valves open and the filling of the ventricle starts such that the volume in the ventricles increases but you see the pressure still declines why it declines because the ventricles are relaxing. So despite filling there is decrease in the left ventricular pressure and then with filling there is little bit rise in the pressure. So initially it decreases and then there is little bit rise in the pressure but not that much. So this phase is the filling phase. So basically pressure volume loop is depicting the relationship between the ventricular volume and ventricular pressure in one beat. Thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel Physiology Open. Thank you.