Capillaries: Study with Video Lessons, Practice Problems & Examples

Capillaries, the smallest blood vessels, facilitate nutrient and gas exchange between blood and tissues. There are three types: continuous, fenestrated, and sinusoid capillaries, each varying in permeability and distribution. Continuous capillaries are the most common but least permeable, while sinusoid capillaries are the most permeable, allowing larger substances to pass. Capillary beds, networks of capillaries, enable microcirculation, with blood flow regulated by arterioles and precapillary sphincters, ensuring efficient tissue perfusion and nutrient delivery.

Introduction to Capillaries

In this video, we're going to do just a little bit more of a formal introduction to capillaries. And so recall from our previous lesson videos when we overviewed the major types of blood vessels that we already defined capillaries as the smallest blood vessels of them all. In fact, these capillaries are only about 5 to 10 micrometers in diameter, which is super tiny. And recall that erythrocytes, or red blood cells, are about 7.5 micrometers in diameter, which means that these capillaries, even at their largest, are just barely large enough for these red blood cells to flow through them in a single file line. And in many cases, the red blood cells actually need to fold up upon themselves just to squeeze through these tiny capillaries. Now, in addition to being the smallest, capillaries are also the most numerous of the blood vessels. It's estimated that there are billions of capillaries in the average adult, and I've seen the estimates go from about 20,000,000,000 up to about 100,000,000,000 capillaries in 1 adult, which is just mind-blowing to think we have that many capillaries inside of each and every one of us. Now functionally, capillaries are important for facilitating exchanges between the blood that they carry and the tissues that surround them. And so these exchanges include things such as nutrients, like glucose, and gases like oxygen and carbon dioxide. Now structurally, capillary walls are very thin, and they lack smooth muscle, which is very important because it means that capillaries lack the ability to change their diameter through vasoconstriction and vasodilation. And capillary walls also only have one single very thin tunic, and that is the tunica intima. And so if we take a look at the image down below on the left-hand side, notice that we're showing you an individual capillary. And you can see that the endothelium is being shown as this layer highlighted here in the middle that is in direct contact with the blood that passes by, and then it has a basement membrane that is surrounding it that you can see highlighted here in green. And so, really, that is about it for the capillary structure, and so it is very, very thin and only consists of the tunica intima. Now what's really important to note is that capillaries do not function independently. These capillaries only exist in extensive networks of numerous branched and connected capillaries that are called capillary beds. And so notice on the right over here, we're showing you a capillary bed, and notice that the image on the left is actually just a zoom-in to just one of the capillaries of this capillary bed. And so, in this capillary bed, again, it consists of numerous branched and connected capillaries. And so, in this particular capillary bed, what you'll notice is that the red side over here is carrying oxygenated blood. So, blood would be flowing in in this direction as you see here. It would continue to flow through the capillaries where exchanges would occur between the blood and the tissues, which is why we have this purplish color here. And then the blood would continue to flow and collect into these venules, and start to make its way back to the heart. And so, again, these capillary beds can flow from right to left in this direction, or they can flow from left to right, or really any direction, as long as it's flowing from the oxygenated side to the deoxygenated side. And so, moving forward in our course, we're going to be able to talk a lot more about capillaries and capillary beds, including the different types of capillaries, which we'll talk about in our next video, so I'll see you all there.

Types of Capillaries

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.

Capillaries Example 1

So here we have an example problem that asks, which type of capillary would you not expect to find in a tissue where relatively large molecules need to be exchanged between the blood and surrounding tissues, and smaller molecules need to be exchanged rapidly. And we've got these 3 potential answer options down below, which are the 3 types of capillaries that we talked about in our last lesson video. Now, of course, when we look at this problem, it is asking us about the diffusion of relatively large molecules, and it's also asking us about smaller molecules diffusing rapidly. And so, really, in terms of the capillary we would not expect to find in these types of tissues, we would not expect to find the capillary that are the least permeable. And recall from our last lesson video that the capillary that is the least permeable is the continuous capillary. And so we can go ahead and indicate that answer option a, continuous capillaries, is going to be the correct answer to this problem because we would not expect to find them in these areas where large molecules need to diffuse and smaller molecules need to diffuse rapidly. And so, we can go ahead and indicate a is correct because they are the least permeable.

Now looking at answer option b, it says fenestrated capillaries. The fenestrated capillaries have small fenestrations in them, which are these relatively small pores. So they may not allow for the diffusion of relatively large molecules, especially in comparison to the sinusoid capillaries. However, they would allow for the diffusion of smaller molecules to occur more rapidly, and so we would expect to find them more so than, we would expect to find the continuous ones. And so for that reason, we can eliminate answer option b.

And then, of course, answer option c says sinusoid capillaries. These are the most permeable amongst these three types of capillaries. So they would certainly allow for the diffusion of large molecules, and they would allow for smaller molecules to diffuse more rapidly. And so, for sure, we would expect to find them. And, again, we're looking for the answer where we would not expect to find them. And so, answer a, continuous capillaries, is the correct answer to this example. That concludes this example, so I'll see you all in our next video.

Which type of capillary is characterized by a discontinuous basement membrane, allowing for the easy passage of larger molecules and cells?

• A. Continuous capillaries.
• B. Fenestrated capillaries.
• C. Sinusoid capillaries.
• D. Muscular capillaries.

General Structure of Capillary Beds

So now that we've covered the types of capillaries in our previous lesson videos, in this video we're going to focus on capillary beds, more specifically, the general structure of capillary beds. Recall from our previous lesson videos that capillaries do not function independently. Instead, capillaries exist within capillary beds, which are an extensive network of numerous branched and connected capillaries. A typical capillary bed could have anywhere between about 10 to about 100 capillaries. Notice that in our image below, we're showing you the typical structure of a capillary bed on the left-hand side, and all of its components are labeled as well. Over here on the far left of the capillary bed, we have an arteriole, which is a small artery that carries blood away from the heart and towards these capillaries. We know that blood vessels are able to branch, including arterioles, and there is a branch right here. The arteriole continues downward, but it's branching over here toward this capillary bed that consists of an extensive network of numerous branched and connected capillaries. Just before this capillary bed, it's being labeled as the terminal arteriole. The terminal arteriole, as its name implies, is the region that is near the end of an arteriole, so it's going to be feeding blood immediately into the capillary bed. Once blood transitions from the terminal arteriole into the capillary bed, exchanges will occur between the blood that the capillaries are carrying and the tissues that immediately surround these capillary beds. From the capillary bed, the blood transitions into what's being labeled as the postcapillary venule. The postcapillary venule receives blood from the capillary bed and is the smallest of all the venules. Blood from the postcapillary venule transitions into the venule, and the blood in the venule starts to carry the blood back towards the heart. That covers the typical structure of a capillary bed.

It is also important for you all to know the term microcirculation. When the term circulation is used, it refers to the blood flow throughout our entire body. However, the term microcirculation refers to the blood flow through a capillary bed from the terminal arteriole through the capillary bed and into the postcapillary venule. That is exactly what we discussed moments ago. What can help you remember this is that the root micro means small, and so microcirculation refers to the blood flow through these really small capillary beds. Remember, capillaries and capillary beds do not have smooth muscle, so capillaries are not capable of changing their diameter through vasoconstriction or vasodilation. However, arterioles can have smooth muscle, meaning that arterioles can vasoconstrict. Notice that on the right-hand side, we're showing you the vasoconstriction of these arterioles and the terminal arterioles with these yellow arrows. Vasoconstriction of these arterioles can redirect blood flow away from the capillary beds. This redirection may occur to redirect the blood flow to other areas of the body where the blood may be needed more. It's amazing that our body has evolved the ability to redirect blood to other areas of the body in this way. So that's pretty fascinating. This here concludes our brief video on the general structure of capillary beds. As we move forward in our course, we'll be able to apply these concepts, and we'll also be able to continue to learn more about capillary beds and about blood vessels in general. See you all in our next video.

Capillaries Example 2

So here we have an example problem that wants us to choose one of the 4 potential answer options down below that best fills in the blanks in the sentence. And so notice that the sentence says, if the body requires blood in skeletal muscles, blank in the digestive system will blink. And so recall from our last lesson video that if one area of the body needs blood more than another area of the body, then the body is able to redirect the blood flow toward the area of the body that needs the blood more. And so I've gone ahead and sketched out something that I think may be helpful to visualize this scenario. Notice that over here on the left, what we have is tissue number 1, and over here on the right, what we have is tissue number 2. Tissue number 1 is in a scenario where it needs more blood, whereas tissue number 2 is in the scenario where it does not really need more blood. Because tissue number 1 needs more blood, the arteries that are going toward tissue number 1 are going to dilate, and dilate means they will enlarge in order to allow more blood flow to rush towards this tissue. Whereas in tissue number 2, because it does not need more blood, it is going to have its arteries that are feeding blood to it, constrict, and that means that it will narrow down to reduce blood flow going to tissue number 2, and that will ultimately help to redirect the blood flow to tissue number 1, who needs that blood even more.

When we reread this problem here, what you'll notice is that it says that if the body requires blood in the skeletal muscles, what that's saying is that tissue number 1 must be the skeletal muscles. So we can label it as so, and, it needs more blood. So its arteries will dilate. But tissue number 2, in this case, must be the digestive system, so we can go ahead and label it as so. What you'll notice is that it does not need more blood. And so, what we can see is that its arteries are going to constrict. And so what we would expect to see is something here that says something about arteries or arterioles or something of that nature, and then over here, we would expect that it's going to constrict.

Taking a look at the answer options, notice option a says, veins in the digestive system will constrict. But veins are taking blood away from the digestive system, and veins are not going to have the ability to vasoconstrict nearly as much as arteries. So for that reason, we can eliminate answer option a. Option b says, capillaries in the digestive system will dilate, but we know that capillaries lack smooth muscle, and so they lack the ability to vasoconstrict and vasodilate. So we're going to eliminate answer option b for that reason. And so now we're between either answer option C or answer option D. Notice they both are referring to terminal arterioles in the digestive system, but option c says that they will dilate, whereas option d says they will constrict. Again, in the digestive system, as we just discussed, the arteries, or terminal arterioles in this case, are going to constrict. And so that means that answer option d, terminal arterioles in the digestive system will constrict, is going to be the correct answer to this example problem. And answer option c is not going to be correct. And so, that concludes this example problem. Option d is correct, and I'll see you all in our next video.

Which of the following areas of the body has the highest concentration of capillary beds?

• A. Skeletal muscle.
• B. Joints.
• C. Connective tissue.
• D. Cartilage.

Which of the following best describes the microcirculatory flow of blood (microcirculation)?

• A. Terminal Arteriole → Arteriole → Capillary Bed → Postcapillary Venule.
• B. Arteriole → Terminal Arteriole → Capillary Bed → Postcapillary Venule.
• C. Postcapillary Venule → Capillary Bed → Terminal Arteriole → Arteriole.
• D. Capillary Bed → Postcapillary Venule → Arteriole → Terminal Arteriole.

Structure of Mesenteric Capillary Beds

So now that we've talked about the general structure of capillary beds in our previous lesson videos, in this video, we're going to focus on the structure of mesenteric capillary beds. The mesenteries are the serous membranes of the digestive system that hold our intestines in place. These mesenteries have a specialized capillary bed structure that includes the presence of a vascular shunt, which is important for the precise and fine-tuned control of blood flow through the capillary bed. Let's take a look at our image down below so that we can visualize this vascular shunt. Notice on the left over here, we're showing you the capillary bed structure of the mesenteries, which looks very similar to the capillary beds that we showed you in our previous lesson videos. However, notice that it has this structure that we are highlighting here in green and zooming into up above so that you can see the details more clearly. This is the vascular shunt. The vascular shunt allows for blood to be shunted or for blood to pass directly from the terminal arteriole through to the postcapillary venule. This vascular shunt actually consists of 2 parts. The first part is the metarteriole, and the second part is the thoroughfare channel. The first part of the vascular shunt is the metarteriole, highlighted here and bracketed is the metarteriole, and then the thoroughfare channel is going to be the remainder of the vascular shunt. The metarteriole is a transitional blood vessel because it has intermediate characteristics of an arterial and a capillary. The most key defining feature of these metarterioles is that they contain what are known as precapillary sphincters on its branches into the capillary bed network. These precapillary sphincters are smooth muscle rings that act as valves or blood flow control into the capillary bed network. So taking a look at our image down below, notice that the precapillary sphincters are being circled with these dotted circles that you see here. Again, these are rings of smooth muscle that wrap around the branches of the metarteriole into the capillary bed network. When these precapillary sphincters contract, they narrow down the branch here and prevent blood from flowing into the capillary bed, and so that redirects the blood flow further into the vascular shunt. The second part of the vascular shunt is the thoroughfare channel, and the thoroughfare channel is really just the continuation of the metarteriole. But the biggest difference is that the thoroughfare channel lacks smooth muscle, including the precapillary sphincters. As soon as the metarteriole loses its precapillary sphincters, it then is considered the thoroughfare channel. The thoroughfare channel will directly connect the metarteriole to the postcapillary venule. The reason that this vascular shunt is so important is because it allows for the precise and fine-tuned control of blood flow through the capillary bed, and that's exactly what the image on the right is focusing on. Notice that the image at the top right up here is focusing in on the vasoconstriction of just the arteriole and the terminal arteriole, just like what we saw in previous lesson videos. Notice that the vasoconstriction of the arteriole is being indicated by these yellow arrows highlighted here, and the vasoconstriction of the terminal arteriole is being indicated by those yellow arrows that you see highlighted right there. The vasoconstriction is going to reduce blood into the arteriole, reduce blood flow into the entire capillary network and vascular shunt, and it's going to redirect blood flow in other areas of the body, very similar to how we saw blood flow redirected in our previous lesson videos. In this top image, it's really nothing new. It's very similar to what we saw in our previous lesson videos, where the blood flow is

Capillaries Example 3

So here we have a pretty tricky example problem, but let's break it down and walk through it. So it says, which scenario results in dilation of arterioles and precapillary sphincters in mesenteric capillary beds? And we've got these 4 potential answer options down below. Now, we already know from our previous lesson videos that blood vessels can have the ability to vasoconstrict and vasodilate. And the vasoconstriction and vasodilation can effectively reroute the blood to other areas of the body where the blood may be needed more, where there's a greater demand for blood. And so here in this problem, it's mentioning the dilation of arterioles and precapillary sphincters in the mesenteric capillary beds. And so, the dilation of the arterioles will enlarge the diameter of these arterioles, and the dilation of the precapillary sphincters will relax the precapillary sphincters so that blood will rush into the capillary network of the mesenteries. And so, the dilation term here is suggesting that the mesenteries have a high demand for blood, that there's a great need for blood in the mesenteries. And so we need to choose one of these answer options that suggest that there is a great demand for blood in the mesenteries. And so, the first answer that we can eliminate right off the bat is answer option d, which suggests a greater need for blood elsewhere in the body other than the mesenteries. But again, if there was a greater need for blood elsewhere in the body, then we wouldn't expect the dilation of these things in the mesenteries because we would expect the constriction of these things because that would allow for blood to be rerouted to other areas of the body where there was a greater demand for the blood. And so, again, this is not the case. There is not constriction. The problem mentions the dilation, and so there's not a greater need for blood elsewhere in the body. In this particular scenario, this is not going to be the best answer, so we can eliminate answer option d. Now answer option b suggests that there is adequate oxygen gas concentration in the mesenteries. But if there is already adequate oxygen gas concentration in the mesenteries, then there's no need to dilate and allow more blood to rush to the mesenteries, and so answer option b is not going to be the best answer. So now we're between either answer option a or answer option c. And answer option c says that there's increased pH levels near the mesenteries. Now, an increased pH is correlated with lower metabolic activity, and so if there's lower metabolic activity, then there's not going to be a greater demand for blood. And so for that reason, we can eliminate answer option c since it's not the best answer here. And so this leaves answer option a as the only option, which is the correct answer, and it says increased CO₂ gas concentration in the mesenteries or increased carbon dioxide gas concentration in the mesenteries. Now, increased carbon dioxide gas is an indicator of greater metabolic activity. And if there is greater metabolic activity, then there's going to be a greater demand for blood. And that would be a scenario that we would expect the dilation of arterioles and precapillary sphincters in the mesenteric capillary beds. So answer option a is the best answer for this example problem, and that concludes this example, so I'll see you all in our next video.

Which scenario results in constriction of arterioles & precapillary sphincters in mesenteric capillary beds?

• A. Increased metabolic activity in mesenteries.
• B. Decreased pH level near mesenteries.
• C. Low $O_2$ concentration in mesenteries.
• D. A greater need for blood elsewhere in the body.

Which structure regulates blood flow distribution in mesenteric capillary beds & diverts blood from one area to another based on local metabolic needs?

• A. Precapillary sphincters.
• B. Arteriole valves.
• C. Capillary gates.
• D. Venous pumps.

Which of the following correctly describes the flow of blood through a mesenteric capillary bed when the precapillary sphincters are constricted?

• A. Terminal arteriole > metarteriole > capillaries > thoroughfare channel > postcapillary venule.
• B. Terminal arteriole > metarteriole > thoroughfare channel > postcapillary venule.
• C. Terminal arteriole > vascular shunt > capillaries > postcapillary venule.
• D. Terminal arteriole > thoroughfare channel > metarteriole > postcapillary venule.

Here’s what students ask on this topic:

What are the main functions of capillaries in the circulatory system?

Capillaries are the smallest blood vessels in the circulatory system, and their primary function is to facilitate the exchange of nutrients, gases, and waste products between the blood and surrounding tissues. This exchange includes oxygen and carbon dioxide, glucose, amino acids, and other essential substances. The thin walls of capillaries, composed of a single layer of endothelial cells, allow for efficient diffusion. Additionally, capillaries help regulate blood flow and pressure within tissues, ensuring that each cell receives adequate nutrients and oxygen while removing metabolic waste products.

What are the different types of capillaries and their characteristics?

There are three main types of capillaries: continuous, fenestrated, and sinusoid. Continuous capillaries have tightly joined endothelial cells with small intercellular clefts, making them the least permeable. They are found in the skin, muscles, lungs, and central nervous system. Fenestrated capillaries have pores (fenestrations) in their endothelial cells, increasing their permeability. They are located in areas of active filtration, secretion, and absorption, such as the kidneys, endocrine glands, and small intestines. Sinusoid capillaries have large gaps and a discontinuous endothelium, making them the most permeable. They are found in the liver, bone marrow, and spleen, allowing large molecules and cells to pass through.

How do capillary beds function in microcirculation?

Capillary beds are networks of interconnected capillaries that facilitate microcirculation, the flow of blood through the smallest blood vessels. Blood enters the capillary bed from arterioles and exits through venules. Within the capillary bed, exchanges of gases, nutrients, and waste products occur between the blood and surrounding tissues. Precapillary sphincters, rings of smooth muscle at the entrance of capillaries, regulate blood flow into the capillary bed. When these sphincters contract, blood flow is reduced, and it is redirected through a vascular shunt, bypassing the capillary bed. This mechanism ensures efficient tissue perfusion and nutrient delivery based on the body's needs.

What is the role of precapillary sphincters in regulating blood flow?

Precapillary sphincters are rings of smooth muscle located at the junctions between arterioles and capillaries. Their primary role is to regulate blood flow into the capillary beds. When the precapillary sphincters contract, they constrict the entrance to the capillaries, reducing or stopping blood flow into the capillary bed. This action redirects blood through a vascular shunt, allowing it to bypass the capillary network. Conversely, when the sphincters relax, blood flows freely into the capillaries, facilitating nutrient and gas exchange. This regulation ensures that blood is directed to tissues based on their metabolic needs, optimizing tissue perfusion and nutrient delivery.

Where are fenestrated capillaries commonly found, and what is their function?

Fenestrated capillaries are commonly found in areas of the body where active filtration, secretion, and absorption occur. These locations include the kidneys, where they filter blood to form urine; the endocrine glands, where they facilitate the secretion of hormones into the bloodstream; and the small intestines, where they absorb nutrients from digested food. The presence of small pores, or fenestrations, in the endothelial cells of these capillaries increases their permeability, allowing for the efficient exchange of larger molecules and fluids between the blood and surrounding tissues.