Hello. Welcome to Byte Size Med. This video is on the cardiac cycle and in part one, we're going to look at the events that take place. The cardiac cycle includes events that happen in the heart from one heartbeat to the next. It repeats over and over, hence its a cycle. The duration of each cycle can be calculated from the heart rate. If we assume a heart rate of 72 beats per minute, which we usually do, the cycle length would be 1 over the heart rate. 1 over 72 is 0.013 and that's in minutes. It's a very small number, so if we multiply it with 60, that gives us the number in seconds, which would be around 0.8 seconds. That's 0.8 seconds for each cardiac cycle. If the heart rate increases, that means the cycle length is shorter. If the heart rate decreases, the cycle is longer. During that time, there is both contraction and relaxation of the heart. The contraction phase is called systole and the relaxation phase, diastole. And they happen both in the atria and in the ventricles. There's an atrial systole, an atrial diastole, a ventricular systole and a ventricular diastole. If we use the term systole or diastole without mentioning which chamber is actually happening in, usually we're talking about the ventricles. The atrial systole happens before the ventricular systole. In that 0.8 seconds, atrial systole would take around 0.1 seconds. So the rest of the cycle, that's 0.7 seconds, the atria spend in diastole. Ventricular systole is longer, around 0.3 seconds, and the remaining 0.5 seconds is in diastole. The numbers would vary depending upon the duration of the cycle. But atrial diastole overlaps with ventricular systole, and atrial systole overlaps with the end of ventricular diastole. The rest of the cycle is when both the atria and the ventricles are relaxing in diastole. In both chambers, the duration of diastole is longer than systole. What exactly needs to happen between heartbeats? Blood from the body and from the lungs reaches the atria. This blood then flows into the ventricles. The ventricles pump out that blood through the pulmonary trunk and the aorta to reach the lungs and the rest of the body. To simplify the events of the cardiac cycle, I'm going to be using this schematic heart. So we have the right atrium, the left atrium, the right ventricle and the left ventricle. The superior and inferior vena cava open into the right atrium, the pulmonary veins into the left atrium. The pulmonary trunk emerges from the right ventricle and the aorta emerges from the left. In between the atria and the ventricles are the atrioventricular valves. That's the tricuspid valve on the right and the mitral valve on the left. Between the ventricles and those big vessels are the semilunar valves. That's the pulmonary valve and the aortic valve. Deoxygenated blood from the whole body has to reach the right atrium through the superior and inferior vena cava, and oxygenated blood from the lungs reaches the left atrium through the pulmonary veins. This happens during atrial diastole. Now blood from the atria actually passively flows into the ventricles as they fill. That's when the atrioventricular valves are open. The tricuspid valve on the right and the mitral valve on the left. During atrial systole, the atria contract in order to push the last 10 to 20 percent of blood into the ventricles. So that's what happens during atrial diastole and atrial systole. The atrial fill and the atria empty. At the end of atrial systole, these atrioventricular valves close. Now the ventricles are filled with blood and they have to pump that blood out. The right ventricle into the pulmonary trunk and the left ventricle into the aorta. That's through the semilunar valves, the pulmonary valve and the aortic valve. They contract in order to do that, and this happens during ventricular systole. So they have to contract and eject the blood out. The first phase is called isovolumetric contraction, also called isometric contraction. During this phase, both the atrioventricular and the semilunar valves are closed. The ventricles are contracting, the tension in the muscle increases, but the muscle length doesn't change. That's why you'll also see this phase labeled isometric contraction. Isovolumetric contraction is because no blood leaves the ventricles, since the valves are closed. The volume doesn't change but the pressure in the chamber rises. Once the pressure in the right ventricle is higher than the pulmonary trunk, and the left ventricle is higher than the aorta, the semilunar valves open. The valves open and blood is pumped out of the ventricles. First quickly, that's the phase of rapid ejection, and then slowly. That's the slow or reduced ejection phase. Those are the three parts of ventricular systole. Isovolumetric contraction, rapid ejection and reduced ejection. At the end of the ejection phase, those semilunar valves close. During diastole, the ventricles relax and fill with blood. So there's isovolumetric relaxation, because again the volume doesn't change. During this phase, the atria are in diastole and they are filling against closed valves. The pressure is slowly rising in the atria, the ventricles are relaxing, the pressure is dropping. Once the ventricular pressure goes below the atrial pressure, the atrioventricular valves open and the ventricles start filling. There is rapid filling and then slow filling of the ventricles. The slow filling phase is also called diastasis. So there's isovolumetric relaxation, rapid filling and slow filling. Before isovolumetric relaxation, the first part of ventricular diastole is also called protodiastole. This is when the ventricular pressure starts to drop and at the end of protodiastole, the semilunar valves close. Remember that the last bit of filling happens from atrial contraction, which is during atrial systole. So atrial systole overlaps with the end of ventricular diastole, and we're back at the start of the cycle again. We can look at the phases between the opening and the closing of the valves. The atrioventricular valves open and close. The semilunar valves open and close. Once the atrioventricular valves close, none of the valves are open. The ventricle contracts, that's isovolumetric contraction. Then the semilunar valves open. Blood is ejected out in the ejection phase, which has a rapid and a slow part. Once the semilunar valves close, all the valves are closed again and the ventricle relaxes. In isovolumetric relaxation, the atrioventricular valves open and the ventricles fill. First rapid filling and then slow filling. Isovolumetric contraction and the ejection phase is ventricular systole. Isovolumetric relaxation and the filling phase is ventricular diastole. I just drew this to remember the names of the phases in relation to the valves opening and closing, but recall that the duration of diastole is longer than systole. So the duration of each phase is different. But what about the atria? Atrial systole happens just before the atrioventricular valve closes, the last bit of filling, and the rest is atrial diastole. I'm going to put all of it together now on a straight line. This is one cardiac cycle. If we start with atrial systole, that's short. So the rest is atrial diastole. Ventricular systole starts after atrial systole. It's longer, and the rest is ventricular diastole. This is one cardiac cycle. So when the atria are in systole, the ventricles are in late diastole. So we can fill up the whole cycle. Late ventricular diastole is when the ventricles are just relaxing, but the atria are actually doing something. So we'll bring that down. Atrial systole. Ventricular systole has three parts: isovolumetric contraction, rapid ejection, and slow ejection. Ventricular diastole, again three parts: isovolumetric relaxation, rapid filling, and slow filling. This is what you'd see at the top of Wiggers diagram, which is the big diagram with all the waves and stuff that tells you everything you need to know about the cardiac cycle. Since it's going to be on repeat, sometimes you'll see another atrial systole at the end. And that's what i'm going to be doing. Sometimes you may see the whole cycle again. For the valves, an easy way to remember it is that everything happens at the beginning and the end of the isovolumetric phases. For these phases to happen, all the valves need to be closed. So at the start of isovolumetric contraction, the atrioventricular valves close. And at the start of isovolumetric relaxation, the semilunar valves close. At the end of these phases, valves open. So now reverse it. The semilunar evolves open, then the atrioventricular valves open. And those are the events that take place during a cardiac cycle. In part two, we'll look at how the volume changes during these phases. I hope this video was helpful. If it was, you can give it a like and subscribe to my channel. Thanks for watching and I'll see you in the next one! :)