 So let's look, let's remind ourselves of the cardiovascular system as a whole. We've already looked at the heart. This is a very diagrammatic view of the whole thing. And we know that it's just a series of tubes that connect one side of the heart to the other side of the heart and enable a flow of blood through the lungs to pick up new oxygen and get rid of carbon dioxide and a flow of fresh blood to all body parts and then back again to the heart so that we can go back to the lungs and do our exchange. There are five main kinds of vessels that are found in this whole system. So we have to look at the different anatomy of each of the vessels in order to appreciate the function of the capillaries. So and the mechanics of fluid flow throughout this whole tubular, totally tubular system. So look at this chart that I have drawn for you. I've listed out my five kinds of vessels that we're going to talk about. We have arteries, arterioles, capillaries, venules, and veins. And they move the veins flow into the heart and the arteries flow away from the heart, and the arteries, once they move away from the heart, they branch into arterioles, which branch into capillaries, which don't branch but come back into venules, which then come back into veins. And there are two numbers that I can give you right now just for perspective on size and significance, just anatomical perspective. So the diameter, the diameter of an artery can range from, and I'm just going to use black because I'm going to try to stay disciplined, pay attention to my units here, from 0.1 to 10 millimeters. This is the diameter of the tube. In a second, I'm going to draw all these tubes and that'll give you a sense. I mean, 10 millimeters, that's a centimeter. The diameter of an artery, the lumen, the diameter of the lumen of the artery is, can be big. You can stick a finger in that thing. Arterioles, on the other hand, are 10 to 100 micrometers. And I'm just going to throw down here because I had to look it up. 1 millimeter equals 1,000 micrometers. And so you can see that this is like, 100 micrometers is 0.1 meter. So it makes sense that our arteries are going to become the biggest arterioles and then arterioles are going to get smaller as we go. Capillaries, on the other hand, the diameter of a capillary is anywhere between 4 and 10 micrometers. Seriously, that's small, you guys. That's tiny. Venuals go from 10 to 100 micrometers. And again, this is the diameter. And check this out. This will blow your mind. The diameter of a vein goes from 0.1, and again, that's equal to 100 micrometers, 0.1 to 100 millimeters. Dude, that's gigantic. Millimeters. That's crazy talk. That's huge. That's 10 centimeters. What? That seems crazy. I might want to go back and check that thing. Let's talk about the thickness of each of these because that's another number that is going to change with each of them. The thickness of an artery can be up to one millimeter. This is just the wall of the artery. The thickness of an arteriol, now we're back into micrometers, just six micrometers. So there's a huge difference between an artery and an arteriol when you talk about thickness. And we'll look at the structure of them and draw them here and see why that is. A capillary is half a micrometer. And a venuola, just one micrometer. And veins, super thin. These guys are half a millimeter. Okay? Now, so what? Let me tell you what they're made out of. Every single one of them is lined with a structure called endothelium. And this, I'm going to draw it in all of them and I'm drawing it so that I can include all my other structures. I'm trying to show you the lumen and I'm not drawing it to scale. The thickness is going to be, I'll show you my thickness in a second, but the diameter is the diameter right here. When I measure the diameter of a blood vessel, I'm measuring from the edge of endothelium to the edge of endothelium. That's across the lumen. So these are in no way drawn to scale, but I want to fit everything on here. Endothelium is simple, squamous epithelial tissue. And its name endothelium is because it's special and all lining of blood vessels is called endothelium, but it's made out of simple squamous epithelial tissue. And in some of our blood vessels, okay, I'm going to have to go with some kind of an orange here, I think, we have an elastic tissue or an elastic layer. And your arteries actually have a really big elastic layer. The other vessel that has an elastic layer but not as big is a vein. Anybody else doesn't have an elastic layer. Now, at the same time, okay, I got a little bit distracted, but I'm going to show you over here like a little bar graph that gives you an idea of amounts of each of these layers or like how big it is. So in case you can't see my drawing very well, you can still appreciate the fact that everybody has the same amount of endothelium. However, veins have elastic tissue and so do arteries and arteries clearly have much more. So my drawing over here is on purpose. From anatomy, you might remember these two, I'll do it up here. One of the two structures put together was the, I can do it, tunica intima, tunica intima. So the tunica intima included the endothelium plus the elastic layer. And we just looked at the arteries and veins in anatomy. Not everybody, but some people also have smooth muscle. Arteries have a lot of it. And you know who else has a lot of it? Our arterioles. Okay, arterioles don't have as much. Can you tell the difference? I'm going to just draw it in here. Arteries have a lot of smooth muscle surrounding them. Arterioles have some but not as much as arteries. And veins have some. Veins actually have about the same amount of the arterioles but not as much as arteries. Arterioles are where you can do a lot of changing of diameter. Vasoconstriction, vasodilation can happen in arterioles. But you'll notice capillaries and venules do not have any smooth muscle at all. And then all of them, not all of them, never mind. Not all of them are surrounded by a fibrous layer. So let's do this, fibrous layer, which is just a connective tissue capsule that surrounds the whole thing. Those vessels that have the connective tissue capsule, the fibrous layer, have the same amount. So we're going to find this on our arteries and our veins. And they have the same amount of fibrous tissue. Okay, venules. I needed something else in there. I need some fibrous tissue surrounding my venules. Because the only thing that is only endothelium are the capillaries. Oh, does this work for you? Now, we're going to look at different mechanics of flow through these vessels. And then we're going to preview some, what, control homeostatic control tools in the blood pressure homeostasis toolbox. But as I already said, we're not going to talk about blood pressure homeostasis until we've looked at the kidneys because they play a big role in the whole system. Okay, so let's look at some physics of blood flow because physics is fun.