 All right, there's a couple of fellas that we must meet. Meet Erno Starling. Erno had a buddy named Otto. Meet Otto Frank. Erno and Otto got together and came up with the Frank Starling law of the heart. It would have been cooler if it was the Erno Otto law of the heart. And in fact, I've decided that I think I need to purchase some ducks. I need two ducks because they need to be named Erno and Otto. Because how cool is that? My duck friend Erno and his brother Otto. And then I can go out and call them. Erno, Otto, come get some snacks, boys. Erno and Otto gave us this fantastic law of heart contraction that I'm going to tell you right now. You're going to be like, dude, seriously, why did we have a whole law? Why did we have to learn two whole new people just to talk about something that, of course, we already knew this? Frank Starling, Erno, Otto, law is comparing, essentially saying the amount of stretch in cardiac muscle increases the amount of tension generated by the contraction. Okay, follow that again. The amount of stretch experienced by cardiac muscle cells, stretch, is going to stretch apart our sarcomeres. It's going to put our actin and myosin myofilaments in optimal position, in a better position. The more stretched the cardiac muscle is, the greater the tension, the greater the force of contraction that you're going to generate. That really, I mean, up to a limit, there is a limit where we could stretch the cardiac muscle so far out that we can't generate a proper contraction anymore because we aren't getting any overlap at all or little overlap. But your heart really isn't set up to go there. And I want you to think about this for a second. Let's jot it down. That increased stretch equals increased force of contraction. That says force of contraction. Now, think about your heart. How could you increase the stretch of your heart? Well, I'm going to tell you right now, all you have to do to increase the stretch of your heart is you could fill it fuller, more full. Fill it up more. Fill it, like, push more blood in. And now it's going to stretch your heart. Do you totally agree with that? Now, what concept think Wiggers? What concept have we done and learned that would allow us to quantify a fuller heart? Push pause and go look at Wiggers and see if you can figure it out. Where on Wiggers could we see an actual visual of a heart that was filled more full? I'm about to tell you that. Stretch can be quantified by end. Let's write stretch first. And it can be quantified by end diastolic volume. Remember end diastolic volume? It's the volume of the ventricle, the volume of blood in the ventricle at the end of diastole. Right before systole, right before contraction happens, the more blood you push in, the greater the end diastolic volume. So end diastolic volume being really big, like let's say, I don't know, 400 versus being very small, let's say zero, or I don't know, I think my number is 135 kind of in the middle. Not really, oops, not really in the middle, but somewhere like normal, like relaxed chilling heart is going to have an end diastolic volume of 135 milliliters. But if you push in more volume, and how could we do that? There's all sorts of ways that we could do that. Here's what they said. You're going to have more force. Now, what is some way that you could quantify the amount of force that you're generating? Think about this and see if this works for your brain. Could we quantify force or like, not quantify the actual concept of force, but like have something that we could make. Okay, this is how we're going to measure force. How about stroke volume? Think about that. The amount of blood that you push out, can you agree that if I have a small contraction, a small force, I'm going to have a low stroke volume. I'm going to push out very much blood. But if I like push a huge force of my ventricle contraction, then I'm going to have a bigger stroke volume. So force is estimated by stroke volume. Or it's, I don't know, what's that word? Like it's represented by stroke volume. Okay, so if our stroke volume at like, a regular is like 70 milliliters, here's what we see. At a regular stretch, you know, a stretch of 135 mils, that's how much we filled, we stroke volume, we contract up to 70 mils out. And what we see is that we get this little curve. Thank you, Erno, and I forgot my other guy's name. We end up with this curve where the more we stretch, the bigger the end diastolic volume, the greater the stroke volume that we actually push out. Now, I know that kind of makes sense, but the reason why that happens is because the stretch itself literally generates a bigger contraction in the muscle. The muscle fiber, like the myofilaments, have a stronger pull when they're stretched apart more. And we definitely hit a peak, we definitely hit a physiological barrier to this law, but in general, this is something that happens. If you have a substance that increases the strength of the contraction, if you can increase the force of the contraction, this substance is called inotropic. Inotropic substances increase the force of contraction. And we can see that one of the ways to increase the force of contraction is to increase the end diastolic volume. If you have a greater end diastolic volume, that will increase the force. Now, how do you increase end diastolic volume? Somehow you have to get more blood in your heart. I'm not going to even go here, but think about this. If you have greater venous return, increased venous, now think about that word, return, or think about that concept. Venous return is a way to think about the amount of blood that's coming back into the heart from the veins. If you have increased venous return, then you're going to have possibly increased end diastolic volume, which will lead to an increased stroke volume. Ultimately, how is an increased stroke volume going to affect blood pressure? It makes perfect sense that you have an increased force represented by stroke volume, and that's going to increase your blood pressure. All right, now let's talk about the concepts of preload and afterload, because those are also related to this idea of how is our heart actually functioning to change the force of contraction, knowing that the force of contraction is what is creating the blood pressure in the first place.