Define cardiac cycle, and follow the events of one cycle.

Video transcript

Hi, everybody, everybody. Welcome back. Let's look at our next question, which of the following correctly describes the sequence of events in a cardiac cycle. We're given four answer choices, all of them contain the same terms, but in different orders. So rather than read out four answer choices with all of the same terms over and over again, we'll read through choice a to have those terms in our mind and then I'll walk through a cardiac cycle and we can go back to our answer choices to determine which one has the terms listed in the correct order. Remember, this is a cycle meaning that you can start at any point. So the starting point doesn't matter just that the order of events is in the correct sequence. So where I like to start just personally as I'm trying to think through the cycle is with passive ventricular filling because somehow I imagining the heart filling up with blood is sort of seems like a logical starting point to me, but you can start anywhere you like. So in passive ventricular filling blood is going into the atria. I drew blood with a little arrow to Atria due to low pressure inside the heart. The A V valves between atrium ventricles are open. So blood is also flowing from the atria into the ventricles. That's why this is passive ventricular filling because nothing is being done. You aren't having any squeezing. You just have blood flowing from the atria through the open valve into the ventricles. And at this point, the aortic and pulmonary valves are closed. These are the semi lunar valves or S L valves. Now, at a certain point, the aorta which has been relaxed contracts. So the next step is that the atria contract and this propels the rest of the blood into the ventricles. And this would be active ventricular filling. You're actually ejecting blood by means of a muscle contraction into the ventricles. Once that is done, you then have the atrium begin to relax, the ventricles begin to contract. So ventricles contract as a result, the pressure inside the ventricles increases a little up arrow there rather quickly. That means that the A V valves close due to that increased pressure, but the S L valves are still closed as well. The ventricle is sealed closed but contracting. So this stage is called isovolumic contraction or isovolumetric contraction because is still meaning same, the volume of blood in the ventricles is remaining the same. Despite the ventricles contraction at a certain point, the increasing pressure due to that contraction forces the semi lunar valves open. So pressure opens the semi Lunar valves. And at this point, the ventricles contracting causes the blood to be ejected out into the aorta. So blood is ejected from the ventricles, which obviously would be ventricular ejection. At this point, the ventricular contraction is over the ventricles, relax the pressure drops inside the heart. This causes blood to begin to flow back towards the heart which closes the S L V S L valves, clues, but we know our A V valves are still closed as well. So we have again, a similar situation where the ventricles are completely sealed off. So we have another situation of iso volumic relaxation. And in our final step, now, the atria which have been relaxed and therefore filling with blood fill with enough blood that the A V valve is forced open, allowing once again passive ventricular filling. So we go back to the beginning with the opening of the A V valves. So A V valves open leading us back to passive ventricular filling. So that's the completion of entire cardiac cycle. Now, I just want to go through and highlight and I'll highlight in blue the terms that match the terms in our answer choices. And then as our last step, we'll go and find the one that's in the correct order. So passive ventricular filling in the first step we listed matches the term in our question. The second step we listed was active ventricular filling. The third step we listed was the isovolumic contraction. Have the ventricles. Then the following step after that was ventricular ejection and then finally, isovolumic relaxation leading back to passive ventricular filling. So let's start with choice A and look for the order of events. Choice A begins with isovolumic contraction. So just follow along comparing it to our particular cycle. So next is ventricular ejection which is correct uh following that isovolumic relaxation also correct, then passive ventricular filling, then active ventricular filling. So we've actually hit on the correct order right away. In choice. A. In choice B it has isovolumic relaxation, then ventricular ejection, then isovolumic contraction. Those two isovolumic steps should be reversed. Contraction comes first before ventricular ejection and then relaxation. So that's what's wrong with choice. B those two first and third terms should be switched. In choice C we have the same problem with the isovolumic steps being in the wrong order surrounding ventricular ejection. In addition, it has active ventricular filling happening before passive ventricular filling. And they should also be in the other reverse order. In choice D, the isovolumic steps and the ventricular ejection are correct, but it has active ventricular filling happening before passive ventricular filling. So those two are in the wrong order and that's why choice D is incorrect. So again, which correctly describes a sequence of events in a cardiac cycle and choice A is in the correct order. Again, isovolumic contraction, ventricular ejection, isovolumic relaxation, passive ventricular filling, active ventricular filling. See you in the next video.