Here it says that heart sounds are made at the transition from one stage of the cardiac cycle to the next. We want to identify which heart sound would be heard at the start of each phase of the cardiac cycle. And if there is no distinct heart sound at the beginning of the phase, we can write none. Alright. So we have our 4 phases here. And for each phase, we see a diagram of the heart, reminding us what's going on. So let's go through them one by one. Remember, these phases start with either the opening or the closing of a valve, and so let's think which ones we would be able to hear. So we'll start with ventricular filling. Do you think you could hear the start of ventricular filling and why? Well, ventricular filling is really sort of defined as these AV valves being open. When those AV valves are open, blood from the atria can flow through the AV valves and fill up the ventricles. The ventricles are in diastole. During that time, they are relaxed. Well, the opening of those AV valves is what starts ventricular filling. Can you hear the opening of a valve? No, you can't. So there's going to be no heart sound, or I'm going to write none, that you can hear at the start of ventricular filling.
That brings us to isovolumetric contraction. Will you be able to hear the start of that phase, and if so, why? Well, isovolumetric contraction. Contraction, this means that the ventricle goes into systole, that systole squeeze. The pressure shoots up, and that pressure shooting up is what causes these AV valves to snap shut. When they snap shut, you can hear that. That's that first heart sound. We can write that first heart sound as S_1, or we can write it's the lub of the heart, if we're being a little less technical.
All right. Now, isovolumetric. Remember, it's snapped these AV valves shut, but the pressure hasn't risen enough to push these semilunar valves open yet, so the blood can't go anywhere because all the doors, all the valves to the ventricles are closed. Alright. That brings us to ventricular ejection. What starts that and will you be able to hear it? Well, in ventricular ejection, your heart your ventricle is still in systole, and it is now squeezing enough. The pressure goes enough that it pushes open these semilunar valves, and blood is ejected or pushed out of the heart. So could you hear that? No. You cannot hear the opening of a valve, so you can't hear that, so we're gonna write none.
Alright. That brings us to isovolumetric relaxation. There you go. Relaxation behind my head. Alright. Isovolumetric relaxation. Can you hear that? And if so, why? Well, relaxation. That means that the heart is going into diastole. The ventricles are relaxing. That's gonna lower pressure. As that pressure lowers, well, you're gonna have some back pressure, from the arteries here, and that's gonna snap the semilunar valves closed. The snapping of those semilunar valves closed, that's gonna be heart sound. We're gonna call that our second heart sound or what we can sometimes just write as S_2. Or if we want to be a little less technical about it, we can call it the dub in that lub dub of the heart. Alright. Remember, isovolumetric. That's because now all 4 valves are closed. All 4 valves are closed, but relaxation, it's still in diastole, but because the valves are closed, there's no change in volume in that ventricle.
Okay. Again, remember, the heart sounds are the valves closing, and that's, those valves opening and closing are the beginning and the ends of these phases, so those heart sounds should line up with the starts of both of those isovolumetric phases. Alright. More practice problems to follow. You should give them a try.