Today we're going to be talking about the cardiac cycle. The first phase of the cardiac cycle that we're going to take a look at is called the atrial contraction phase. In this phase, the atria is contracting and pumping blood into the ventricle, which is already filled with 80% blood. There is a valve that separates the atria from the ventricle called the mitral valve, and this remains open throughout this phase, allowing the blood to move in. The other valve that separates the ventricle from the aorta, that remains shut through this process. In the next phase, the mitral valve closes, and the semi-lunar valve remains closed. As the ventricle starts contracting, the pressure inside starts building up, going from 0 millimeters of mercury up to 80 millimeters of mercury, this phase is called the isovolumetric contraction phase, meaning the volume does not change. In the next phase, the ventricle continues to contract. As the pressure builds up inside, it causes the semi-lunar valve to pop open, and the blood rushes out of the ventricle into the aorta. This phase is called the rapid ventricular ejection phase. Blood continues to be pushed from the ventricle into the aorta, but not as rapidly, because the pressure inside the ventricle slowly starts to drop. This phase is called the slow ventricular ejection phase. The pressure that had built up all the way to 100 millimeters of mercury slowly starts to drop, eventually going back to 3 millimeters of mercury. In the next phase, the semi-lunar valve closes, and the mitral valve remains closed. This phase is called the isovolumetric relaxation phase, so the ventricle starts relaxing, and the pressure goes all the way back down to 0 millimeters of mercury. In the next phase, the mitral valve opens once again, and blood moves passively into the ventricle. There's a portion of blood that remains in the ventricle, called the in systolic volume that remains after every cycle and does not change. This phase is called the rapid passive ventricular filling phase. In the following phase, the mitral valve still continues to remain open and blood, slowly moves into the ventricle passively, and this phase is called the slow passive ventricular filling phase. In a normal individual, when a valve opens, there should not be a sound heard. However, when a valve closes, that is when the heart sounds are heard. The first heart sound, called the S1, is heard during the isovolumetric contraction phase, and S2 is heard during the isovolumetric relaxation phase. S3 and S4 are abnormal heart sounds and should not be heard in healthy individuals. Sistally refers to the contraction of either the ventricle or the atrium, and diastole refers to the relaxation. So the atrial contraction phase is when the atria is undergoing systole, and the isovolumetric contraction and rapid ventricular ejection phase is when the ventricle is undergoing systole. Thank you for tuning in to biology made simple. Please don't forget to subscribe, like, and comment.