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 billion up to about 100 billion capillaries in one 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 gasses like oxygen and carbon dioxide gas. 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, which you'll notice is that the red side over here is carrying oxygenated blood. So, uh 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 venues and start to make its way back to the heart. And so, uh 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. Uh As long as it's flowing from the oxygenated side to the deoxygenated side. And so, uh 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.
2
concept
Types of Capillaries
Video duration:
9m
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In this video, we're going to talk about the types of capillaries. And so structurally, there are three types of capillaries that we have numbered down below in our text, 12 and three. 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 penetrable 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 macro molecules 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 two. 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 clefs, which are these relatively small gaps that can be found in between the cells. And so it's also important to note that these intercellular 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 intercellular clefts can vary. And so notice that down below in the bottom left of the image, we're labeling the intercellular 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 see 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 uh 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. Uh 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 intercellular clefts of the continuous capillaries. And really that is it for the fenestrated capillaries. So 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 you 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 uh more blood, a whole 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 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 uh make them very, very permeable. And they also have much larger fenestrations, these larger holes in the endothelium. And these uh 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 uh either of these other two types of capillaries. So that's what keeps it together, it's not going to fall apart. And so uh what's important to note is that because it has such a discontinuous nature in these much larger holes. Uh it's going to allow for the diffusion of relatively large substances, relatively large uh molecule macro molecules. And even uh 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 uh in the liver, uh 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 year 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.
3
example
Capillaries Example 1
Video duration:
2m
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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 three potential answer options down below which are the three 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, uh 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 uh 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, uh 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, uh they would allow for the fusion of smaller molecules to occur more rapidly. And so, uh we would expect to find them more so than uh 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, uh 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 uh 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.
4
Problem
Problem
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.
5
concept
General Structure of Capillary Beds
Video duration:
5m
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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. And so recall from our previous lesson videos that capillaries do not function independently. Instead, capillaries exist within capillary beds, which recall are an extensive network of numerous branched and connected capillaries. And so a typical capillary bed could have anywhere between about 10 all the way up to about 100 capillaries. Now notice that down below in our image, we're showing you the typical structure of voc capillary bed on the left hand side and notice that all of its components are labeled as well. And notice that over here on the far left of the capillary bed, we have an arterial, which recall is a small artery that carries blood away from the heart and towards these capillaries. And of course, we know that blood vessels are able to branch including arterials. And so notice that there's a branch right here. So the arterial continues on downward. But again, it's branching over here toward this capillary bed that again consists of an extensive network of numerous branched and connected capillaries. And what you'll notice is that just before this capillary bed, it's being labeled as the terminal arterial. And so the terminal arterial as its name implies with the term terminal is the region that is near the end of an arterial. And so it's going to be feeding blood immediately into the capillary bed. Now, of course, once blood transitions from the terminal arterial into the capillary bed, this is where the exchanges are going to occur between the blood that the capillaries are carrying and the tissues that immediately surround these capillary beds. And so, uh from the capillary uh bed, the blood is going to transition into what's being labeled as the postcapillary venue. And so the postcapillary venue is going to be receiving or it receives blood from the capillary bed. And so this postcapillary venue is really the smallest of all of the venues and blood from the postcapillary venue is then going to transition into this venue that you can see right here. And of course, the blood in the venue is start is going to start to uh carry the blood back towards the heart. And really that is it for the typical structure of a capillary bed. Now, it is also important for you all to know the term micro circulation. So what when the term circulation is used, it's referring to the blood flow throughout our entire body. However, the term microcirculation refers to the blood flow through a capillary bed from the terminal arterial through the capillary bed and into the postcapillary venue. And so uh that is exactly what we had just discussed here moments ago. And so what can help you remember this is that the root, micro is a root that means small. And so micro circulation refers to the blood flow through these really, really small capillary beds. Now, it's important to remember that capillaries and capillary beds do not have smooth muscle. And so capillaries are not capable of changing their diameter through vasoconstriction and or vasodilation. However, recall that arterials can have smooth muscle. And so this means that arterials can vasoconstrict. And so notice that over here on the right hand side, we're showing you the vasoconstriction of these arterials and the terminal arterials with these yellow arrows that you can see. And so vaso constriction of the these arterials is capable of redirecting and it can redirect blood flow away from the capillary beds. And so again, on this right hand side of the image, notice we're telling you that vasoconstriction of the arterial and the terminal arterial is capable of reducing the blood flow through the arterial and reducing blood flow into the capillary bed and redirecting most of the blood flow to other areas of the body. So vasoconstriction can redirect blood flow away from capillary beds and it may do this in order to redirect the blood flow to other areas of the body where that blood may be needed much more. And so it's amazing that our body has evolved the ability to redirect blood to other areas of the body in this type of way. So that's pretty fascinating. And so this year concludes our brief video on the general structure of capillary beds. And 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 more about blood vessels in general. So I'll see you all in our next video.
6
example
Capillaries Example 2
Video duration:
4m
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So here we have an example problem that wants us to choose one of the four 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. So notice that over here on the left, what we have is tissue number one and over here on the right, what we have is tissue number two. And so tissue number one is in a scenario where it needs more blood. Whereas tissue number two is in a scenario where it does not really need more blood. And so uh because tissue number one needs more blood, uh the arteries that are going toward tissue number one 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 two, because it does not need more blood. Uh it is going to have its arteries that are feeding blood to it, uh constrict. And that means that it will narrow down to reduce blood flow, going to tissue number two. And that will ultimately help to redirect the blood flow to tissue number one, who needs that blood even more. And so 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 one must be the skeletal muscles. Uh so we can label it as so and uh it needs more blood so its arteries will dilate. But tissue number two, in this case must be the uh digestive system. So we can go ahead and label it as so. And uh what you'll notice is that it does not need more blood. And so uh 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 arterials or something of that nature. And then over here, we would expect that it's going to constrict. So 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 uh 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 uh 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 or 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 arterials in the digestive system. But option C says that they will dilate whereas option D says they will constrict and again in the digestive system, as we just discussed, the arteries or terminal arterials in this case, are going to constrict. And so that means that answer option D, terminal arterials 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.
7
Problem
Problem
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.
8
Problem
Problem
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.
9
concept
Structure of Mesenteric Capillary Beds
Video duration:
6m
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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. And so the mesentery are these se membranes of the digestive system that hold our intestines in place. And these mears 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. And so let's take a look at our image down below so that we can visualize this vascular shunt. So notice on the left over here, we're showing you the capillary bed structure of the mesentery, 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 and this is the vascular shunt. And so the vascular shunt allows for blood to be shunted or for blood to pass directly from the terminal arterial through to the postcapillary venue. And so this vascular shunt actually consists of two parts. The first part is the Met arterial and the second part is the thoroughfare channel. And the numbers that we have up above in the text correspond with the numbers that you can see down below in the image. And so again, the first part of the vascular shunt is the met arterial. And so you can see highlighted here and uh bracket it is the met arterial and then the thoroughfare channel is going to be the remainder of the vascular shunt. Now, the med arterial is a transitional blood vessel because it has intermediate characteristics of an arterial and a capillary. And the most key defining feature of these met arterials is that they contain what are known as precapillary sphincters on its branches into the capillary bed network. And so 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. And again, these are rings of smooth muscle that wrap around the branches of the met arterial into the capillary bed network. And when these precapillary sphincter 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. And so, uh if we take a look at the image down below. Again, you can see those precapillary sphincters being circled right here. And again, when they contract, they redirect the blood flow only through the vascular shunt. Now, the second part of the vascular shunt is the thoroughfare channel and the thoroughfare channel is really just the continuation of the met arterial. So it is a med arterial continuation. But the biggest difference is that the thoroughfare channel lacks smooth muscle, including the precapillary sphincter. So the met arterial has smooth muscle with precapillary sphincter. But the third thoroughfare channel does not have any smooth muscle or precapillary sphincters. So as soon as the met arterial loses its precapillary sphincters, it then is considered the thoroughfare channel. And so the thoroughfare channel will directly connect the met arterial to the postcapillary venue. Now again, 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. And so notice that the image at the top right up here is focusing in on the vaso constriction of just the arterial and the terminal arterial just like what we saw in previous lesson videos. And so notice that the uh vaso constriction of the arterial is being indicated by these yellow arrows highlighted here. And the vaso constriction of the terminal arterial is being indicated by those yellow arrows that you see highlighted right there. And of course, the vasoconstriction is going to reduce blood into the arterial, 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. And so 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 going to bypass uh all of the capillaries and the vascular shunt as well. Now down below, in this bottom right image, this is where the new material comes into play because vaso constriction of just the precapillary sphincters allows for more precise and more fine tuned regulation of the blood flow through the capillary uh network here. And so what you'll notice is that here we are showing a yellow background behind those precapillary sphincters to show that they are constricted. And again, when they constrict, it will prevent blood flow from going into the capillaries. And so notice that here, only the capillaries are transparent because there's no blood flow going through them. And so when the precapillary sphincter constrict baso constrict, they prevent blood from going into the capillaries and reroute the blood flow through the vascular shunt. And so in this image, blood flows via the vascular shunt only bypassing the capillaries instead of bypassing the capillaries and the vascular shunt. And so the vascular shunt again, allows for more precise and more fine tuned control. So it's pretty fascinating that our bodies have been able to evolve these mechanisms of uh redirecting blood flow. And so, uh that is what the vascular shunt allows for. And so this year concludes our brief lesson on the structure of mesenteric capillary beds. And as we move forward, we'll be able to apply some of these concepts and also learn more about blood vessels. So I'll see you all in our next video.
10
example
Capillaries Example 3
Video duration:
3m
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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 arterials and precapillary sphincters and mesenteric capillary beds. And we've got these four 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 arterials and precapillary sphincters in the mesenteric capillary beds. And so the dilation of the arterials will enlarge the diameter of these arterials and the dilation of the precapillary sphincters will relax the precapillary sphincters so that blood will rush into the capillary network of the meters. And so uh the dilation term here is suggesting that the meta have a high demand for blood that there is a great need for blood in the meta. And so we need to choose one of these answer options that suggests that there is a great demand for blood in the meter. 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 meta. 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 mesa because uh we would expect the constriction of the, 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. And this particular scenario, this is not gonna 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 mesa. But if there is already adequate oxygen gas concentration in the meta, then there's no need to dilate uh and allow more blood to rush to the meta. 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 mesa. 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 meta or increased carbon dioxide gas concentration in the meta. 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 would we would expect the dilation of arterials and precapillary sphincter 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.
11
Problem
Problem
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 O2 concentration in mesenteries.
D
A greater need for blood elsewhere in the body.
12
Problem
Problem
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.
13
Problem
Problem
Which of the following correctly describes the flow of blood through a mesenteric capillary bed when the precapillary sphincters are constricted?