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 CYO atrial node along with a complex cardiac conduction system that runs through your heart coordinating your heartbeat in this video we're going to 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 cavers and other fac images provided by anatomage the creator of the world's first virtual dissection table so you can see all these structures are arranged three-dimensionally in the body and by the end of this video you're going to know this whole 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 blood the left side in 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 venne cavis 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 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 mitol valve into the left ventricle and then the 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 and 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 ventrical 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 a whole lesson on all that check out my 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 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 and pumps blood but it also conducts signals as well the card conduction system which is just about 1% of the total kind of cells in here that's going to conduct the signals very quickly whereas the cardiac muscle it will take a little bit longer for the signals to pass 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 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 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 and into two sections that we call the atrial sensium as well as the ventricular sensium a sensium is just a big sort of hardto pronounce word that means a group of cells that are all electrically connected to each other so all of the cardiac muscle in this atrial sensium 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 cuz they're all electrically connected same thing in the ventricular sens 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 can contract as one unit but again the atrial cisum will depolarize first Contracting the Atria and then the ventricular sensium will depolarize Contracting the ventri Les now let's take a look at the individual parts of the cardiac conduction system first here we have the Sino atrial node now the Sino atrial node is autorhythmic meaning that it's going to be sending pulses by itself even without input from some other source autorhythmic meaning self- rhythmic in other words that saay note 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 saay node or to slow down that 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 and it prevents pottassium 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 reaches a threshold membrane potential the SA node will send an action potential now the signals will eventually get to another node called the atrio ventricular or AV node more on that in just a moment but next let's talk about the internodal pathway this is going to be how the signal transmits from the SA node to the AV node and if you look at that internodal pathway there's three branches of it and they're all passing through the right atrium now on my diagram they look sort of planer or flat with each other but if we look on the anatomage images we'll see that these are actually three-dimensional um running through that right atrium which we can see right there so the SA node depolarizes sends a signal through the internal 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 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 deol ize it most of the diagrams 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 off of one of the internodal 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 going to travel through the interatrial pathway as well as the internodal pathway depolarizing both Atria so that they can contract that signal then is going to make it to the AV node now we've talked about the cardiac conduction 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 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 hiss also known as the atrio ventricular 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 His and the bundle branches are going to transmit that 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 and the aorta this way the tricuspid and mitro valves will also snap shut during this time to prevent the blood from back flowing into the Atria now extending out of the left and right bundle branches we have something called the Peri fibers and the pingi fibers are going to take that signal traveling through the bundle branches and spread it out throughout the muscle of the right and left ventricles that's going to conduct that signal many many times faster than if we were only relying on the ventricular Sensi or the connections between all of the cardiac cells so quick recap of all 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 internodal pathways 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 hiss and the left and right bundle branches on the way back up they'll pass through the pingi fibers that's going to stimulate the cardiac muscle and the ventricles to contract and pump the blood out 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 if it stops you hear it go beep because 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 QRS 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 Q complex in the t-wave you can see that happening one more time there I just really like that animation so what we're going to do is we're going to connect this to the cardiac conduction pathway looking at the p-wave QRS complex and the t-wave we're going to 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 the PR interval is going to start at the beginning of the p wve 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 qwave 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 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 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 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 Contracting of the Atria will really start early on in the p wve 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 noal delay happening because as soon as the QRS complex hits then we're going to be depolarizing 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 His and the bundle branches and then through the bingi fibers stimulating all of this cardiac muscle that electrical activity is going to be much greater in the ventricles because the ventricles have more cardiac tissue they also have to pump the blood a lot farther the Atria just had to pump the blood from one chamber to another the ventricles have to pump the blood to the lungs and then also Al 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 bundle branches and through theingi 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 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 QRS 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 that big depolarization of the ventricles but both of those things are happening during that QRS complex up next we have something called the St interval that 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 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 but those ventricles will be contracting during that St interval now when the ventricles contract that's where we're going to hear our first heart sound the love of the ldub ldub we represent that with S1 for the first sound of the heart and that sound is caused by the tricuspid and mitol valves snapping shut right before the 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 pull AR 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 repolarization 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 ldb 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 valve snapping shut 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 all right so a lot going on in that process let's do a quick recap we have the soo atrial node where the signal will start we have the interatrial pathway the signal will travel there to depolarize the left atrium we have the internal pathway the signal will travel through the noal Pathways to depolarize the right atrium the signal will travel to the AV node where it is slowed down or delayed so the Atria can finish Contracting before the ventricles get depolarized and contract we have the bundle of His which is going to separate into the left and right bundle branches the bundle and the branches are going to transmit the signal very quickly because we want the ventricles to contract as one contract all unit as quickly as possible signal passes down the septum and then on the way back up it's going to pass through pereni fibers which are going to distribute the signal throughout the heart muscle to help the ventricles 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 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 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 mitro 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 going to have the second heart sound which is the dub of love dub and that's caused by the aortic and the pulmonary semilunar valves snapping shut now let's take a look at some video from anatomize 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 going to 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 than the Contracting of the ventricles later on the signals passing through the Atria into the AV node where we have that AV noal delay and then the signals passing through the bundle branches and the pingi 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 signal from the SA node and we get a new p wve 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 pause 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 electroc cardiogram and here's all that information back so you can check and see how you did thanks again to anatomage for sponsoring this video they make these amazing virtual dissection tables they have a science table they also have anatomage lessons lots of awesome stuff go check those 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 uh check those out on the channel if you're interested and may your Sino atrial node continue sending signals for years and years to come and I'll see you in the next video