Today we're going to continue our discussion of the cardiac cycle. The first thing that we're going to take a look at is called the ECG or the electrocardiogram. The p-wave refers to the depolarization of the atria. The q-r-s complex refers to the depolarization of the ventricle. And the t-wave refers to the repolarization of the ventricle. The repolarization of the atria is not seen because it is hidden by the q-r-s complex. In the last video, we talked about the seven different phases of the cardiac cycle and saw what happens in the atrium and the ventricle. The first phase was called the atrial contraction phase when the mitral valve is open and the blood rushes into the ventricle. The next three phases, the isovoimetric contraction phase, the rapid ventricular ejection phase and the slow ventricular ejection phase is when the ventricle is contracting and pumping blood into the aorta. The next phase, we see the isovoimetric relaxation phase. That is when both the valves are closed and the ventricle is relaxing. Then it lasts two phases, the mitral valve opens and blood rushes into the ventricle filling it once again and the cycle repeats itself. The first wave that we're going to take a look at is called the aortic pressure wave. The pressure of the aorta is 80 millimeters of mercury until the rapid ventricular ejection phase when the semi-looner valve opens and the blood from the ventricle rushes into the aorta, increasing the pressure all the way up to 120 millimeters of mercury. The pressure remains until the semi-looner valve closes during the isovoimetric relaxation phase and the pressure returns back to 80 millimeters of mercury. The next phase that we're going to take a look at is called the ventricular pressure wave. We see a huge spike in the pressure of the ventricle during the isovoimetric contraction phase because when the ventricle contracts, the pressure inside the ventricle builds up. It goes from zero millimeters of mercury all the way up to 120 millimeters of mercury. The pressure drops when the ventricle stops contracting and that is during the isovoimetric relaxation phase. The next wave we are going to take a look at is called the atrial pressure wave and we notice that the atrial pressure does not increase significantly. The only time that it does increase is during the atrial contraction phase. The phonocardiogram shows the heart sounds. The first heart sound S1 is heard during the isovoimetric contraction phase when the mitral valve closes. S2 is heard during the isovoimetric relaxation phase when the aortic valve or the semi-looner valve closes. S3 and S4 are abnormal as was stated in the previous video and they should not be heard in healthy individuals. Looking at the ventricular volume, we see a huge drop in the volume during the rapid ventricular ejection phase because the blood is being pumped out of the ventricle into the aorta. So it makes sense that the volume of the ventricle would drop. The volume starts slowly increasing when the mitral valve opens and the blood is pumped from the atria and the ventricle where it accumulates once again. Thank you for tuning into biology made simple. Please don't forget to subscribe, like and comment.