 Hey everybody, Dr. O. In this important video, we're going to cover the conducting system of the heart, the actual electrical system that allows the heart muscle to pump. I'm going to show you the six steps here on this image and then I'll actually show you where these things are in the heart. So step one is pretty straightforward, nothing's happening. You can see there are the EKG on the left, there's no activity yet. So the SA node is resting, this is when the heart is taking a break. So step two, now the SA node, which is called the pacemaker of your heart, it's in the back wall of the right atrium, is going to initiate the action potentials that's going to sweep across the entire atria. So it's going to depolarize the entire atria. So the SA node is sending a signal to what's called the AV node. But as it does, so imagine like throwing a big rock and a pond, it's going to stimulate all the cells in the atria and that's that purple coloring that you can see there. So this would correlate with the P wave on the EKG. So that's step two, the SA node has done its job. It's gotten the signal to the AV node. The cardiac skeleton is what insulates the rest of the heart and makes sure that the stimulus only goes there. It only goes from the SA node to the AV node. So now we're at step three and this is a very important one. There's a delay of 100 milliseconds. So it goes SA node to AV node, atrioventricular node, but then there's a delay of 100 milliseconds. The reason that delay is so important is because that's what allows the atria to have time to contract before the ventricles get depolarized. So it's very, very important. All right, so then we have number four. Now the signal after this 100 millisecond delay, the signal is now going from the AV node to what's called the atrioventricular bundle, which when I was in school was called the bundle of his. You might still hear it called that. And then it's going to go into your left and right bundle branches all the way down to the apex of the heart at a word called the Purkinje fibers and they're going to travel back up. The other area that's being stimulated there, I'll circle it here for you, is called the moderator band. The moderator band is a little string of tissue that's going to actually connect to the papillary muscles. Hopefully remember them. The papillary muscles are the nipple-like muscles inside the heart. They're going to tense up right before the rest of the ventricle contracts to make sure that the AV valves are good and stiff. So that's called the moderator band. So that's step four. Step five, so now the ventricles have been depolarized. That was the QRS complex on your EKG there. Step five, this impulse is going to travel all the way through the ventricles. And then step six, the ventricles are going to contract. Now an important thing here to note is that the reason that the stimulus goes all the way to the apex of the heart and then turns around, that means that the heart muscles are going to depolarize and contract from the bottom up. So imagine ringing out a tube of toothpaste. You're going to squeeze it from the bottom and squeeze the whole heart up and ring it out. Ring every drop of blood that you can out of the heart. All right. That's the conducting system. I'm going to show you the same pathway here, but we're actually in the heart. So we start again with our SA node, our synoatrial node, our pacemaker. And it's going to fire off that signal to the AV node. So SA node to AV node. While it does that, it's going to depolarize all the atrial muscle cells and then your two atria are going to contract. And their job is to help fill the ventricles. SA node to AV node. It's going to sit there for 100 milliseconds, buying the two atria time to contract. Then the AV node is going to fire off that signal to the AV bundle, also known as the bundle of his. And then to your two bundle branches, the left and right bundle branches, then to the brachinji fibers, which you see there, then that signal is going to turn around and it's going to depolarize the entire ventricle from the bottom up. Then when the ventricles are going to contract, it's going to ring out the heart from the bottom like we already mentioned. One last thing to note here, usually not a big deal unless someone is on drugs or having serious problems. The AV node can only send a signal about 220 beats per minute, which that's, the AV node is what's going to set the maximal heart rate. So again, unless for extreme, except for extreme circumstances, your heart should not be able to beat faster than the AV node can allow, which is, which is 220 beats per minute. Okay. I think that's going to be it. Just remember that that's why those brachinji fibers worked their way back up from the bottom. So you ring the heart out from the bottom up and it's much more fist, you know, that's how you brush your teeth. Try squeezing the tube of toothpaste in the middle. You'll see it doesn't work very well. Squeeze it from the bottom and it's going to be a much more efficient pump. Okay. So that's the conducting system of the heart. I hope this helps. Have a wonderful day. Be blessed.