 the message has been received. We had essentially a, really? We essentially had the depolarization pass all the way through the ventricles, and now the next inevitable step is that we have ventricular contraction. So that is ventricular systole. One of the things that is true at this point, is that the ventricles begin to contract. However, that snaps the AV valves shut as the ventricles start to contract. They push the blood up. The blood's going to want to go backwards. Dude, why not just go into the giant atrium? That'd be way easier. But the AV valves say nice tripal. You've got to stay where you are. The AV valves snap shut. That's the first heart sound. That's the lub that you hear with the heart. The AV valves snapping together. At this point, we are in early systole. We're contracting. We've snapped those closed, but we haven't pushed through the semilunar valves yet. So at this exact moment, we have a steady volume. The volume does not change, even though the pressure is increasing in this space. Eventually, we're going to have the amount of pressure being placed on the blood inside the ventricle is going to become greater than the force that's holding the semilunar valves closed. You can imagine the next step is going to be that the pressure of the blood in the ventricle is going to overcome the pressure that's keeping the semilunar valves closed, and it's going to snap those semilunar valves open, which allows all the blood to leave the ventricle. Now, that's in our next section. I think I stole my own thunder, but there's something in the last section that I wanted to tell you and make sure that you were clear about. Here's this moment where volume is not changing, where we pushed the maximum amount of blood into the ventricles as we possibly could, and we kind of started to contract, but the volume hasn't changed yet. That moment, this maximum blood volume in ventricles is called the end diastolic volume, or EDV. So the end diastolic volume is the maximum amount of blood. It's the biggest volume that you can have in the ventricle at that moment. Now, here's the thing that's crazy. Your end diastolic volume can totally vary. If you're out exercising and your heart is beating really fast, your blood is moving through your body really fast, that actually is going to stretch your heart muscle even more, which ultimately, the more your heart muscle stretches, the more forceful the contraction is in response. So exercise actually leads to more volume being thrown around the body than would otherwise be the case. That's because the end diastolic volume is greater when you exercise. If you're just chilling or sleeping, your end diastolic volume can be smaller. So this isn't a number that's the same every single time, but it is a number that helps us evaluate various scenarios. If we have an end diastolic volume, that makes you think that we might have another kind of volume that we might want to look at. In the next section, let's look at what happens when we do bust through the semi-lunar valves and get out, the blood can get out of the ventricles.