 There are two main characteristics that we want to talk about when thinking about the movement of fluid or blood in this case or really anything that acts fluid like even air. And those two characteristics are flow. So we know that fluids can flow. And then we also intuitively have a sense of resistance, which is basically friction that opposes flow. And here's the fact, fluids, all fluids, flow down pressure gradients. We go down pressure gradients. So blood is going to move always from a high-pressure zone to a low-pressure zone. This is, thankfully, incredibly intuitive. This makes sense to us. If you imagine me squeezing a balloon, the high-pressure zone where I'm applying the squeezing force is the air is going to move out of that zone. It's going to move out of the high-pressure zone if it can. It's going to move and pop the balloon out in the area where there's less pressure because my hands aren't squishing it. Wind is nothing more than atmosphere moving from areas of high-pressure to areas of low-pressure. Resistance, on the other hand, resistance is opposition to flow. So any force that opposes the flow is considered resistance. In this situation, almost 100% of the time, we're talking about friction. Now think about that for a second. My little analogy, what? I have an analogy. I know this is shocking to you. My analogy, shocker, is imagining my children playing on a slide that is made, that is lined with shag carpet. Seriously? Even if it's like a super steep slide. Go sit on the slide made of shag carpet. Go climb inside that slide and sit down. Shag carpet is like the super long, awesome, like make it like pea soup green or something like that and throw it in your house. Fantastic. I actually would like to have a room that was full of shag carpet even on the walls because then I wouldn't stress about my children beating the holy living tar out of each other pea soup green shag carpet room. Because if they like slid down the shag carpet room or the shag carpet slide, there's so much friction that they're going to go really slow. If you take the shag carpet out or we race, we've got the shag carpet slide and we've got the regular slide. There is still friction in the regular slide. There still is resistance to downward movement in the regular slide. There's resistance to flow, but the resistance is much less in a regular slide and you're going to go faster. There's less friction opposing your movement down the hill. In the blood vessels, more resistance could be atherosclerotic plaques. If you have thickening hardening arteries and you're decreasing the diameter of your blood vessel because you've got this crap that's filling your blood vessel, lumen, that's going to create more resistance to flow. And just think for a second about what could that possibly do to the pressure. If you increase resistance to the flow, that automatically increases the pressure. Knowing that flow always moves, flow is created by pressure gradients. If we didn't have pressure gradients, if you didn't have a difference in pressure in your blood vessels, your blood would not move anymore. What creates the pressure gradient in your heart? I mean, I just told you the freaking answer. Dang it. What creates pressure gradients in your blood? Oh, you guys are so smart. Your heart creates the pressure gradients. I knew you guys were paying attention out there. Good job. So your heart, because it's this awesome, beating pump that's creating a high pressure zone, holy cow, go review your Wiggers diagram and see what happens during ventricular contraction. See what happens to the pressure inside the ventricle. You create the super high pressure zone. The fluid, the blood is like, dude, get me out of here. I got to find a low pressure zone to head to that creates flow. Your vessels can create resistance, but the flow is generated by your heart. So let's take a second to rewind and figure out some characteristics of how your heart creates flow and how your heart, the mechanism of heart muscle contraction is a little bit different than skeletal muscle contraction, and that allows for some interesting regulation that can happen from your actual little heart. Pat your heart, everybody. Aw, I love your heart. Okay, I'll be right back and we'll talk about heart contraction.