If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:9:43

Video transcript

here's a diagram of the four chambers of the heart so let's let's name them to get started so we've got the right atrium up here we've got the right ventricle down here we've got the left atrium and the left ventricle so these are the four chambers and blood is going to flow through all of them and then get out to the body so to do this and to do this right the heart has got to coordinate how it squeezes and and we know that the way that it kind of squeezes down is you have a cell and that cell is usually negatively charged and it will at some point become more positively charged and we call that process depolarization right so depolarization is the idea of going from a negative membrane potential to something much more positive and when you depolarize is when the muscle cell can squeeze down so where does that begin let's actually draw it into our our diagram so if you were to look there's actually an area here where little cells can actually depolarize by themselves and that's actually quite unique because most of the cells in the body are going to depolarize when the neighbor when that neighboring cell depolarizes so these are actually really unique cells because they're depolarizing all by themselves and we call that area the sinoatrial node sinoatrial node sometimes called the SA node sinoatrial node and the fact that they can actually depolarize by themselves we we have a word for that too we call it automaticity automaticity so it just means that they can kind of automatically depolarize without having a neighbor do it first so once they depolarize what happens after that well when when these cells depolarize they immediately they're connected through little gap junctions to the neighboring muscle cells and so they're going to start sending out waves of depolarization in all directions and so it's almost like going to a football game and watching the wave start you know it just goes on and on and on and so all the neighboring cells are going to start depolarizing as well and they're going to that orange arrow is moving kind of slowly right that depolarization of wave is moving kind of slowly relative to how fast it could be moving if it went through a specialized band of tissue so this band of tissue that I'm drawing this blue band is almost like a highway compared to that orange arrow which is like a little road and that highway is going to take that same depolarization wave over to the other side over to the left atrium and all these cells begin to do the same thing they start depolarizing as well so yet depolarization happening both in the right atrium and the left atrium in a coordinated way so it's happening all very very evenly and this this band or bundle is called Bachmann's bundle so it's like a little bundle of tissue right so it's called Bachmann's bundle so now we've named two things the sinoatrial node and Bachmann's bundle and actually just like Bachmann's bundle they're actually a few more little bands of tissue ohms like little highways that take that signal down to another node called the atrial ventricular node so this right here is the atrioventricular node and the atrioventricular node is really the only major connection I shouldn't say only major only connection in most of us between the atria and the ventricles atrial ventricular node and this is actually sometimes called the AV node so the AV node is going to get the signal and actually I didn't even tell you what a signal came through it came through this is kind of a generic name internodal just meaning between two nodes tracts and that's that's kind of the name for all three of them so the signal went from the SA node through the intra nodal tracks down to the AV node and they're kind of an interesting thing happens so if you actually take a step back and look at the AV node let's imagine we're now kind of focused in on exactly what's happening there and to figure out what's happening there I'm going to give you a little scenario so let's say that you've got I don't know let's say a little timeline here and that timeline is let's say one two three seconds three seconds and your job is just to watch the atria and see how they contract so you just watch the atria and you say wow you know I saw one contraction that happened right there and one contraction that happened right there and one contraction that happened right there so the atria as they get the wave of depolarization are contracting now three times in three seconds so for the atria you saw three contractions now you do the exact same thing but you do it for the ventricles so for the ventricles you kind of just keep an eye and you watch you know exactly what happens and you notice that there's a contraction of the ventricles there and again there and one more there so both the atria and the ventricles are both contracting the same number of times but the unique thing is that there's like this little delay between the two right they're not actually contracting at the same moment in time there's this tiny delay and if you measured it it would be about 0.1 seconds so just a tiny little fraction of a second but the reason that there's that delay is due to the AV node so one of the kind of interesting things about the AV node is that it creates a delay a delay between the atria and the ventricles so delay between atria and ventricles and the reason that's really important is that if the atria and the ventricles were actually contracting simultaneously then they would actually be squeezing blood against each other they would be actually doing work that wouldn't actually move the blood in the right direction so by creating the delay the atria can squeeze the blood can move from the atria to the ventricles and then a tenth of a second later the ventricles can squeeze and then the ventricles can move that blood onwards so the reason for the delay is actually to make sure the blood moves in a coordinated way through the heart so now this signal has delayed by a tenth of a second but then it continues on all right continues on and it goes to a little area right there and this is called the bundle of hiss bundle kind of funny names I know bundle of hiss and even those you know spelled H is you you would you don't say his it's hiss almost like what a snake does and then it continues from the bundle of his through one track down here and this is considered the right bundle and then it goes through the left bundle and actually the left bundle splits there's like a forward part that goes up to the front and a part that goes to the back and I'm going to draw the back part kind of dashed like that so this is called the the left posterior because posterior means back posterior fascicle fascicle and this is called the left anterior because it's coming forward anterior fascicle and you got to kind of imagine that it's going forward and back because obviously in two dimensions it's hard to show that and then this is just called the right bundle and actually just so you're not ever mistaken this part right here is called the left bundle where it's still combined and it's not broken into the two fascicles so you have the left in the right bundle and then the left bundle splits again and then all of the all the fibers get really kind of split up here at the end and these are called the Purkinje fibers and it happens on both sides Purkinje fibers and from this point basically the electrical signal can kind of dash out in all directions right so you can finally get all the muscle cells involved so up until now it's been part of the electrical conduction system meaning these are all the highways but now you have all of the waves of depolarization going through all the little tiny roads and I'm using the idea of roads and highways just to point out the idea that through the electrical conduction system the signal moves really fast and when you get down into the muscle itself then the signal moves slightly slower but you can see that that's important because that's the only way to get all the muscle cells on the same page so this is how the electrical signal moves from the SA node all the way through the electrical conduction system so that the atria are beating together and then goes into the AV node where there's a little delay and then down into the ventricles where again the ventricles are going to beat together