In this video, we're going to talk about the types of capillaries. Structurally, there are 3 types of capillaries that we have numbered down below in our text, 1, 2, and 3. These are the continuous capillaries, the fenestrated capillaries, and the sinusoid capillaries. Notice that down below in the image, there is a section for each of these three types of capillaries. These three types of capillaries are based on their structure, but they're also based on their prevalence or how widely they are distributed throughout the entire body, and on their permeability as well, or how penetrable their boundaries are to diffusing substances.
The continuous capillaries are the most common type of capillary in terms of their prevalence and are very widely distributed throughout the entire body. However, they are actually the least permeable amongst these three types. Before we continue, it is important to note that these are all capillaries, meaning they are all fairly permeable, especially in comparison to other types of blood vessels like arteries and veins. Although the continuous capillaries are the least permeable, their permeability is sufficient for the exchanges that are needed for most of our tissues, allowing for the diffusion of fluids and small substances. It's generally just the large substances, such as macromolecules and proteins, that have a harder time getting through the continuous capillary boundaries. The reason these capillaries are the least permeable is due to their endothelium forming a relatively continuous tube; the endothelial cells are held tightly together with many tight junctions, represented in the image below as blue circles.
Despite this, the continuous capillaries have intercellular clefts—small gaps between the cells. These gaps are also found in the fenestrated and sinusoid capillaries, although the size of the gaps can vary. These intercellular clefts allow some substances to seep through. Continuous capillaries are found in areas such as the skin and in organs like the brain, lungs, nervous tissue, connective tissue, and muscle tissue, including skeletal and smooth muscle tissues.
Next, we discuss the fenestrated capillaries. As indicated by the term "fenestrated," these capillaries have endothelial cells containing fenestrations—small pores within the endothelial cells that increase permeability more than continuous capillaries but less than sinusoid capillaries. These capillaries also have fewer tight junctions compared to continuous capillaries, resulting in slightly larger intercellular clefts. Fenestrated capillaries are commonly found in areas of active filtration, secretion, and absorption, such as the kidneys, near endocrine glands, and in the small intestines.
Last but not least are the sinusoid capillaries. These have a discontinuous endothelium with relatively large holes in its structure, usually irregular in shape and often curvy, with lumens much larger than those of continuous or fenestrated capillaries. This allows them to store more blood and hold larger volumes. Sinusoid capillaries are the least common and most permeable. They are found in areas where rapid and large substance diffusion is required, such as in the liver and lymphoid organs like the bone marrow and spleen. In the red bone marrow, hematopoiesis occurs—new blood cells formed here enter circulation through the sinusoids, demonstrating that the gaps are large enough for cells to pass through.
This concludes our lesson on the types of capillaries. As we move forward in our course, we'll continue to apply these concepts and learn more about blood vessels. I'll see you all in our next video.
