Capillaries: Types

by Geoffrey Meyer, PhD

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    00:01 So let’s look at the structure of a capillary. There are actually three types of capillaries, and I will describe them briefly here, but then I’ll emphasize them more when we look at their functional role in other organ systems. On the left-hand side, you can see a picture or an image taken through a connective tissue. And there are blood capillaries running through them, very, very thin.

    00:30 You can make out just the very thin walls, you can see through them, and just make out the elongated structures that represent the nuclei of the endothelial cells. Well, probably the most common type of capillary is what we call a “continuous capillary.” And this is illustrated on the diagram, particularly, on the bottom diagram on the right-hand side.

    00:57 A continuous capillary is one that I’ve referred to you before. This is where the endothelial cells join together to make up the capillary lining. And they join together by very strongly occluding junctions. The strongly designed occluding junctions that include tight junctions. So the junctions between endothelial cells is very very solid, very strong. Nothing passes through those junctions. And if you look back on your knowledge of epithelia, these occluding junctions are very important in many epithelial tissues because they restrict movement of fluid and other substances, pathogens, between cells.

    01:48 So in many organs of the body, skin, and particularly the brain, you have these continuous capillaries.

    01:58 And the only way in which substances are transported across the capillary wall is by pinocytosis, by these substances being invested by the cell, taken in by the cell, wrapped up in little membranes and transported across the endothelial cell surface in cytoplasm, and then release on the other side into the interstitium. It’s called transcytosis, the movement of fluid and other substances by endocytosing that material at the lumen, and by those vesicles moving across and releasing the products on the other side. That enables these capillaries to restrict the sorts of substances that they later pass across them. The basal lamina wrapped around the endothelium is always continuous. Well, another sort of capillary is one where the actual cytoplasm of these very very thin endothelial cells have little fenestrations, little windows, little pores, and that allows substances to pass out through those pores, all be it restricted and all be it very finely controlled. We’ll see an example of these fenestrated capillaries in the kidney because they form part of the filtration component of our blood, forming a glomerular filtrate. In that situation, there is a very thin diaphragm between the fenestrations because they need to be further controlled of what passes from the blood in the kidney. But just to summarize these little capillaries, again, the basal lamina is continuous, but there are little windows or little pores within the cytoplasm allowing substances to pass through them. Well, the final type of a capillary, we call a “discontinuous capillary” that has a discontinuous basal lamina. Sometimes we refer to this as being sinusoidal. There are large gaps in the cytoplasm, in the lining of the epithelium of the blood vessel, the endothelium. And these large gaps are quite typical in some endocrine tissues. They are very leaky. They allow substances to pass out of them, and also into them. And in some organs, such as the spleen, cells actually pass through these large gaps. The liver is another example of these sorts of capillaries. And again, let me stress that we will look at these types of capillaries when we look at the different organ systems where they have a functional role.

    About the Lecture

    The lecture Capillaries: Types by Geoffrey Meyer, PhD is from the course Cardiovascular Histology.

    Included Quiz Questions

    1. ...kidney
    3. ...spleen
    4. ...endocrine organ
    5. ...brain
    1. Brain.
    2. Liver.
    3. Spleen.
    4. Adrenal glands.
    5. Kidney.

    Author of lecture Capillaries: Types

     Geoffrey Meyer, PhD

    Geoffrey Meyer, PhD

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