 Okay, here we are. We've generated an action potential that's sending the message to the ventricle, muscle, contractile cells. Dude, do your job, contract. They begin to contract. The volume is decreasing as the pressure is increasing. Eventually, the pressure overcomes the semilunar valves, and the blood pours out either into the aortic arch or into, there's my aortic arch, or into the pulmonary trunk. Either way, the blood is now being pushed out of the heart by the cardiac muscle tissue. There are some really interesting things about the way cardiac muscle tissue is organized inside the heart. It's actually kind of spiraled, so that it's almost like ringing out the heart. The contraction is such that it rings out the heart and pushes all that blood up and out those arteries. This process of squeezing out the blood, you don't squeeze it all out. You actually leave some in there, and that is what do you think? End systolic volume. So the end systolic volume is a small number that is essentially the amount of blood that's left after ventricular contraction has completed. This is still ventricular contraction. We still are summing up this electrical activity in the form of the QRS complex. Look at the size of that thing. Seriously, it's huge compared to the P wave and the T wave, and that's because the ventricles are huge. If you've taken anatomy, you know that if you grab yourself a cow heart or a sheep heart, pretty much most of it is ventricle. And it makes sense because the ventricles, this moment, ventricular ejection, is the moment where you're pumping everything. That's the push that sends the whole blood stream, like all your blood, all whatever, 10 liters, pints, 10 pints, not liters, 10 pints of blood through your system. Like it's all because of this moment of ventricles contracting, and it's kind of wild if you think about it. This is a strong muscle contracting 100,000 times every single day. What? That's crazy talk. Okay, we squeezed out all of our blood. We've completed our contraction. Look where we're headed next. Go ahead and try to speculate on what the next step is.