 The function of the blood vessels is directly related to, oh, imagine this, its structure. There are five different types of blood vessels that we're going to look at, and we've already talked about the biggies. Arteries carry blood away from the heart, while veins carry blood to the heart. Arteries, when they carry blood away from the heart, they branch into arterioles, which branch into capillaries, so tiny little blood vessels where exchange of materials takes place. In fact, let's just make a note of that. It's kind of like the whole point of the blood vessel system is to allow for exchange at the capillaries. Every single cell is within, I don't know, like a micrometer of a capillary in order for that cell to survive. We're going to have an entire lecture on capillary exchange, so we don't need to go into a great deal of detail right now. What we do want to know is the structure of the capillaries that would therefore enable their function of exchange. And here's the scoop. Capillaries literally are nothing but endothelium. And so endothelium is this special inner layer. It's a specialized epithelial tissue. And it's called endothelium, and I'm trying to draw simple squamous epithelial cells. That was a really impressive job, wasn't it? All of our vessels have endothelium, and the structure of the endothelium, I'm going to have to draw lots of other layers on these guys, so I'm going to make their diameters small, or rather than my capillary, but in actuality, their diameters are actually bigger, so I'm going to note the diameters here just so you can keep perspective. But every single vessel type has that inner layer of simple squamous epithelium called the endothelium. But capillaries don't have anything else going on. They don't have elastic tissue or smooth muscle or fibrous tissue. They don't have any other layers, which makes them perfect for their function of exchange of nutrients and gases. The thickness of the wall of the vessel itself is the thing I'm going to make note of in this category or in this column. The thickness of a capillary is half of a micrometer, half 0.5 of a micrometer. Let's make sure that's super clear. That's tiny doggies, really tiny. And the diameter of a capillary is somewhere between four and 10 micrometers. Now, maybe that doesn't mean a whole heck of a lot to you, but let's compare it to our other vessels and hopefully come up with some good comparisons. I'm also, in addition to drawing a vessel over here, I'm going to do a little graph that you will see like amounts of each of the tissue types in each vessel so we can compare. The amount of endothelium in each vessel, just like the thickness of the endothelium in the vessel, is the same in every single kind of vessel. That's not going to be the case with our other tissues. Let's look at the elastic tissue next. The elastic tissue is something that we are going to find in only two of our vessel types. We find elastic tissue layers in the artery and in the vein, but if you look at the size of the elastic tissue or the amount of elastic tissue that you find, the amount is different. So the amount of elastic tissue in a vein is much smaller than the amount of elastic tissue in an artery. So does that work for you? There's more elastic tissue. I'd like to be able to draw it in my picture, but I just don't have enough room to do that. Elastic tissue isn't found anywhere else. What would elastic tissue allow? In fact, elastic tissue allows you to maintain pressure. Pressure, there's a reason why hypotension, low blood pressure is bad. If you don't have enough pressure, you can't push the blood around your body. So you have to have a way to maintain pressure. The arteries, because they have so much elastic tissue, they're actually able to absorb the pressure from the pump of the heart. They're able to absorb that pressure and stretch and then contract, even though they're not really contracting, they're just elastically recoiling and that provides an additional push to push that blood onward through the body. Okay, smooth muscle. Who of our good buddies has smooth muscle? Interestingly, the arteries have the most smooth muscle out of all of these guys. The smooth muscle layer comes next and you can imagine that we could look at that histologically. Arterials also have a rockin' layer of smooth muscle. What is smooth muscle? What's the function? Our veins do have some smooth muscle. Now, the smooth muscle, the amount of smooth muscle in my veins, which are bigger vessels, is about the same as the amount of smooth muscle in the arterials. And it's nothing compared to the amount of smooth muscle in the arteries. The arteries allow bigger stretch because of the elastic tissue and they allow constriction, vasoconstriction, which, again, that pushes the blood onward. Arterials allow a regulation of blood to various locations because they are surrounded. They're the site of the majority of vasoconstriction and vasodilation, which is functionally a very important thing to be able to do. Arterials can help regulate the amount of blood that goes to various body parts. We've already talked about how, in a sympathetic nervous response, we're probably going to want to supply blood to certain places like skeletal muscles, more than other places like digestive system. And in fact, it's true. Vasodilation takes place in skeletal muscles during a sympathetic nervous response, whereas vasoconstriction takes place in the digestive system in order to save your blood energy, like why spend our energy digesting when really we should be running away from the bear. Okay, I think I'm still good. The next tissue that we want to add is the fibrous tissue and here's the scoop. Arteries are big and they've got a chunk of fibrous tissue. It's not as thick of a chunk as what I drew here. Sorry about that, but it's a healthy chunk. Venuals also have a little chunk of fibrous tissue surrounding them. The fibrous tissue is going to give them some additional structural support and my veins also have fibrous tissue surrounding them. Veins are floppy little things, which how are we going to know if they're floppy or not? We can look at the differences in thickness and diameter to be able to see things like that. So let's compare our thicknesses and diameters. The thickness of a vein, this is interesting. The thickness of a vein goes up to a half of not a micrometer, a half a millimeter. Make sure you get your M's proper. This is a mu sign, which is different than the M sign, and that means micro as opposed to milli. Veins have a thickness of about half of a millimeter and they can range in diameter, which makes perfect sense. We can range from 0.1 millimeters up to 20 millimeters in diameter, so my little purple circle here can go up to 20 millimeters, nothing else gets that big. In fact, the diameter of an artery can go from 0.1 to just 10 millimeters. But 10 millimeters, dude, that's a, that's big. That's a, that's that big. That's a wide, you can stick your finger in that. That's a big old diameter for a big old artery. The thickness of the artery walls can go up to one millimeter. A millimeter thick wall, so this is something that since you've taken anatomy and you've hung out with my dead bodies, you've seen the difference between the floppy, thin veins and the thick, round arteries. And it's like you can bounce on the artery because the wall of the artery is so thick. Why? It has much more smooth muscle. It has a lot more elastic tissue. It's surrounded in this nice fibrous tissue. It just carries a lot more structure and holds its round shape. Thin, big diameter, that's not a word, but the big diameter and thin wall of a vein is evident by these numbers as well. Now, venules, you're going to expect them to be smaller. We're going back into micrometer land. The venules have a thickness of about one micrometer. Makes sense, not half a micrometer like a capillary, but one micrometer because we added a half a micrometer of fibrous tissue around the capillary. The diameter of a venule can range from 10 to 100 micrometers. Guess what 100 micrometers is? 0.1 millimeters, same thing. Arterials, on the other hand, we're going to go from 10, that's our diameter, 10 to 100 micrometers. And again, our relationship here is the same, look at that. The thickness of an arterial can be up to six micrometers. Okay, so that gives you a big picture. It's important because regulating blood pressure is going to happen in vessels that have the smooth muscle. It's also important because blood pressure is maintained by the blood vessels that have that elastic tissue in them for stretching. Next, we're going to look at some characteristics of blood flow. And these flow characteristics, these things that we're going to pay attention to when we're talking about flowing blood, we're also going to look at them when we're talking about flowing air in the respiratory system or even flowing PP when we talk about the urinary system. So let's look at some flow issues in the next one.