In this video, we're going to begin our lesson on the subcategories of arteries. And notice behind me is our lesson worksheet, and we're going to break this lesson down one step at a time, starting with this section that you can see boxed in red at the top. So let's zoom in and get started. So recall that arteries are blood vessels that always carry blood away from the heart. And it turns out that there are 3 subcategories of arteries that are based on their size and their function. And these 3 subcategories of arteries are the elastic arteries, which are the largest arteries that are closest to the heart, then we have the muscular arteries, which are medium-sized arteries. And then last but not least, we have the arterioles, which we already know from previous lesson videos are the smallest of the arteries. And we also have an image for each of these corresponding arteries showing you the details of their wall structure. And so in this video, we're going to discuss each of these 3 subcategories of arteries, starting with the elastic arteries. So, again, these elastic arteries are the largest of the 3 subcategories of arteries with the largest diameter, and they are located near the heart. Which is why down below in the image, we're showing you an elastic artery with the largest diameter located near the heart. Now in this box over here on the left-hand side, we can label the title elastic arteries. And, again, although these elastic arteries are the largest with the largest diameter, it's important to note that their wall thickness is relatively thin in comparison to their diameter. And you can kind of see that in this image down below, but if we zoom into the wall structure even more, what you'll notice is that we're labeling the lumen diameter as 15 millimeters, but we're only labeling the wall thickness as 1 millimeter. And, of course, there's variation when it comes to the wall thickness and the lumen diameter of these elastic arteries, but what you can gather from this is that when it comes to these elastic arteries, usually their lumen diameter is far greater than their wall thickness. Now, these elastic arteries are also sometimes referred to as conducting arteries, which is why we have "conducting" in parentheses. And this is because these arteries will conduct or facilitate the movement of blood directly from the heart into the medium-sized muscular arteries that we'll get to talk about here shortly. Now these elastic arteries, as their name implies with the term elastic, have lots and lots of elastin, which recalls the protein that's found in elastic connective tissue. And all of this elastin makes these elastic arteries elastic, meaning that they have the ability to stretch significantly, but also recoil back to their original shape after stretching. In fact, these elastic arteries have more elastin than any other type of blood vessel, and the elastin is distributed all throughout the wall of these elastic arteries in every tunic, including the tunica intima, the tunica media, and the tunica externa. However, it's the tunica media, the middle layer with the smooth muscle, that contains the most amount of elastin, which is why we're showing you all of these yellow squiggles throughout the smooth muscle of the tunica media to represent all of that elastin in these elastic arteries. Now these elastic arteries, although they do have smooth muscle, it's important to note that they are relatively inactive in terms of vasoconstriction, facilitating extreme amounts of vasoconstriction and vasodilation. The vasoconstriction and vasodilation are more so going to be a role of the muscular arteries and arterioles. Now these elastic arteries may actually have an internal and elastic lamina, which recall from our previous lesson videos, are these yellow structures that you can see I'm highlighting here. And it's important to note that there's so much elastin in the walls of these elastic arteries that often these internal and external elastic lamina are not really considered to be there because they are just embedded within all of this elastic tissue present throughout the wall. Now the elasticity of these elastic arteries is very important because it allows the walls of these elastic arteries to forcefully eject blood out of the heart into these arteries with relatively high blood pressures. And without stretching, these elastic arteries could become damaged in that case, but the stretching helps to accommodate the blood ejecting forcefully into these arteries. However, just as important as the stretching is the recoil that occurs during diastole, or during the relaxation period of the heart. And so the recoil of these elastic arteries back to their original shape helps to keep the blood flowing continuously even when the heart is in its relaxation period, and that helps to prevent downstream pressure surges that could otherwise cause damage.
Now, this here concludes our lesson on the elastic arteries, so we can go ahead and check them off and move on to the next subcategory of the artery, which is the muscular artery. So these muscular arteries are medium-sized arteries with diameters that are smaller than elastic arteries but larger than arterioles. And these muscular arteries account for most of the main arteries that are named and studied in anatomy and physiology courses. And as their name implies with the term muscular, these muscular arteries have lots of smooth muscle. They have more smooth muscle than elastic arteries, giving them greater ability for vasoconstriction and vasodilation, but they have less elastin than those elastic arteries, which means that they have less ability to stretch and recoil. However, these muscular arteries are further away from the heart, so they don't need that elasticity as much as the elastic arteries do from being closer to the heart. Now, these muscular arteries have so much smooth muscle that the smooth muscle accounts for about 75% of the mass of a muscular artery. So that's a lot of smooth muscle. And so if you take a look at the image down below, notice that there is quite a lot of smooth muscle in the tunica media, and that's going to be the defining feature of these muscular arteries. Now although they have less elastin than elastic arteries, they still can have some elastin, and so these external and internal elastic lamina may still be present. And notice that the lumen diameter is being indicated as 6 millimeters, and the wall thickness is being indicated as 1 millimeter. And so the wall thickness is about the same as the wall thickness of elastic arteries, but notice that the lumen diameter is smaller in muscular arteries than it is in elastic arteries. Now these muscular arteries will receive blood from the elastic arteries, and they will deliver or distribute the blood towards specific organs and muscles. And this is why muscular arteries are also sometimes referred to as distributing arteries because of their ability to distribute or deliver blood towards specific organs and muscles.
Now this here concludes our lesson on muscular arteries, so we can go ahead and check these off. And let's move on to the last subcategory of the artery, which is the arterioles. So we already know from previous lesson videos that arterioles are the smallest of the arteries with the smallest diameters. But not only are they the smallest, they're also the most numerous of the arteries. In fact, it's estimated that there are hundreds of millions of arterioles in the average human body in various locations. And most of these arterioles are not named, so you don't need to worry about memorizing them. Now these arterioles are important for regulating blood flow directly into the capillaries. Now later in our course in a separate video, we'll talk more about the capillaries, but recall from previous lesson videos that the capillaries are important for the exchanges that occur between the blood and the surrounding tissues. Now, in this image over here on the right-hand side, you can see in this dotted box, we're highlighting the arteriole, the smallest of the arteries, and it's feeding blood directly into these capillaries where, again, the exchanges occur between the blood and the surrounding tissues, and that's indicated by this color change to purple. Now the arterioles actually vary in terms of their size and structure, and so the largest arterioles will have all 3 tunics in their wall. However, those tunics are relatively thin and they contain more smooth muscle than elastin, giving them greater ability to vasoconstrict and making them not so elastic. Although arterioles are quite far away from the contracting heart, so they don't really need to be as elastic as the larger arteries that are closer to the heart, like the elastic arteries. Now the smallest of the arterioles are not much more than just the endothelium and a single layer of smooth muscle around them. Now these arterioles will actually vasoconstrict and play a huge role in increasing what's known as resistance throughout the cardiovascular system. Now later in our course, we're going to talk more about this resistance. However, the resistance is any opposition to blood flow, or in other words, it's a measure of how difficult it is for blood to flow through the blood vessels. The greater the resistance, the more difficult it is for blood to flow through. And ultimately, resistance is a measure of the amount of friction that the blood encounters with the walls of the blood vessel as the blood travels through the cardiovascular system. And so resistance is greatly impacted by the diameter of the blood vessels, and so vasoconstriction will increase the resistance. And because arterioles are the smallest and the most numerous, the vasoconstriction of the arterioles has the greatest impact on resistance, and because that's the case, these arterioles are also sometimes referred to as resistance arteries. Now the vasoconstriction of these arterioles can also redirect blood flow away from capillaries to avoid exchanges in those capillaries. And the redirection of blood flow can redirect blood to areas of the body that may have a greater demand for blood. Now, in this image down below, notice that we are zooming into the wall structure of this arteriole, and notice that the thickness of this wall is being indicated as 0.006 millimeters, which is quite small it's the smallest thickness of the wall that we've seen so far. And notice that the lumen diameter is being indicated as 0.037 millimeters. And, again, there is variation in the wall thickness and lumen diameter, but, in general, these arterioles or resistance arteries are going to have significantly smaller lumens and significantly smaller wall thickness. And so this here concludes our lesson on the arterioles, so we can go ahead and check them off.
So now that we've covered the 3 subcategories of arteries, we're now going to talk about the size ranges of each of these 3 subcategories of arteries and compare them to regular household items that you may be more familiar with. And so recall that the conducting arteries are the largest of the arteries, and so they can be as large as about a garden hose, and so their diameter can be as large as about 25 millimeters. And on the smaller side, they can be about the diameter of your pointer finger, which, of course, can vary, but is about 10 millimeters. Now anything smaller than 10 millimeters is starting to get into the range of muscular or distributing arteries, which we call are medium-sized arteries with lots of muscle. And so on the larger end, these muscular arteries are about the diameter of a pencil, which is about 7 millimeters. And on the smaller side, they're about the diameter of a string, which is about 0.3 millimeters. Now, again, anything smaller than 0.3 millimeters is starting to get into the range of these arterioles or resistance arteries, which recall are the smallest and the most numerous of the arteries. Now, on the larger side, these arterioles are about the diameter of your hair, which is about 0.18 millimeters. And on the smaller side, these arterioles are microscopic down to about 15 micrometers in diameter, which is why we have the microscope here. And so this here concludes our lesson on the subcategories of arteries, and moving forward in our course, we'll be able to apply these concepts and continue to learn more about blood vessels. So I'll see you all in our next video.
