In this video, we're going to begin our lesson on the general blood vessel structure. And behind me is our lesson worksheet, and we're going to break this lesson down one step at a time, starting off with this section that you can see boxed in red at the top. So let's zoom in and get started.
Over here on the left-hand side, we're showing you an image of the general blood vessel structure, and notice that the layers are all peeled back so that you can see the components more clearly. And over here on the right-hand side of the image, we're showing you a transverse cross section of this same image of the general blood vessel structure. Moving forward in this video, we'll be utilizing both of these images to help you better understand the general blood vessel structure. Before we continue, it's important to note that there are some structural variations amongst blood vessels. For example, there are some structural variations amongst the different types of arteries that we'll get to talk more about moving forward in our course in a separate video, and there are also some structural variations amongst arteries as a whole, capillaries, and veins.
In separate videos, we'll get to talk more details about those structural variations. But for now, in this video, what we can say is that, although there are some structural variations, many blood vessels, especially the arteries as a whole and the veins as a whole, are made up of 3 distinct structural layers or 3 distinct structural tunics. Moving forward in this video, we're going to be color coding each of these two images in order to highlight the components of each of these tunics.
These 3 distinct structural tunics are the tunica intima, the tunica media, and the tunica externa. Let's shrink these images down a little bit so that we can make some room for our lesson text, and we're going to focus on each of these 3 tunics one by one, starting with the tunica intima. The tunica intima, as its name kind of implies, is the internal or the innermost layer or tunic that is in intimate contact or direct contact with the blood that flows through the blood vessel.
The tunica intima can contain 3 components: the endothelium, the sub endothelium, and the internal elastic lamina. The endothelium is a layer of simple squamous epithelium, or a single layer of these squished or flat epithelial cells that lines the lumen of all blood vessels, and so this is an area where there is no structural variation amongst blood vessels since, again, all blood vessels have an endothelium. This endothelium creates a relatively slick, smooth, and flat surface that minimizes friction with the blood that passes over it. The endothelium is actually a continuation of the endocardium, which, recall from previous lesson videos, is the epithelial tissue that lines the heart. The endocardium will actually extend outward from the heart as the endothelium in order to line the lumens of all blood vessels.
Functionally, endothelium has several different functions, including physically affecting the blood flow since it's in direct contact with the blood, but also the endothelium can secrete chemicals that affect physiological activities, such as vasoconstriction and vasodilation of some blood vessels. As we'll learn moving forward in our course when we discuss capillaries in more detail, the endothelium plays an important role in the permeability of capillaries, which is going to allow for the exchange of substances between the blood and the surrounding tissues. Down below in the image on the left-hand side, you can see the endothelium labeled as this layer of simple squamous epithelium that lines the lumen. And over here on the right image, you can see the endothelium labeled as this layer right here.
Now, next, what we have is the sub endothelium, and the root sub is a root that means below. The sub endothelium is found immediately below the endothelium. The sub endothelium consists of 2 components: the basement membrane, which, recall from previous lesson videos, is an extracellular layer that is found immediately beneath almost all epithelial tissue, and the second part of the sub endothelium is the loose connective tissue, mainly areolar loose connective tissue. This areolar or loose connective tissue can help to bind or attach the endothelium to the rest of the blood vessel wall and can also help to facilitate diffusion through that layer of areolar loose connective tissue. Also, recall that areolar loose connective tissue can harbor immune cells, and so, this loose connective tissue can actually aid in the immunity of the blood vessel wall itself. Down below in the image on the left-hand side, you can see the basement membrane labeled as this light blue layer immediately beneath the endothelium, and then you can see the loose connective tissue as this tealish turquoise color here, which is the areolar loose connective tissue.
And over here on the right-hand side of the image, you can see, again, the basement membrane as this light blue color right underneath the endothelium, and then this other blue color that you see separated by the dotted line is going to be the loose areolar connective tissue. Now last but not least, we have the internal elastic lamina, which can be abbreviated as IEL, and sometimes it's also referred to as the internal elastic membrane. As its name implies with the term elastic, this is going to be a layer of elastic connective tissue, which, recall, contains lots of elastin proteins, which allow for elasticity or the ability to stretch, but then also recoil back to the original shape. And so this internal elastic lamina is usually only going to be present in larger arteries that are relatively close to the heart and need to be able to have that elastic ability to stretch and recoil back.
The reason that these larger arteries need to be more elastic is because they are closer to the heart, which generates forceful contractions and ejects blood out of the heart with relatively high pressures. And so these larger arteries need to be able to have this elastic ability in order to prevent damage to those vessels. Down below in this image on the left hand side, you can see the internal elastic lamina labeled as this yellow structure that kind of looks like Swiss cheese, and that's because it actually does have holes in it that allows for improved diffusion and also allows for substances to penetrate more easily as well. And over here on the right-hand side of the image, you can see the internal elastic lamina labeled as this yellow structure that goes all the way around. And so this here concludes the tunica intima, so we can go ahead and check it off, and we can move on to the next tunic, which is the tunica media.
Now, the m in tunica media can remind us that this tunic is the middle tunic or the middle layer in between the tunica intima and the tunica externa. The tunica media is often going to be the thickest layer or the thickest tunic, especially in arteries. Again, the m in tunica media can remind us that this layer contains mostly smooth muscle. Notice down below in the image, we've got the smooth muscle labeled on the left-hand side as this relatively thick layer in pink. And on the right-hand side, you can see the smooth muscle of the tunica media as this thick layer in pink as well.
In addition to the smooth muscle, the tunica media may also contain some elastic fibers, especially in the arteries that are closer to the heart. In fact, the tunica media may contain an external elastic lamina or an EEL, which is sometimes also referred to as an external elastic membrane. This is really similar to the internal elastic lamina that we discussed earlier in this video, but the main difference is that, as its name implies with the term external, it's going to be closer to the exterior surface in comparison to the internal elastic lamina.
Again, it's going to be found mainly in the larger arteries that are closer to the heart that need to be able to have the elastic ability to stretch but then recoil back to its original shape. So notice down below in the image, we have the external elastic lamina labeled as this Swiss cheese-looking structure here in yellow that has these holes to allow for substances to diffuse and penetrate more easily. And over here on the right-hand side, we are labeling the external elastic lamina as this structure in yellow that is relatively close to the exterior surface, and the external elastic lamina will actually separate the tunica media from the tunica externa when it is present. Now, the smooth muscle of the tunica media will actually contract, or it contracts, to allow for vasoconstriction, the narrowing of the diameter of the blood vessels, and the smooth muscle actually relaxes to allow for vasodilation or the enlargement of the diameter of the blood vessels. And so the smooth muscle of the tunica media is critically important for the regulation of blood flow and blood pressure throughout the entire cardiovascular system.
Again, it's the arteries that usually contain larger, thicker tunica medias with more smooth muscle, and so it's the arteries that have a greater ability to vasoconstrict and vasodilate. And so this here concludes our lesson on the tunica media, so let's move on to the last tunic, which is the tunica externa. Now, the tunica externa is also sometimes referred to as the adventitia. As its name implies, the tunica externa is the external or the outermost layer or tunic, and it's actually composed mostly of collagen fibers. These collagen fibers are either of loose connective tissues or dense irregular connective tissue, depending on the type of blood vessel and depending on the location of the blood vessel within the body. These collagen fibers help to provide structural support to the tunica externa and to the blood vessel.
Now down below in the image, you can see that we're labeling the collagen fibers in both images. What's really important to note is that the internal layers and tissues of the blood vessel wall that are closer to the lumen of the blood vessel will actually receive their nutrients directly from the blood that passes through the lumen. However, those nutrients will only diffuse so far toward the external tissues of the blood vessel wall. And so the exterior parts and tissues of the blood vessel wall also need to be nourished, and so this is where the vasa vasorum comes into play. This is a system of tiny blood vessels that actually nourish the external tissues of the blood vessel wall.
And so notice that in the image on the left-hand side, we're labeling the vasa vasorum as these red circles and also these blue circles, and you can see them extending upward here. These are the tiny blood vessels of the vasa vasorum that help to nourish the blood vessel walls on the exterior parts. And you can also see the vasa vasorum labeled on the right-hand side of the image as well as these blue and red dots. Now, in addition to the collagen fibers and the vasa vastarum, the tunica externa can also have nerves and lymphatic vessels and even elastic fibers as well, especially in the larger arteries that are closer to the heart.
And so notice that we're actually showing you some nerves here in yellow, and that can help stimulate the smooth muscle to contract. Now, the tunica externa is going to be important for protecting and reinforcing the blood vessels, again, because it has those collagen fibers that can provide support. And also, the tunica externa is important for anchoring the blood vessel to its surrounding tissues. And so that concludes our lesson on the tunica externa.
Now, we've covered all 3 tunics, and we are basically done with the general blood vessel structure. So here are all of the labels, and you can see them all labeled here in both images. That concludes this video, and moving forward, we'll be able to apply these concepts and continue to learn more about blood vessels. I'll see you all in our next video.
