Heart Conduction System & ECG (EKG)

[00:00] Your heart has beat continuously for your entire life from that very first heartbeat back when you were a fetus developing in the uterus until this very day. That constant rhythm that keeps the blood pumping through your arteries is thanks to a small mass of cardiac tissue called the sinoatrial node, along with a complex cardiac conduction system that runs through your heart, coordinating your heartbeat. In this video, we're going to be talking about cardiac conduction.

[00:20] build out the cardiac conduction system piece by piece, learn how it all works, and then use that to understand the different parts of an ECG or EKG. And throughout the video we'll look at real human cadavers and other cadaveric images provided by Anatomage, the creator of the world's first virtual dissection table. So you can see all of these structures are arranged three dimensionally in the body. And by the end of this video you're gonna know this whole

[00:40] process by heart, well by brain because that's where memories are stored, but you know what I mean. Let's jump to the whiteboard and get started. So let's start by drawing out the heart. Here we have an outline of the main structure of the heart on the cardiac muscle. We've got the right atrium and the right ventricle and the left atrium and the left ventricle. And of course we have the right side in blue because it's low oxygen

[01:00] blood, the left side is red because it's high oxygen blood that's just come from the lungs. And remember, our blood is always red, never blue. That's just the colors we use in the diagrams. Blood is going to come into the right side through the superior and inferior vena cavas into the right atrium. It'll pass through the tricuspid valve into the right ventricle and the right ventricle is going to pump it out through the pulmonary artery to go to the lungs.

[01:20] to receive some oxygen. The blood is going to get back to the heart through the pulmonary veins and go into the left atrium. From the left atrium it'll pass through the bicuspid or mitral valve into the left ventricle and then the left ventricle is going to pump it very forcefully out through the aorta so it can travel throughout the rest of the body and deliver the oxygen and nutrients and hormones and all the other stuff that's in our blood. In between the left side

[01:40] in the right side of the heart we have the septum that really divides the heart into those two halves. And here we can see in the anatomage models the right atrium and the right ventricle sitting sort of anterior to the left side where we have the left atrium and the left ventricle. Now for the rest of the video I'm going to assume you know those structures pretty well, but if you want to refresh your whole lesson on all of that, check out my

[02:00] Pathway of Blood Through the Heart video. Links down below for that. Now in the heart there's three different types of tissue that we're concerned about in this video. First we have the cardiac conduction system itself that's going to be in yellow on this diagram and that's non-contractile cardiac tissue. In other words these aren't muscle cells that are contracting. They're going to be more like nervous tissue that's going to be conducting signals throughout

[02:20] out the heart. Second we have the cardiac muscle tissue. This is going to be contractile tissue. It's going to be tissue that contracts and pumps blood, but it also conducts signals as well. The cardiac conduction system, which is just about 1% of the total kind of cells in here, that's going to conduct these signals very quickly, whereas the cardiac muscle will take a little bit longer for the signals to pass.

[02:40] through the muscle tissue itself. And the cardiac muscle tissue that's going to be the vast majority of tissue in the heart. Now there is some non-conductive tissue in the heart and that's going to be the fibrous tissue and that's going to run from the atrial floor between the atrium and the ventricles. Now the fact that this fibrous tissue right there is non-conductive is very important. We don't want the atria and the ventricle to pass through.

[03:00] and the ventricles to be contracting at the same time. We want the atria to contract and push the blood into the ventricles and then the ventricles to contract and push all that blood out of the heart. If this tissue right here was conductive, then we would have the atria and ventricles contracting at the same time, wouldn't be good. And the only way that signal can pass through here is through the cardiac conduction system through this yellow section.

[03:20] right there. That's the only part where the signal travels through that fibrous connective tissue. So that fibrous connective tissue separates the atria from the ventricles into two sections that we call the atrial syncytium as well as the ventricular syncytium. A syncytium is just a big sort of hard to pronounce word that means a group of syncytians.

[03:40] group of cells that are all electrically connected to each other. So all of the cardiac muscle in this atrial syncytium are connected electrically, meaning that if one of the cells depolarizes, it depolarizes the next cardiac muscle cell and that depolarizes the next one and eventually they'll all be depolarized because they're all electrically connected. Same thing in the ventricular syncytium.

[04:00] when one of those cells depolarizes, that'll depolarize the next cell and the next cell and the next cell until it's all depolarized. Now the signal passing between cardiac muscle cells is sort of slow. I kind of mentioned that earlier. And that's why we need this cardiac conduction system to conduct those signals very quickly so that the atria can contract as one unit and the ventricles especially.

[04:20] can contract as one unit. But again the atrial syncytium will depolarize first, contracting the atria, and then the ventricular syncytium will depolarize contracting the ventricles. Now let's take a look at the individual parts of the cardiac conduction system. First here we have the sinoatrial node. Now the sinoatrial node is autoarrhythmic, meaning that it's going to be syncytial.

[04:40] sending pulses by itself even without input from some other source. Autorhythmic meaning self-rhythmic. In other words, that SA node is the pacemaker of the heart. It sends a signal every time our heart beats. Now there will be input from the brain, from the cardiac regions of the brain. They'll be sending signals to the heart to speed up that SA node.

[05:00] or to slow down the SA node. But even without input from the brain, the SA node is going to be sending signals itself, causing our heartbeat rhythm. This consistent rhythm happens by increasing permeability in the SA node of sodium ions and calcium ions, so those sodium and calcium ions are slowly entering into the SA node.

[05:20] and it prevents potassium from leaving the cells in the SA node. So there's a slow buildup of positive charge over time as sodium and calcium come into the SA node, and then as soon as it reaches a threshold membrane potential, the SA node will send an action potential. Now the signals will eventually get to another node called the atrioventricular, or AV node. More on that in just a moment.

[05:40] Next, let's talk about the internotal pathway. This is going to be how the signal transmits from the SA node to the AV node. If you look at the internotal pathway, there are three branches of it and they are all passing through the right atrium. On my diagram they look sort of plainer or flat with each other, but if we look on the anatomage images, we will see

[06:00] that these are actually three-dimensional running through that right atrium, which we can see right there. So the SA node depolarizes, sends a signal through the interneural pathway to the AV node, and that's going to depolarize the right atrium. Now eventually that depolarization would make itself over to the left atrium, but that's going to take a long time without the interatrial path.

[06:20] pathway, which is going to run from the SA node over into the left atrium. That interatrial pathway will conduct the signal very quickly into the left atrium and depolarize it. Most of the diagrams that I looked up have the interatrial pathway coming directly from the SA node, but one thing I noticed in the anatomage images is that the interatrial pathway is actually branching

[06:40] off of one of the internotal branches. Even though most of the diagrams you look up on this show it coming from the SA node directly. Great, so the signal comes from the SA node, it's gonna travel through the intraatrial pathway, as well as the internotal pathway, depolarizing both atria so that they can contract. That signal then is gonna make it to the AV node. Now we've talked about the cardiac conduction system and the diag

[07:00] system needing to send these signals very quickly, but the AV node sort of does the opposite. There's going to be a delay in the AV node. Now what would be the benefit of that? Well like I said earlier, we want the atria to contract before the ventricles contract. So that delay is going to really separate the atrial contraction from the ventricular contraction. That way we can get all the blood from the atria.

[07:20] atria to the ventricles and then the ventricles can pump it all out. From the AV node the signal is going to pass into the bundle of hyps, also known as the atrioventricular bundle. That bundle is immediately going to separate into the right and left bundle branches. Now unlike the AV node, which passes the signal very slowly, the bundle of hyps and the bundle branches are going to transmit

[07:40] signal very quickly down the septum of the heart. Also as the signal is traveling through the septum it's not going to be stimulating the ventricles to contract just yet. The ventricles will be stimulated to contract when the signal is passing its way back up. That's going to allow the pumping to happen from the apex of the heart on the way back up to kind of force the blood out through the pulmonary artery.

[08:00] and the aorta this way. The tricuspid and mitral valves will also snapshot during this time to prevent the blood from backflowing into the atria. Now extending out of the left and right bundle branches we have something called the Purkinje fibers and the Purkinje fibers are going to take that signal traveling through the bundle branches and spread it out throughout the muscle of the right and left ventral.

[08:20] That's going to conduct that signal many, many times faster than if we're only relying on the ventricular syncytium or the connections between all of the cardiac cells. So quick recap of all of that. The SA node or the pacemaker of the heart will send out a signal. That'll pass through the interatrial pathway to stimulate the left atrium. It'll also pass through the internotal pathway.

[08:40] to stimulate the right atrium. The signal will make it to the AV node, where it's going to pass very slowly to cause a delay before the ventricles will contract. The signal will pass through the bundle of hyps and the left and right bundle branches. On the way back up, they'll pass through the Purkinje fibers. That's going to stimulate the cardiac muscle and the ventricles to contract and pump the blood out.

[09:00] through the pulmonary artery as well as the aorta. Now let's take a look at an ECG or an EKG. This is the thing that you've seen in like doctor movies and stuff where you see the beep, beep, and then if it stops you hear it go beep as the heart has stopped beating. But it's a measure of the electrical activity happening in the heart and it's got three regions here. It's got the P wave, the QR wave.

[09:20] RS complex as well as the T wave and these three sections correspond to different things happening in the cardiac conduction pathway So again, we have the P wave the QRS complex and the T wave and you can see that happening one more time there I just really like that animation. So what we're gonna do is we're gonna connect this to the cardiac conduction pathway looking at the P wave QRS complex

[09:40] and the T wave. We're gonna start with the P wave. The P wave is going to correspond to the depolarization of the atria. So we've got the depolarization happening and you can see those signals traveling through those different pathways causing depolarization of the atria. So that's the main thing happening here in the P wave. The atria will depolarize. Next we have what we call the PR interval.

[10:00] The PR interval is going to start at the beginning of the P wave and last all the way really until the Q part right here. We call it the PR interval I think because sometimes on ECGs the Q wave might be hard to identify or might not show up. So we refer to this as the PR interval. You also might see something called the PR segment. So just as a quick clarification, the PR interval starts at the beginning of the Q wave.

[10:20] starts at the beginning of P and lasts until R, whereas the PR segment is just from the end of P to the beginning of R. So PR interval would be this, PR segment would just be this. Now during the PR interval the atria are going to contract and that's going to send blood from the right atrium into the ventricle and the blood from the left atrium into

[10:40] the left ventricle. Basically any blood that was still left in the atrium is going to get squeezed out through the contraction of the atria. The contraction of the atria will really start early on in the P wave and last throughout this section right here. Also during this section of the PR interval, which is the PR segment, is where we have that AV nodal delay happening. Because as soon as the QRS complex hits then we're going to be

[11:00] polarizing the ventricles. Speaking of which, let's move on to the QRS complex. In the QRS complex you see this huge R spike. That's because of the signal passing through the bundle of fist and the bundle of branches and then through the brachinje fibers stimulating all of this cardiac muscle. That electrical activity is going to be much greater in the ventricles because the ventricles have

[11:20] more cardiac tissue, they also have to pump the blood a lot farther. The atrium just had to pump the blood from one chamber to another. The ventricles have to pump the blood to the lungs and then also through the aorta throughout the whole body. So they need a very strong contraction. So we need a lot of electrical activity to cause that to happen. So during the QRS complex that signal is going to pass through the left and right

[11:40] bundle branches and through the kabrikinje fibers. That's going to depolarize the ventricles. Also the atria are going to repolarize. Repolarization is the opposite of depolarization. Depolarization is when tissue becomes more positive and in this case causes it to contract. Repolarization is when it returns back to its resting membrane potential and that muscle is going to relax and stop contracting.

[12:00] So we have depolarization of the ventricles as well as repolarization of the atria. Basically, ventricles contract, but the atria will be stopping their contraction. Now that QS complex, that big spike in the R, is caused by the ventricles depolarizing. We can't really see the effect of the atria repolarizing on the EKG because it's sort of hidden by

[12:20] that big depolarization of the ventricles. Both of those things are happening during that QRS complex. Up next we have something called the ST interval. The ST interval is going to start with S and last all the way to the end of the T wave. A subset of that is the ST segment which would just be this section in right there lasting until the beginning.

[12:40] of the T wave. During the ST interval we're going to have the ventricles contracting. That's going to cause blood to be pumped through the aorta as well as blood to be pumped through the pulmonary artery. That very forceful contraction is going to be starting kind of at the end of the QRS complex and lasting until the T wave happens. The T wave is when we're going to be repolarizing the ventricles and stopping the contraction.

[13:00] action, but those ventricles would be contracting during the ST interval. Now when the ventricles contract, that's when we're going to hear our first heart sound, the lub of the lub-dub. We represent that with S1 for the first sound of the heart, and that sound is caused by the tricuspid and mitral valves snapping shut right before the

[13:20] ventricles will contract and pump the blood out. We need those valves to close, of course, because we don't want the blood to rush back into the atria. We want all that blood to be forced out through the aorta and pulmonary artery. So again, that first heart sound is going to kind of happen right around the end of that QRS complex as the valves are snapping shut. Finally, the last section of this is the T wave and the T wave is going to be the re-wave.

[13:40] polarization of the ventricles, or in other words sort of the turning off of ventricular contraction. After the ventricles are finished contracting, we're going to have the second heart sound, the dub of LUB dub. Just like the first heart sound, the second heart sound is going to be caused by valve snapping shut, but in this case that's going to be the pulmonary valve snapping shut as well as the aortic valve snapping shut.

[14:00] So the ventricles are relaxing and we don't want the blood that's been pumped out of the ventricles to pass back into them through the aorta or pulmonary artery. So we snap those valves shut to keep the blood out of the ventricles there. And that second heart sound is going to be happening kind of right at the end of the T wave, somewhere right in there. Alright so a lot going on in that process. Let's do a quick recap. We have the sinoatrial node

[14:20] where the signal will start. We have the interatrial pathway, the signal will travel there to depolarize the left atrium. We have the internodal pathway, the signal will travel through the interodal pathways to depolarize the right atrium. The signal will travel to the AV node where it is slowed down or delayed so that the atrioc and finish contracting before the ventricles get depolarized and contract.

[14:40] We have the bundle of hyps, which is going to separate into the left and right bundle branches. The bundle on the branches are going to transmit the signal very quickly because we want the ventricles to contract as one contractile unit as quickly as possible. The signal passes down the septum and then on the way back up it's going to pass through perkinje fibers, which are going to distribute the signal throughout the heart muscle to help the ventricle.

[15:00] contract all at once from the apex up. All of this electrical conduction is going to cause the ECG, the electrocardiogram. The ECG will start with the P wave. This is where the atria are depolarizing. Up next we have the PR interval. This is where the atria are contracting and it's going to be pushing blood from the right atrium to the right ventricle and from the left

[15:20] atrium to the left ventricle. Next is the QRS complex. This is going to be where the ventricles are depolarizing and it's going to be where the atria are repolarizing or sort of turning off. Once the ventricles are depolarized we move into the ST interval and this is where the ventricles are going to be contracting, causing blood to pass up through the aorta and pumping blood out through the pulmonary artery.

[15:40] artery as well. And at the beginning of that ST interval is where we have the first heart sound which is caused by the tricuspid and mitral valves snapping shut. Up next we have the T wave. The T wave is going to be where the ventricles are repolarizing or turning off or stopping their contraction. And as those ventricles relax we're gonna have the second heart sound which is the dub of LUB dub and that's caused

[16:00] by the aortic and the pulmonary semilunar valves snapping shut. Now let's take a look at some video from Anatomage so we can see all of this pumping and contracting and stuff happening in action. So we have the signal starting in the SA node and we're gonna see the depolarization of the atria. The atria are going to be contracting and it's hard to see that contracting of the atria. It's going to be much less forceful.

[16:20] than the contracting of the ventricles later on. The signal is passing through the atria into the AV node where we have that AV nodal delay and then the signal is passing through the bundle branches and the braching fibers which is going to cause that QRS complex and then we have the ventricles contracting during the ST interval and finally the ventricle is relaxing until we have another

[16:40] signal from the SA node and we get a new P wave and this process starts all over again. And now let's watch that process happening in real time. It's just a cool process. Imagine this is happening in your heart like every time it beats multiple times per second. It's just wild. Now the only way to really learn this stuff is to practice yourself. So here's the diagram that you can use.

[17:00] the video, test yourself, see if you can label all the parts of the cardiac conduction system, as well as explain what's happening through the different parts of the electrocardiogram. And here's all that information back so you can check and see how you did. Thanks again to Inadimage for sponsoring this video. They make these amazing virtual dissection tables. They have a science table. They also have Inadimage lessons. Lots of awesome stuff. Go check the

[17:20] out in the website link below. And special thanks to my supporters on Patreon. Link in the description if you're interested in joining. All my supporters on Patreon get access to the diagrams both labeled and unlabeled from all my videos, including this one. Thanks for learning about the heart in this video. I've got more videos on the heart and cardiovascular system and other parts of the body, so check those out on the channel if you're interested. And may your sinoatrial nose

[17:40] continue sending signals for years and years to come, and I'll see you in the next video.