 At the end of atrial and ventricular diastole relaxation, we had the firing of the SA node. The SA node passed the action potential all through the myocardium of the atria, and that is received, that information, that action potential is received by the contractile cells, and now they go. So we have atrial contraction or atrial systole. Notice what it looks like. We have little red arrows here indicating that we're decreasing the volume of the atria. What's going to happen? We are no longer passively filling our ventricles. Blood is now forced into the ventricles, and this is going to do two things. It's going to increase the pressure, and it's going to increase the volume. Remember that when we were passively filling our ventricles, we were increasing the volume, but we had no change in the pressure, because it was just a passive process. The ventricles were relaxing to accommodate the increase of volume that was coming in, so there was no change in the pressure. 80% of ventricular filling happens this way, passively. So most of the blood gets moved into the ventricle in that passive period. However, the last 20% is smashed into the ventricles when the atria contract. Now, you can imagine that, yeah, that's totally going to increase the pressure. The contraction is electrical, so keep in mind that we have action potentials running through contractile cells, and that's where we get this P wave. P wave is associated with ventricular contraction, I mean atrial contraction. Um, yeah, yeah, yeah. Now, we have to accommodate, or we have to address the fact that, yeah, we fired, my color choice is just terrible. We fired the SA node, and the message goes where next. You know it's true. The message is going to go to the AV node. Now, AV node is somehow we have to get the message of the order to contract to the ventricles if wouldn't you expect that ventricular contraction is going to happen next. And remember the anatomy, we have to go through the AV bundle, which remember this isn't in the chamber, it's actually in the interventricular septum, it's going to get into my bundle branches and down into the Purkinje fibers. And while the atria contract, the AV node hangs on to the message. That's why the AV node slows it down. Because imagine that if the atria are contracting and the ventricles are contracting at the same time, can you imagine how basically there's going to be like a push pull, like who's going to win? And if you're voting on the ventricles, they're a little bigger and tougher than the atria, if you know what I'm saying. So, it's not a great strategy. Let's go ahead and have the atria contract first so they can squeeze all that blood into the ventricles, hold on to the message, wait for it ventricles, wait for it, and then we'll send the message. Now, the message is really fast. Again, anatomically, all the action potential is held in a certain space. So, the action potential isn't allowed to spread right away. It isn't allowed to spread until it gets to the bottom of the ventricle. This right here is ventricular, the message is passing down into the ventricles, the action potential, and we're getting ready to depolarize the ventricles. We're getting ready for this stage of the EKG. We're going to sum it all up with the QRS complex. But we're not quite there yet, we're just spreading the action potentials and we're getting ready. So, what is coming up next? Go ahead and guess.