Description

Complete Cheat Code for Heart Physiology Series:

• 1st Video: Types of cardiac muscle, action potentials of pacemaker cells and contractile myocardium, and general properties of cardiomyocytes.
• 2nd Video: Detailed exploration of the cardiac cycle, including phase-by-phase events, valve operations, and pressure differences.
• 3rd Video: Cardiac output.
• 4th Video: Regulation of heartbeat.

Phases of the Cardiac Cycle:

The cardiac cycle consists of mechanical events in one heartbeat, including contraction (systole) and relaxation (diastole).

• Atrial Systole (0.1s): Atria contract, pushing blood into ventricles.
• Isovolumetric Contraction (0.04s): Ventricles contract, but valves remain closed.
• Ventricular Ejection (0.26s): Ventricles push blood into the aorta and pulmonary artery.
• Isovolumetric Relaxation (0.08s): Ventricles relax, no blood flow into ventricles.
• Passive Filling (0.32s): AV valves open, ventricles fill passively.

Valve Operations and Pressure Differences:

Bottle Cap Phenomenon: Demonstrates how pressure controls valve function.

Pressure Curve and Graphical Representation:

• Vertical Axis: Blood pressure (mmHg).
• Horizontal Axis: Time (seconds).
• Red: Left ventricular pressure.
• Blue: Left atrial pressure.
• Yellow: Aortic pressure.

Heart Sounds:

• S1 (First Heart Sound): Closure of AV valves, start of systole.
• S2 (Second Heart Sound): Closure of semilunar valves, start of diastole.
• S3 (Third Heart Sound): Passive ventricular filling.
• S4 (Fourth Heart Sound): Strong atrial contraction.

Ventricular Volumes:

• End Diastolic Volume (EDV): 110-150 ml.
• Stroke Volume (SV): 60-100 ml.
• Reserve Volume: 20-40 ml.
• Residual Volume: Volume never ejected.
• Ejection Fraction (EF): Normal 50-70%, reduced in heart failure.

#cardiacphysiology #heartfunction #cardiaccycle #ecg #medicaleducation #usmlepreparation #nursingeducation #premed #heartanatomy

Sources:

• University lectures and notes.
• Barrett, K. E., Barman, S. M., Boitano, S., & Brooks, H. L. (2016). Ganong’s Review of Medical Physiology (25th ed.). McGraw-Hill Education.
• Hall, J. E. (2016). Guyton and Hall Textbook of Medical Physiology (13th ed.). Elsevier.
• Mohrman, D. E., & Heller, L. J. (2014). Cardiovascular Physiology (11th ed.). McGraw-Hill Education.
• Biorender.

Transcript

Introduction 0:08 What s up, Taim talks med here. Let s continue our Complete Cheat Code for Heart Physiology. 0:13 We re doing this in 4 segments. The 1st video was about the different types of cardiac muscle, 0:18 the action potentials of pace maker cells and contractile myocardium, and the general 0:22 properties of our cardiomyocytes. In this video we will look detailed into the cardiac cycle, 0:28 and draw a curve and understand what happens at each phase. In the 3rd video we ll cover 0:33 everything you need to know regarding the cardiac output. And in the 4th video where we ll cover 0:38 the regulation of the Heartbeat, covering all the most important mechanisms that actually change the 0:43 contractility and heart rate. Alright awesome. Cardiac cycle is basically just the beginning What is Cardiac Cycle? 0:50 of one heartbeat to the beginning of the next. And it includes all the mechanical 0:55 events happening within one heartbeat. So If you look here, this is the heart, 1:00 and here it s pumping continuously. Showing you one cardiac cycle after the other, 1:05 and if we remove the walls as it s pumping, you ll see that there are specific events happening. 1:11 And if you just stare at this heart, focus on one chamber, you ll notice that one cardiac 1:17 cycle consists of two periods: one during which the heart muscle relaxes and refills with blood, 1:23 called diastole, following a period of contraction and pumping of blood, called systole. 1:29 What we re going to cover now are what happens within each phase, what valves open, 1:35 when do they close, how long does these phases take, and how pressure differences are controlling 1:40 all of these events. I ve simplified everything to you don t worry. At the end of this video 1:45 everything will make perfect sense. Now, we divide the cardiac cycle into 5 Phases of Cardiac Cycle 1:51 5 phases according to what the heart is doing at that specific moment. Saying it simply. 1:57 First phase is the beginning, it s the atrial systole. In atrial systole both 2:02 atria contract and pushes the blood into the ventricles. This lasts for about 0.1 second. 2:09 Next phase is something called Isovolumetric Contraction, where the ventricles contract, 2:15 but there s no blood flow out of the ventricles since both valves are closed, this lasts for 2:20 0.04seconds, Then after that the ventricles are going to push the blood into the great arteries, 2:27 into the pulmonary trunk and the aorta, lasting for about 0.26s, so the ventricles contract for 2:33 about 0.3 seconds if you add up the isovolumetric contraction and the ejection phases. After that 2:40 we got Isovolumetric relaxation, where all the chambers relax, there s no blood flow into the 2:46 ventricles, but there s blood passively coming into the atria, lasting about 0.08 seconds. 2:53 Then lastly there s the passive filling phase, where the av valves open, and blood is passively 2:59 flowing into the ventricles without the atria contracting, lasting for about 0.32 seconds. 3:06 And then after that we go back to the first phase, where atria contracts, squeezing the 3:14 remaining blood into the ventricles. Now. Take the parts where the ventricles are contracting, 3:20 what can we call them? Ventricular Systole. Take the parts where the ventricles are relaxing, 3:26 what do we call them? Ventricular Diastole, or common diastole, or common pause. 3:32 Now I ll give you a little cheat code in understanding Bottle Cap Phenomenon 3:35 how the valves work during each phase. Here you have a bottle, just an empty bottle 3:41 with a cap on. The bottle has a specific pressure within, compared to the pressure outside. Let s 3:47 say you grab this bottle, and give it a good squeeze, what happens to the pressure within 3:51 now? It increases. Now you re super strong, let s say you squeeze extra hard, what happens now? 3:59 The pressure inside increases even more. And you squeeze so hard that the cap of the bottle pops 4:05 open. Doesn t it then make sense to say that the bottle cap popped open because the pressure within 4:11 the bottle were much higher than the pressure outside the bottle? Now let s add the heart. 4:17 Which way does the valve go? Goes to the ventricles, so we can say that the 4:22 ventricles are outside the bottle cap, and the atria is underneath the bottle cap. 4:28 If the pressure in the atria and the ventricles are equal, what will happen? Nothing. 4:34 If the pressure in the ventricles are higher than the pressure in the atria, what happens? Nothing, 4:40 the bottle cap won t pop open because of that. Now if the pressure in the atria is much higher 4:46 than the pressure in the ventricles, what happens now? The valve is gonna pop open. That s basically 4:53 how the valves in each phase is controlled by the pressure changes. Let s put this to practise. Pressure-Time Graph 4:59 Now the cardiac cycle is all dependent on the pressure of the atria, ventricles and 5:04 the great arteries. So let s make a pressure curve. Here in the vertical axis we got blood 5:09 pressure expressed in mmHg, and time on the horizontal axis in seconds. Let s now divide 5:16 this curve so that it correspond to each of these 5 phases. And we ll draw the pressure 5:22 in the left ventricle in red, pressure in the left atrium in blue and pressure in the aorta 5:28 in yellow. Now that we have everything set up, let s start with the atrial systole. Atrial Systole 5:34 During atrial systole, the pressure in the ventricle is lower than the pressure in the 5:39 atria, right? Ventricular pressure is lower, and because it s lower, the AV valves are open, and 5:45 blood is being squeezed into the ventricles. While this is happening, the pressure in the ventricles 5:52 are lower than the pressure in the great arteries, right? That means that the semilunar valves are 5:58 closed. Remember pressure inside is lower than pressure outside. So how does this look like? 6:05 Imagine now that during atrial systole, while atrium is contracting, 6:09 there s no blood from the veins coming into the atria, but since it s contracting and pushing 6:15 blood through the AV valves, the pressure is going to rise initially, before decreasing 6:20 again since it s getting more and more empty. When it comes to the left ventricle, blood from 6:26 atria is flowing into ventricle, so pressure is going to rise. How much? In the right ventricle 6:32 about 4-6 mmHg, and in the left ventricle about 5-10 mmHg. As we re approaching the end of atrial 6:42 systole, you ll notice that the pressure in the left ventricle goes a little bit down, 6:47 partly because once the atrium is empty, the plane of the AV valve rise a little bit upwards, 6:53 and also while all of this is happening, the pressure in the Aorta keep decreasing 6:57 aswell since blood is leaving the Aorta towards the periphery, so the plane of the semilunar 7:02 valves may rise a little bit aswell. Right after the atria is done contracting, Isovolumetric Contraction 7:08 we enter the isovolumetric phase. In the isovolumetric phase the ventricular pressure is 7:14 higher than the aortic pressure. So the AV valves are closed. And the ventricular pressure is lower 7:20 than in the great Arteries, so the semilunar valves are closed aswell. In this phase the 7:25 ventricles are squeezing without any blood moving. Now how does this phase affect the pressure. 7:32 First off the pressure in the atria is going to rise, and there re two reasons for that. 7:37 The first one is that when the AV valves shut close, it pushes the AV plane a bit up, 7:43 increasing the intraatrial pressure a little bit. The second reason is simply because the ventricles 7:49 are contracting. They re contracting, which also pushes the AV-plane a little bit upwards. 7:54 The pressure in Aorta is just gonna keep decreasing still, because blood is still flowing 7:59 out to the periphery. Now what happens with the pressure in the ventricles? 8:03 It shoots up, the walls are contracting but there s no volume change, blood isn 8:08 t going anywhere, so the pressure rises a lot. So notice, the red line is above the blue line, 8:14 which means the pressure in the left ventricle is higher than in the atria, 8:18 so the AV valves are closed, while the left ventricular pressure is still lower than in 8:23 the aorta, so the semilunar valves are closed. Now we enter the ejection phase. In the Ejection Ejection 8:31 phase, the pressure in the ventricles are higher still than in the atria, so the AV valves are 8:37 closed. The pressure in the ventricles are higher than that of the great vessels. So the semilunar 8:43 valves are open, and blood is being pushed out into the great vessels, in both the pulmonary 8:48 circulation and in the systemic circulation. Now how does that affect the pressure in each part? 8:54 The atrial pressure during the ejection phase is first going to decrease, primarily because while 9:00 the ventricles are contracting, it s going to pull the AV-plane downwards. I can t express how 9:05 important this is for the venous return. Because look here, while the ventricles are contracting 9:11 and pulling the AV plane down, it decreases the pressure in atria which facilitates the venous 9:18 return into the atria, same way as if you re holding a syringe and want to suck up some fluids, 9:24 how do you do that? You pull this part back, decreasing the pressure in the chamber of the 9:29 syringe, which allows for the fluid to move in. That same mechanism is happening here in the 9:35 atria, while ventricles are contracting, it pulls the AV plane down, decreasing the pressure in the 9:42 atria and increasing the venous return. Eventually the pressure will rise a little bit again since 9:48 the atrium is getting filled with blood. Now, the pressure in the ventricles are higher 9:55 than that of the great arteries. So the pressure in the great arteries are about 60-80 mmHg in the 10:02 Aorta and about 10-15 mmHg in the pulmonary trunk. So the pressure in the left ventricle is higher 10:09 than in the aorta and the pressure in the right ventricle is higher than in the pulmonary trunk. 10:15 Because of that, semilunar valves are open and blood is ejecting into the great arteries. Now 10:22 the left ventricle as it contracts, it s going to reach a max pressure of about 125-130 mmHg in the 10:30 left ventricle, and about 25-30 mmHg in the right ventricle. Though we re mostly interested in the 10:37 left ventricle for now. But as the left ventricle is getting less and less blood within it as it s 10:43 pushing it out, the pressure start to decrease. The pressure in the aorta is going to more or 10:49 less follow the left ventricle, so what happens is. Right in the beginning, just as the aorta is 10:54 receiving a lot of blood, it s gonna initially get stretched as blood is filling up here. 10:59 Remember the aortic wall is quite stertchy so it can accumulate a certain amount of blood. While, 11:05 of course some of that blood is going to leave but not all of it. That s why the pressure in the 11:10 aorta rises together with the left ventricle initially. At the end of the ejection phase, 11:16 the pressure starts to decrease as the blood doesn t rush into the aorta as quickly as it 11:20 did in the beginning anymore, since the ventricles don t have that much blood within it after pushing 11:26 everything. So there s still going to be some inflow at the end of ejection phase, 11:31 but not as fast as in the beginning. This happens while blood is flowing into the 11:36 periphery. So the blood inflow is lower than the blood outflow, so the pressure decreases. 11:43 Now we re done with the ejection phase and we enter the isovolumetric relaxation. We ve done our Isovolumetric Relaxation 11:50 job, It s time to relax. All walls are relaxing, we ve entered the common diastolic phase. 11:57 So now the pressure in the ventricles are higher than the pressure in the atria, 12:02 so the AV valves are closed. Pressure in the ventricles are now lower than 12:06 the pressure of that in the great arteries, so the semilunar valves are also closed. 12:12 What s happening now is that the atria are still passively getting filled with 12:16 blood while the walls are relaxing. So the pressure increases a little bit. 12:21 The ventricles are just tired at this point it steeps all the way down. It doesn t contain a lot 12:26 of blood within it, and the walls are relaxing. The pressure in the aorta however is a little bit 12:31 interesting now. The pressure is initially going to increase a little bit at first because when 12:37 the pressure within the aorta becomes higher than that of the ventricles, the semilunar valves are 12:43 gonna close, and what happens now is that there s gonna be a brief moment where the blood flows 12:50 back and fills the pockets of the semilunar valves with blood before it s reflected back again. That 12:56 s why it initially rises just a little bit before it starts to decrease again as blood continues to 13:02 flow out towards the periphery. Now we ve come to the last phase, the passive filling phase, Passive Filling 13:09 where the pressure in the atria is now higher than the pressure in the ventricles, 13:13 so the AV valves are open. The pressure in the ventricles are lower than in the great arteries, 13:21 so the semilunar valves are closed. What s happening now is that blood 13:25 from the atria pours into he ventricles so the pressure decreases a little bit in the beginning, 13:30 and then it remains more or less stable. The ventricles are going to receive 13:36 all of that blood passively, just because of that pressure difference, 13:39 you see that the red line is continuously below the blue line, which means the pressure in the 13:45 atria is continuously higher than the ventricles, so the AV valves are open and blood is filling 13:51 without any parts of the heart contracting. While this is happening, the aortic pressure 13:57 is just gonna continue to decrease because there s gonna be a constant outflow of blood from the 14:03 aorta and out to the periphery. And then we re back at square one again in the cardiac 14:08 cycle. Back to atrial systole where the atria is going to contract and increase the pressure. Active Filling 14:14 It s important to highlight here that during the passive filling, that s when the majority of blood 14:20 is going to fill the ventricles, about 80% of the blood flows in during the passive filling phase, 14:26 while during atrial systole, we call this phase active filling, means it s 14:31 gonna actively squeeze hard trying to push the remaining 20% of blood into the ventricles. 14:37 So that s really it, that s what happens within one cardiac cycle from beginning to end, 14:42 showing the left ventricle, left atria and the aorta in this graph. That s what we re interested 14:47 in in general. And here at the bottom you ll see when the semilunar valves are open and closed, Valves 14:53 and when the AV valves are open and closed. You ll notice everytime the blue line is over 14:59 the red line, the AV valve is open, and when it s below, the AV valve is closed. Same with 15:05 the semilunar valves, when the red line is above the yellow line, the valves are open, 15:10 but when the red line is below the yellow line, the semilunar valves are closed. Awesome. Phonocardiogram 15:17 Let s now expand this diagram a little bit more. Now, if you get a patient, 15:23 and you take a stethoscope, and you put the tip of the stethoscope on the patients chest, 15:28 you can hear some heart sounds, right? What part of the cardiac cycle do you think you 15:33 re hearing? Well let s record the sounds of the heart and make a curve, called phonocardiogram. 15:43 When you look at this diagram, you ll see that we re able to record 4 heart sound, but in reality 15:50 we re really only able to really hear 2 heart sounds only, the 1st and 2nd because those are 15:55 the loudest. The 3rd and the 4th heart sounds can only be recorded only with a phonocardiogram 16:01 with the proper amplification of the sound. Now, the first heart sound can be heard at the 16:07 beginning of the ventricular systole, and that s why we can also call it the systolic heart 16:12 sound. And there are actually three reasons for the systolic heart sound. The first reason for 16:17 the strong systolic sound is because of the sound produced during closure of the 16:22 AV valves at the time of when the pressure in the ventricles are greater than in the 16:27 atria. The second reason for this sound is because of the vibrations of the blood as it 16:32 hits the ventricular walls during isovolumetric contraction. And the third for this strong sound 16:38 is due to the opening of the semilunar valves and blood rushing into the great arteries. 16:44 The second heart sound is heard at the beginning of the diastolic phase, 16:48 so it s also called the diastolic heart sound. And the diastolic heart sound is due to the closure 16:53 of semilunar valves when pressure in the aorta becomes greater than that in the ventricle. 17:00 The third heart sound is recorded at the beginning of filling phase after AV valves opening when 17:05 blood from atria fills the ventricles. While the fourth heart sound is recorded during 17:12 atrial systole, or active filling phase, when strong atrial contraction pushes the blood into 17:18 the ventricles. Kinda makes sense doesn t it? So again, the first and the second heart sounds 17:23 are the ones we can hear. Now let s say you listen to the heart sounds of the patient, 17:28 how do you differentiate these two sounds? Well there re three ways to differentiate them. 17:34 The first heart sound is longer than the second one, and the first one have a lower frequency 17:40 than the second one. But the main difference, the main way to differentiate these to is by listening 17:46 to the time at which they appear. Because look the distance between the first and second heart sound 17:51 is shorter than the distance between the second and the first heart sound. Makes sense, right? 17:57 Knowing this will make it so much more easier to diagnose things like valvular stenosis or 18:03 regurgitation. The third and fourth heart sound is actually pretty hard to record unless the patient 18:09 has a really high blood flow and turbulence. So that s the heart sounds. 1st heart sound 18:15 represents the systolic sound, and the 2nd heart sound represents the diastolic sound. Ventricular Volumes 18:20 The last thing I wanna mention now, is something called ventricular volume. 18:25 How do we classify the different volumes at different times during the cardiac cycle. 18:30 Now, after the passive filling phase and the active filling phase, right after the AV valve is 18:35 closed and the diastolic phase has ended, we have a certain amount of blood within the ventricles, 18:40 right? So we say, the maximum amount of blood that can accumulate within the ventricles at the end of 18:47 the diastole is right around 110-150 ml of blood. We call this value the End Diastolic Volume. 18:56 And we can now divide the end diastolic volume into three parts. I ll just show you quickly 19:01 here before showing it on the graph. Alright. The end diastolic volume, which again is the maximum 19:07 amount of blood within the ventricles, can be divided into three volumes. Now, 19:13 during the systolic phase, blood is being pushed out towards the aorta, that volume of blood that s 19:19 being pushed out during one cycle is called Stroke volume, which is about 60-100 ml. In addition to 19:27 that, there s also something called Reserve Volume, which makes up around 20-40 ml. It s 19:33 called reserve volume, meaning if the ventricles were to give off a stronger contraction, a really 19:38 strong contraction, you d eject the reserve volume as well. So it s kind of in an emergency 19:43 if your body really needs a lot of blood. The third volume is called residual volume, 19:48 which is the volume that s never ejected from each ventricle even during the strongest contraction. 19:54 So the ventricles never really becomes completely empty, mostly to prevent the walls from sticking 19:59 to each other. Remember anatomically, the right ventricle has a weaker contractility 20:04 and a larger internal volume than the left ventricle, so the Residual volume in the right 20:09 ventricle might be a little bit higher that the residual volume within the left ventricle. So, 20:14 reserve volume, only pushed out when needed. Residual volume, never pushed out. 20:20 The last two volumes, the reserve volume and the residual volume, can be combined called End 20:26 Systolic Volume. So, we got end diastolic volume, after the systolic phase when the 20:32 heart has given off the systolic volume, we re left with the end systolic volume, which 20:37 contain reserve volume and residual volume. If you take the end diastolic volume, you can 20:44 make up a percentage of how much of that volume has actually been ejected out of the ventricle, 20:49 we call that, ejection fraction. Normally the amount of blood ejected should be about 20:55 50-70% from total end diastolic volume. Now imagine a patient with heart failure. 21:02 If that heart has cardiac muscle failure, the contractility decrease. And fi the contractility 21:07 decrease, what happened to the ejection fraction? It decreases aswell, becoming even lower than 21:13 50%. So despite the fact that the ventricles are getting filled with blood, they can t even eject 21:19 normal stroke volume anymore. We ll talk about this in detail when we go through the topic of 21:24 heart failures. But now, let s go back to our graph. We got our end diastolic volume, right? 21:32 When pressure of the ventricle rises above the pressure in the aorta, 21:36 the semilunar valves open and blood is pushed out of the heart. That blood that is pushed out 21:42 during one cycle is called Stroke Volume. And now only the end systolic volume is remained 21:48 within the ventricle, which consist of the residual volume and the reserve volume. 21:53 And notice now at the end of the isovolumetric relaxation, when the AV valves open, 21:59 about 80% of the blood is filled passively into the ventricles, and the rest of the 20% is filled 22:05 actively while the atria is contracting. Alright so that was everything I had for the Next video 22:11 Cardiac Cycle. We covered all the 5 phases of the cycle. Tell me, what is the name of the 5 cycles 22:17 again? Atrial Systole, Isovolumetric Contraction, Ejection, Isovolumetric Relaxation, and Passive 22:26 filling phase. Awesome. What else did we cover? We made a pressure graph for the cardiac cycle, 22:32 showing pressure of the left ventricle, left atria and the aorta. And we added the phonocardiogram 22:38 for the heart sounds and covered the ventricular volumes at each phase of the cardiac cycle. 22:44 The next video is going to be about the Cardiac Output. QUIZ 22:47 Thank you all for watching! If you found this information helpful and want a little extra 22:50 practice on Cardiac Cycle, I’ve created a quick follow-up video with quizzes and questions. These 22:56 resources are available exclusively for channel members and I’ve kept the membership fee as 23:00 low as possible to support the channel’s growth. Your support means a lot to me. 23:04 Consider becoming a channel member to access these additional resources and help me continue 23:09 creating more educational content like this. Just click the join button below to get started. 23:13 Thank you again for watching and I hope this was helpful. See you in the next video! Peace.