 There are three concepts that we need to be comfortable with in order to talk about the mechanics of blood, the physics of blood moving through your body. And these concepts are pressure, flow, this is the movement of the fluid, and resistance. And they actually are like totally related to each other. So flow, the flow of blood, or the flow of any fluid type substance, including gas, which is going to be relevant because we're talking about the respiratory system next. Flow is movement. And I'm going to just say fluid. And we have to put this into context because these concepts of pressure and resistance are related to flow. Flow is a movement of fluid due to pressure gradients. Pressure gradients. If this is the central concept that we're looking at, the flow, like how does blood flow through your body? How does it move? It moves because of different pressures. Different pressures that, oh, say, have been during the ventricular systole as compared to ventricular diastole. Hopefully this concept is like, oh, yeah, of course. But it's probably prudent to give ourselves a definition of pressure. And again, this is going to be relevant in the respiratory system as well. And dude, we're pretty much in pressure land now from here on out because there's all kinds of pressures that we're going to talk about. Pressure, our physics definition of pressure is force divided by area or surface area. And there's actually a YouTube lecture of me trying to explain this concept of pressure using, of course, an analogy. And in it, I couldn't remember the words for the parts of the analogy that were relevant. I know the words now. So I wanted to compare for my analogy so that you could visualize the concept of pressure and how force and area come into play. I used an example of stepping on your foot. I mean, on your hand. You put your hand on the floor and I'm going to step on it. And I'm either wearing stiletto heels or diving flippers. And you decide, dude, which one? Okay, first of all, stiletto heels or diving flippers, which one has the biggest area? Okay, let's just do this. Big area are diving flippers, right? And small area are stilettos. I want you to know that it would not matter which ones I was wearing. If I tried to walk in either of them, I would walk exactly the same and it would be equally as dangerous. You know how it is, walking in flippers. That's a disaster waiting to happen. Walking in stilettos for me would be a disaster waiting to happen. The force that I would apply to your hand on the ground when I step on it in either of these is going to be the same. The force is basically my body weight. So we'll just say 1,000 pounds, right? Sure, because I'm that buff and strong. So I'm going to apply 1,000 pounds of force to your hand in high surface area flipper or a low surface area stiletto heel. You tell me which one do you want to have done to you. Let's just hope that you get that this is going to be less pressure because it's a bigger number. If we said the area of the flipper was 100 and the area of the stiletto heels was 10, okay, that works for you, yes. We'd have 100 millimeters of mercury of pressure. I'm making all this stuff up versus, I mean, excuse me, this would be 10 and this would be 100 millimeters of mercury of pressure on your hand. Dude, take the flippers. You'd much rather have me stand my 1,000 pounds of pressure of force upon you with my flippers and not my stiletto heels. And I personally would rather wear flippers than stiletto heels. Okay, so you've got the analogy of what pressure is. We're going to talk about pressure in lots of different scenarios. So make sure you're like, yeah, I have a conceptual idea of what pressure actually is. Now, if pressure is encouraging flow and think about pressure gradients, we're going to go from high pressure to low pressure if we were going to move any fluid. It's going to wind. Wind is air fluid moving from high pressure zones to low pressure zones. That's all wind is. Resistance is anything that, I don't know, the only word I can think of to explain it is it resists flow most of the time. It's friction. If there was something like, do you agree that if I had a tube that was lined with shag carpet, because everybody's got those, and you tried to send some fluid through that tube, there's going to be a lot of friction in there because of all the shag carpet fringes versus a really smooth, same diameter, but a super smooth plastic tube that's going to have less resistance. Does that work for you? We're going to talk about the concept of resistance because diameter changes resistance. Does that work for you? Like, if the tube has a really wide diameter, there's less resistance than if the tube has a really small diameter. And again, all of these things are going to affect the flow of the fluids, the amount of pressure pushing and the amount of resistance is going to affect fluid flow. Did you follow that? Now let's look at blood pressure, not blood pressure homeostasis.