we're breaking down the cardiac cycle making it easier than ever and by the end of this series you'll not only be able to Ace any test on the cardiac cycle it's also going to bring together a bunch of other cardiovascular Concepts so let's do this when we're talking about the cardiac cycle what I'm really talking about is everything that happens from the beginning of one heartbeat to the beginning of the next so the heartbeats we're talking about a single lub dub make sense all right well there's actually a lot of stuff happening here and I want you to understand it fully so much that I created a free guide that you can download to help you solidify the concepts you can click on the link in the description to get that now we're going to start by looking at the first phase of the cardiac cycle and that's called atrial systole the term atrial systole has two parts atrial which refers to the Atria of the heart and systole which means contraction so we're talking about the contraction of the Atria so far so good let's look at where things are right before the Atria begin to contract just to set the scene blood is coming back to the heart on the right side we have blood coming back from the rest of the body and on the left side we have blood coming back from the lungs and to start off the cardiac cycle the Atria are going to contract now the question is what needs to happen in order for the Atria to contract well contraction doesn't just happen on its own we need some kind of signal to cause it to happen and fortunately we have such a signal coming from the pacemaker that's in the heart in the right Atria that's where we find the pacemaker aka the SA node which stands for cyanoatrial node now the SA node generates a signal and since the SA node is located in the right atrium that signal starts right there and it spreads throughout the rest of the Atria so when we look at the electrocardiogram what we're going to see is we have a P wave now that P wave shows the depolarization of the Atria in other words it's showing you the spread of the signal across the Atria now we have a signal and that signal is going to cause the Atria to contract which is atrial systole now when the Atria contract what do you think that's going to do to the pressure in the Atria well think about it this way if you have a container with fluid or even like lotion and you squeeze it what do you expect to happen well the squeezing what it does is it increases the pressure in the container and that pushes whatever is inside out well when the Atria contracts that's basically like you're squeezing the Atria and the squeezing increases the pressure in the Atria and that's going to push the blood through the valves into the ventricles now the name of these valves are like super easy to remember they're between the Atria and The ventricle so we call them the atrioventricular valves man if only everything in biology were named that simply that would be awesome but anyways this brings the last bit of blood into the ventricles in fact about 25 of the volume of the blood in the ventricles comes from this last push from the atrial Contracting now when we look at the pressure in both the Atria and ventricles you can see that it's pretty low I mean it was pretty close to zero millimeters of mercury the increase in pressure that we see isn't huge if you think about it the Atria don't have as much work to do as the ventricles they are sending the blood a very short distance so they just have to send blood to the ventricles and they're like right next door and when you compare what they do to what the ventricles do you'll notice that the ventricles are much stronger so when they contract the increase in pressure is going to be significant more and you'll see that in the next video for now I want you to know that there's a little bump in pressure as a result of the Atria Contracting to send blood into The ventricle and that little bump is going to be seen in the Atria as well as the ventricles there's one more thing I want to point out here now we've got all the blood in the ventricles that we're going to have in the ventricles there's no more coming and what we've looked at so far happens right before the ventricles contract so the ventricles are in their relaxed State getting ready to do what they do contract so that it can send blood throughout the body the amount of blood inside the ventricles at this point is called the end diastolic volume that's the amount of blood that's present at the end of diastole and the word diastole refers to relaxation of the muscle systole refers to contraction and diastole refers to relaxation but we've discussed all the major things that happen during atrial systole and now to test your understanding let's do it if you understand the summary that I'm gonna give next then you're ready for any test on this section so here goes the first phase of the cardiac cycle is atrial systole when looking at the electrocardiogram the P wave shows us the depolarization of the Atria after atrial depolarization we get atrial contraction now when the Atria contract that increases the pressure in the Atria and pushes the last bit of blood into the ventricles we'll also see a bit of a bump in the pressure in the ventricles and now we have all the blood in the ventricles that we're gonna have which gives us the end diastolic volume we're now ready for the ventricles to contract and send that blood throughout the body man that was a lot of big words there but I hope you got it if you did get it type in the comments below Leslie I get it I love seeing your feedback and don't forget to get my free Guide to the cardiac cycle which will be linked up in the description below we're looking at the cardiac cycle this is the second video in the series and we're digging into phase two isovolumetric contraction now let's set the stage the blood has come back from the rest of the body and we're trying to take that blood and pump it back out now in the first phase of the cardiac cycle the Atria contracted and that pushed the last bit of blood into the ventricles it's now time to take that blood and pump it back out to the body the right ventricle is getting ready to pump the deoxygenated blood to the lungs so that it can pick up some oxygen and the left ventricle is getting ready to pump that newly oxygenated blood that just came back from the lungs out to the rest of the body and this next phase is called isovolumetric contraction or more specifically isovolumetric ventricular contraction let's to break down that term we know what contraction is that has to do with muscles the muscles will actually make themselves shorter and Tighter and that's contraction with the heart that basically squeezes the heart to pump the blood out now the term ventricular lets us know that we're talking about the ventricles and the term isovolumetric is made up of two parts we have the iso part meaning the same and volumetric referring to the volume and in this case the volume of blood so when we combine it all isovolumetric ventricular contraction is the phase of the cardiac cycle where the ventricles contract but the volume of blood in the ventricles Remains the Same how and why does this happen I'm glad you asked first let's look at the EKG we looked at the P wave in the last video and how that led to the Atria Contracting the next thing we see is the QRS complex the QRS complex shows the depolarization of The ventricle goals let's look at this more closely the signal for the ventricles to contract starts in a structure called The Av node or the atrioventricular node once that signal comes from The Av node it spreads via these bundle fibers and also the purkinje fibers all throughout the ventricles and the cool thing about the cells that make up the muscle of the ventricles is that they're all electrically connected so if one gets depolarized that depolarization is going to spread to the entire structure and as the ventricles depolarize that's shown on the EKG as the QRS complex as you can see it's much larger than the P wave and that's because the ventricles are larger than the Atria so the signal is also going to be significantly stronger and once the ventricles depolarize now we have the signal that we need for the ventricles to contract now what happens next is a key concept to understand during this phase if we look at the ventricular volume we can can see that it stays constant during this phase and there's an important reason for that when a ventricles start to contract that's going to close the valves let's just look at the left side the left atrioventricular valve that's between the left atrium and the left ventricle that's going to close and the aortic valve also gets shut that's the valve between the left ventricle and the aorta and the aorta is a very large main artery that takes the blood to the rest of the body you might also see this valve in some textbooks called the mitral valve or the aortic semilunar valve it's all the same thing so these two valves are both closed as a result for a relatively short period of time we have a sealed container that's Contracting what's that going to do to the pressure well of course that's going to increase the pressure significantly and that's exactly what we see now that's only going to go up to a certain point and why that is just makes sense let me explain we know that the blood from the the left ventricle is gonna go to the aorta to go to the rest of the body and the aortic valve is between the two structures but there's a certain amount of pressure in the aorta already because there's blood in there already and you can see that it's right around 80 millimeters of mercury so if you have pressure on one side of that valve that's at 80 millimeters of mercury that's inside the aorta and pressure on the other side that's increasing rapidly well the valve in between will stay closed until you pass that 80 millimeters of mercury it's like if you have someone pushing on one side of a door and someone pushing on the other side once you get more force on one side than the other what's gonna happen the door is going to open so at that point the aortic valve will finally open and the blood can be sent into the aorta so that it can go to the rest of the body that's when the isovolumetric contraction phase will come to an end and we move on to the next phase now there are two more things that you need to know about this stage we already know that when the ventricles contract the vowels are going to close but there's something that happens to the atrial pressure during this stage you'll notice this little bump in the atrial pressure that little bump is called the seaweave what causes this well as the atrioventricular valve closes it kind of bulges back into the Atria because of the pressure increase in the ventricles picture the atrioventricular valves like this and as the ventricles contract they don't just close like this imagine them being pushed by The increased pressure and closing like this so there's a little bit of bulging into the Atria and that causes this bump in pressure that we see in the Atria and lastly when the atrial ventricular valves close it's not a quiet event it makes a sound in fact that's when we get the first heart sound you know we have that sound when the heart beats well the lub of that lub dub that's the sound round of the atrioventricular valves slamming shut okay maybe slamming is a bit much but you get the point and that is isovolumetric contraction all right this is the last video in my series on the cardiac cycle and we're gonna finish the cardiac cycle by talking about ejection isovolumetric relaxation and ventricular filling but first let's recap when we talk about the cardiac cycle we're speaking about everything that happens from the start of one heartbeat to the start of the next good okay now in the first two videos we covered the first two phases of the cardiac cycle we had atrial systole AKA atrial contraction once the blood comes back from the rest of the body the Atria contract and that pushes the last bit of blood into the ventricles and then we had isovolumetric ventricular contraction this is when the ventricles start to contract but because the valves close the pressure builds and builds but the volume of blood in the ventricles Remains the Same it's ISO volumetric so this isovolumetric phase happens but only up to a certain point and that's the point we're going to start exploring further in this video now the heart it does a lot of work and the ventricular contraction is actually a pretty strong contraction this pressure buildup that we get in this isovolumetric phase is very important in fact the contraction of the ventricles and the pressure buildup that comes as a result it's enough to pump the blood 30 feet like if you were to take the heart out of the body and it could just function as well out of the body which it can't but humor me that's how far the blood would travel 30 feet so that pressure buildup phase is really important but let's get back to the point where the valves open we're building up pressure and as soon as it reaches this point right here the valves are going to open now what is that point well if we're looking at the left ventricle it pumps blood into the aorta so that it can go to the rest of the body but there's a valve in between the left ventricle and the aorta that valve is the aortic valve aka the left semilunar valve so on one side we have the left ventricle that's trying to pump the blood out and on the other side you have the aorta that's leading to the rest of the body but there's also blood in the aorta and as a result you have pressure in the aorta how much pressure well the average amount of pressure in the aorta is around 80 millimeters of mercury when it's relaxed that's before The ventricle pumps all the blood in there so once the pressure in the left ventricle builds up to that 80 millimeters of mercury what's going to happen to the aortic valve well it's going to pop right open and that starts the ejection phase that's when the blood is going to get ejected from the ventricles now to be clear on the right side we have the right ventricle that's sending deoxygenated blood to the lungs which then comes back to the left side of the heart and the left ventricle sends this newly oxygenated blood to the aorta so that it can go out to the rest of the body in both cases this is ventricular ejection and it's happening at the same time now we can break this ejection phase down into two parts and as you'll see it just kind of makes sense in the beginning of the ejection phase you're going to get blood that's ejecting into the arteries very rapidly this is the rapid ejection part but then as the blood leaves the ventricles the pressure in The ventricle is gonna start to decline not only that but as the blood goes into the vessels it's going to encounter resistance it's going to be bumping against the walls there's going to be friction and the pressure difference between in the ventricles and the blood vessels it's going to decrease this is when you'd get the second part slow ejection by the way if you're loving my cardiac cycle content check out my free cardiac cycle guide so that you can download it and even print it out to help you master the cardiac cycle I'll link to that in the description below now let's briefly touch on the pressure in the arteries we said that the pressure in the aorta is around 80 millimeters of mercury and that's when it's relaxed well now that we've just pumped a bunch of blood into the aorta that pressure is going to increase and the maximum pressure that it'll reach is somewhere around 120 millimeters of mercury at least under normal circumstances now do those numbers sound familiar 120 and 80 well of course they do right 120 over 80 that's what's considered normal blood pressure the 120 millimeters of mercury is the peak value and that's called the systolic pressure and and the 80 millimeters of mercury is the low value and that's called the diastolic pressure and those are the values we measure that give us blood pressure and of course if those values are too high or too low that can cause some issues high blood pressure can cause damage to your vessels because you got all this pressure it can cause them to lose their elasticity it basically can lead to a whole bunch of blood flow problems that affect how blood and oxygen reach the tissues and on the other end of the spectrum you have low blood pressure and as you can imagine if you're not building up enough pressure that's also going to affect how blood and oxygen get delivered to the tissues which can have a whole host of effects on the body now let's take a look at how the volume of blood in the ventricles change during the ejection phase at the end of the isovolumetric phase we have about 130 milliliters of blood in the ventricles that volume is going to go down relatively quickly to around 60 milliliters that means that each ventricle pumps out about 70 milliliters of blood during that ejection phase that's 70 milliliters of blood is called the stroke volume that's the amount of blood that's ejected from the Heart during one stroke or during the ejection phase and the way we calculate that is we take the amount of blood before ejection which is the end diastolic volume minus the amount of blood after the ejection the end systolic volume so 130 minus 60 gives us 70 and that's 70 milliliters of blood all right let's take a look at the EKG to see what happens during this phase of each action there's one thing that we see here and that's the T wave the T wave shows ventricular repolarization now if you remember the QRS complex is what showed ventricular depolarization which was the spread of the electrical signals across the ventricles now that we had that signal the membrane potential of the muscle cells in the ventricles they basically combat down it resets so that we can get another signal and we can have the ventricles contract again so that the cycle can continue that brings us to the end of the ejection phase and the end of systole however it's now the beginning of the next phase isovolumetric relaxation or more specifically isovolumetric ventric particular relaxation and guess what this part will be pretty simple to understand if you understand what happens during isovolumetric contraction because it's somewhat the opposite but let's get into it we have the T wave which we said showed ventricular repolarization well when repolarization happens the natural thing that's going to happen next is that the ventricles are gonna relax there's no more depolarization the signal to contract is gone no more contraction signal no more contraction and the opposite of contraction is relaxation and that's what the ventricles are going to do now what's that going to do to the pressure in the ventricles well blood left the ventricles are relaxing the pressure was already starting to go down looking at the left side when the pressure in the left ventricle gets lower than the pressure in the aorta that's going to cause the aortic valve to close and since the atrioventricular valve was already closed those we now once again have a closed container but instead of the ventricles Contracting they are now relaxing so there's going to be a relatively quick decrease in ventricular pressure and that's the isovolumetric ventricular relaxation phase that's a mouthful wow I like it there's another thing I want you to notice in this phase let's take a look at the aortic pressure in the previous phase we saw that the pressure in the aorta was starting to come down from its peak at around 120 millimeters of mercury however that decrease is briefly interrupted by this slight bump in pressure before it continues to go down what's happening here well we already know that the beginning of isovolumetric contraction is when the aortic valve closes when that happens there's going to be a brief period where the blood in the aorta will flow backwards and hit against that closed valve causing a brief increase in aortic pressure and this is completely normal we call it the dichrotic notch it's when because the aortic valve closes the blood briefly hits against the valve causing a slight bump in aortic pressure but after that brief period the aortic pressure continues going down now we've spoken about the aortic valve closing and just like before when valves close it's not a silent event there's going to be a sound as a result of this valve and the one on the right side closing these two semilunar valves close at the same time resulting in the second heart sound the dub of the lub dub that you hear when you listen to the heartbeat let's bring this ISO volumetric relaxation phase to a close and briefly address the last phase the ventricular pressure is going down and it's gonna reach a point where the pressure in the ventricles is going to be lower than the pressure in the Atria and when that happens the atrioventricular valves they're gonna open ending the isovolumetric relaxation phase and bringing us to the last phase of the cardiac cycle slow ventricular filling or just ventricular filling the atrioventricular valves are open the semilunar valves are closed the pressure in the ventricles is at its low point and as a result the blood that's coming back from the body and from the lungs comes in through the Atria and starts to fill the ventricles that's why we see the volume in the ventricles start increasing even though there's no contraction taking place it's kind of a passive process this then sets the stage for the entire cardiac cycle to start over we had atrial systole isovolumetric contraction ejection isovolumetric relaxation slow ventricular filling and this entire cardiac cycle happens over and over and over to 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