19. The Blood Vessels
Capillaries
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In this video, we're going to talk about the types of capillaries. And so structurally, there are 3 types of capillaries that we have numbered down below in our text, 1, 2, and 3. And these are the continuous capillaries, the fenestrated capillaries, and the sinusoid capillaries. And notice that down below in the image, there is a section for each of these three types of capillaries. Now, again, these three types of capillaries are based on their structure, but they're also based on their prevalence or how widely distributed they are throughout the entire body. And they're also based on their permeability as well or how penetratable their boundaries are to diffusing substances. And so the continuous capillaries are actually the most common type of capillary in the body in terms of their prevalence, and so they are very widely distributed throughout the entire body. But they are actually the least permeable amongst these three types of capillaries. Now before we continue, it is important to note that these are all capillaries, which means that they are all fairly permeable, especially in comparison to other types of blood vessels like arteries and veins. And so although the continuous capillaries are the least permeable, their permeability is sufficient for the exchanges that are needed for most of our tissues. And so they will still allow for the diffusion of fluids and small substances. And really, it's just the large substances, such as the macromolecules and proteins, that have a harder time getting through the continuous capillary boundaries. And the reason that these continuous capillaries are the least permeable is because, as their name implies with the term continuous, their endothelium forms a relatively continuous tube, since their endothelial cells are held really tightly together with lots of tight junctions, which as you can see down below in the image, the tight junctions are being represented as these blue circles that are holding the endothelial cells really tightly together, allowing a very continuous tube to form. And when we take a look at the other types of capillaries over here, notice that they've got these holes in them that actually make them more permeable than the continuous capillaries, which don't have those same holes. Now although the continuous capillaries do form a relatively continuous tube, it is important to note that they still have these intercellular clefts, which are these relatively small gaps that can be found in between the cells. And And so it's also important to note that these intracellular clefts are not unique to the continuous capillaries. They are also found in the fenestrated capillaries and in the sinusoid capillaries as well, although the size of the intracellular clefts can vary. And so notice that down below in the bottom left of the image, we're labeling the intracellular cleft, which again is just going to be these gaps in between the cells that I'm tracing here. And, of course, some substances will be able to seep in between through the intercellular clefts. Now, again, these continuous capillaries are the most common type of capillaries, so they are very widely distributed throughout the body, And they can be found in areas such as the skin, which we know covers the external part of our body from our head to our toe, but they can also be found in other types of organs such as our brain and our lungs and nervous tissue and connective tissue and muscle tissue, including skeletal and smooth muscle tissue. And so this here really concludes our lesson on the continuous capillaries. Let's move on to the next type, which are the fenestrated capillaries. And so the fenestrated capillaries, as their name implies with the term fenestrated, are going to have endothelial cells that contain fenestrations, which are these relatively small pores that are in the endothelial cells. And those fenestrations give the fenestrated capillaries a moderate amount of permeability that is less than I'm sorry. That is more than the permeability of the continuous capillaries, but less than the permeability of the sinusoid capillaries. And so down below in this middle part of the image, we can label this as the fenestrated capillaries, and what you'll notice immediately is that we are labeling all of the fenestrations, all of those small pores, these little holes that you can see in the endothelial cells that make the fenestrated capillaries just a little bit more permeable than the continuous capillaries. And so their permeability falls right in the middle of this permeability scale. And also, in terms of their prevalence, they also fall right in the middle as well. So they are less common than the continuous capillaries, but they are more common than the sinusoid capillaries. And so in terms of where these fenestrated capillaries can be found, they are usually found in areas of active filtration, secretion, and absorption. So for example, these fenestrated capillaries are found in the kidneys, where they actively filter the blood to produce urine. They're also found near endocrine glands, which produce hormones that are secreted into the blood, and they're also found in the small intestines, which absorb nutrients from the diet. And really, that is it for the fenestrated capillaries. One thing to note is that they are going to have tight junctions, but they're going to have less tight junctions than the continuous capillaries. And so notice that their intercellular clefts are just a little bit bigger than the intracellular clefts of the continuous capillaries. And really, that is it for the fenestrated capillaries. Let's move on to the last but not least type of capillary, the sinusoid capillaries. Now the sinusoid capillaries are going to have an endothelium that is discontinuous and has relatively large holes in its structure. And so these sinusoid capillaries usually have an irregular shape to them, so they're often going to be quite curvy, and their lumens are generally much larger than the lumens of continuous or fenestrated capillaries. And so that allows them to store, more blood, hold larger volumes of blood. And so over here on the far right of the image, we can label this as the sinusoid capillaries. And they are actually going to be the least common, the least distributed throughout the body, but they are the most permeable amongst these three types of capillaries. And this is because of their discontinuous nature and the relatively large holes that are found in its structure. And so if we take a look at the image, what you'll notice is that the sinusoid capillaries have these really large intercellular clefts that, make them very, very permeable, and they also have much larger fenestrations, these larger holes in the endothelium. And these, the sinusoid capillary is so discontinuous that it looks like it's going to fall apart, although it does still have some tight junctions, it just has way less tight junctions than, either of these other two types of capillaries. So that's what keeps it together. It's not going to fall apart. And so, what's important to note is that because it has such a discontinuous nature in these much larger holes, it's going to allow for the diffusion of relatively large substances, relatively large, molecule, macromolecules, and even, cells themselves are able to make their way into and out of the sinusoids. And so these sinusoids are going to be found in areas where rapid diffusion is going to be needed, and also the the diffusion of relatively large substances is going to be needed, such as, in the liver, and also in lymphoid organs, such as the bone marrow and the spleen. And what's interesting is that in the red bone marrow, this is where hematopoiesis occurs, the formation of new blood cells. And those new blood cells will actually enter into circulation through the sinusoids that are found in the bone marrow. And so that goes to show that these gaps are actually large enough for cells to be able to pass through them. And so this here concludes our lesson on the types of capillaries, and as we move 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.
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