hey everyone it's sarah with registerednessrn.com and in this video i'm going to be reviewing atria and ventricle repolarization and depolarization so let's get started we need the electrical conduction system to send off these electrical signals that target the cells of the atria and the ventricles so they will contract and relax so in order for the electrical conduction system to do this it has to cause repolarization and depolarization of these specialized heart cells so these are two terms that you really want to make sure that you understand whenever you're studying ekg interpretation because they really lay the foundation for understanding this pqrst complex so here i have a heart cell it's resting meaning it's not really doing anything it's just hanging out until it's told to do something so whenever you have a resting heart cell it is negatively charged on the inside and we can refer to this as it's being polarized and this cell the reason that it's like that is because there's a different concentration of ions on the inside compared to the outside and this cell membrane around the cell is really not permeable right now it's impermeable specifically to sodium there's a lot of sodium outside the cell and it would love to get on the inside so whenever this cell receives a jolt a jolt of electricity from this electrical conduction system it really changes that cell membrane and causes it to be more permeable so that sodium's like hey i can get inside that cell now so the sodium goes inside the cell and it changes its state from being negative to less negative so a little bit more positive because sodium is positive and it causes depolarization and whenever the cell becomes depolarized it causes it to contract so the next step after contraction is relaxation so after we have contraction it relaxes we get repolarization it's going back to its negative state so what you want to take away from this is that depolarization leads to contraction so whenever i'm saying atrial depolarization i'm talking about atrial contraction the contraction of the atria whenever i'm talking about repolarization i'm talking about relaxation and to help you remember that to keep those two straight remember that the word relax starts with re repolarization starts with re so whenever i refer to ventricle repolarization i'm talking about the relaxation of the ventricles so now let's see the heart in action how it actually contracts and relax whenever it is stimulated by this electrical conduction system and creating this pqrst complex by looking at this animation first the sa node fires and this causes atrial depolarization which causes the atria to contract then the signal goes down to the av node and notice the av node does not fire until those atria are empty and the blood is in the ventricle then it's time for ventricular depolarization so the signal travels down through the bundle of his to the bundle branches and then the purkinje fibers causing ventricular contraction followed by the relaxation of the ventricles so now let's recap all that information and look at this pqrst complex and see how depolarization and repolarization is occurring in each part of that complex so here section one is the p wave that represents atrial depolarization beginning so we have the contraction of the atria then two that was our pr segment that was atrial depolarization complete so it's done then three was our qrs complex and it represented ventricular depolarization beginning and in there but you can't really see it is atrial repolarization it's relaxing then four is our st segment and that is ventricular depolarization completing then we have five which represented our t wave and that is ventricular repolarization beginning and then six is that flat isoelectric line and this is ventricular repolarization complete okay so that wraps up this video and if you'd like to watch more videos on ecg interpretation you can access the link in the youtube description below