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Blood Vessel Types
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0:00
We've been focusing on the heart here and mentioning various types of blood vessels, but now we want to take a moment to differentiate among the five types of blood vessels with regard to their structure and function.
0:14
The five types of vessels are, arteries, arterials, capillaries, venules, and veins. Arteries are blood vessels that, ultimately, take blood away from the heart. A trick to remembering this is that artery starts with A, and A is also the start of away. And so arteries bring blood away from the heart. As the arteries travel throughout the body, as you can see here, they branch off into smaller and smaller vessels. Eventually, they becomes small enough that we start referring to them as arterials.
0:51
Those arterials continue to branch till we reach capillaries. The capillaries spread throughout the tissue, allowing for gas exchange and nutrient and waste exchange between the cells and themselves. The capillaries collect again into the venules, and many of them collect together into larger and larger veins, and the veins ultimately bring blood back to the heart. One analogy that you sometimes hear about is that of roads. The arteries and veins represent the largest roads, so highways.
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The arteries allow a lot of blood to flow through them. But then as those highways move along, they branch off into smaller roads. Those smaller roads then branch off into smaller neighborhood roads, where, in this case, the analogy is we're dealing with capillaries that allow for exchange of oxygen and nutrients in the body. But these side streets allow for people to go into their driveways or come out of their driveways onto the small road. Going to venules represent the small side streets
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collecting to larger roads and then back to the vein, which is represented by a highway, letting lots of blood through.
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If we were to take a closer look at the artery by looking at a cross section, we can see that it's made up of three major layers, the tunica intima, tunica media, and tunica adventitia. The tunica intima is made up of an endothelial layer. Endo, again, represents inside, and endothelium is the single layer of cells that is nice and smooth to allow the blood to easily flow through it. The tunica media is made up of elastic tissue as well as smooth muscle.
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The tunica adventitia is made up of connective tissue that allows the artery to connect to the surrounding tissue and also provides protection to it. Taking a look at the vein, it's made up of the exact same three layers with the same constituent layers that make it up. There are some differences here, however, between the veins and the arteries. You'll note that the smooth muscle layer within the veins is much thinner than the smooth muscle
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layer in the arteries, that's because the arteries have much higher pressure blood traveling through them. Well look at that a little more closely in a few moments, but the blood that is going into the arteries is coming right out of the heart, because remember, A for away. So the heart is contracting and squeezing blood under high pressure out into the arteries. We want a nice strong elastic tissue and then smooth muscle layer in order to hold all that pressure in.
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Because the blood in the veins is under lower pressure, the vein has valves within it. The valves prevent backflow within the veins, because the blood is under such low pressure. Moving on, we can compare the arteries and veins to the capillaries. As this image indicates, the capillaries are made up of only one layer, the endothelial layer. As mentioned earlier, the endothelium is only one cell thick, and this is because, ultimately, the capillaries
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are the site for gas and nutrient and waste exchange.
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If we look at the capillary here, it's where oxygen will go out from the blood into the cells, where water will go out, where electrolytes and glucose will go out into the cells surrounding the capillary. It's also the site where wastes and carbon dioxide come from the surrounding cells into the vessel. The analogy here is, this is the restaurant, the site where we're getting nutrients and where wastes are being brought away from the cells. Ultimately, the capillary is made up
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of only the endothelial layer, so that we can move materials out of the capillary or into the capillary very easily. To compare that to capillary here to the veins and to the arteries, the veins and arteries have three layers that are quite thick, that would be very inefficient at letting nutrients in and waste out. And so the capillary is only this one layer, the endothelium, that is one cell thick, to allow for this exchange. So how does this exchange occur?
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Let's start by looking at an analogy of this needle here. Imagine this needle, where there'd been holes drilled into the side of the needle. As there was a force applied to the plunger of the needle, what would happen is that, the water that was originally inside the needle would move out both the tip of the needle and all the holes within the needle itself.
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A similar thing goes on with the capillaries. What we see here is that there is a force from the heart pushing the blood through our blood vessels. As that blood travels through our capillaries, because they're thin and there are spaces between the cells, that pressure allows for pushing out of water and glucose and electrolytes and other materials that those cells would need. This type of pressure is called hydrostatic pressure, and it allows for outward movement which is called filtration.
6:17
Because theres' high pressure within the capillary, the fluid and the components within the fluid actually move out of the capillary to the surrounding cells. Another type of pressure that's important is oncotic pressure. It's involved with osmosis, and this is inward movement. As materials move out from the capillaries and a bit of glucose electrolytes leave and a lot of water is pushed out because of the hydrostatic pressure, what happens is that, we end up with blood left
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behind that has a high amount of protein within it and other dissolved materials.
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This is now at a larger concentration than the surrounding tissue, and so the water is attracted back towards those plasma proteins and towards those electrolytes that have remained behind within the blood. Ultimately, these two types of movement equal out, where there's filtration ongoing, but eventually the blood gets to a point where it is concentrated enough that there's an inward movement of water back into the blood, drawing that water from the tissues into the blood.
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Earlier, we looked at the three layers of the veins and the arteries. Here, if we take a look at an arterial, we can see the smooth muscle that makes up the tunica media can actually relax, causing vasodilation.
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Or what we can have is that, the smooth muscle contracts causing vasoconstriction. The vaso in both of these terms is referring to the blood vessel, dilation referring to relaxation or getting bigger, and constriction referring to getting smaller. This is an important concept in a number of ways. One is our body controlling our blood pressure. Our blood pressure can be changed based on contracting or relaxing our blood vessels. If we relax our blood vessels, if we undergo vasodilation,
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our blood vessel gets bigger and decreases our blood pressure. If we need higher blood pressure for some reason, then what can happen is vasoconstriction, and our blood vessel will get smaller, not allowing as much space for the blood to travel through and therefore increasing the pressure of the blood pushing against the walls of the blood vessel. A second way this is important is that our body can control what parts of our body more or less blood is flowing to.
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If we need more blood in a certain area relative to another, we can have vasodilation. If we need less blood in an area relative to another, we can have that have vasoconstriction, closing off how much blood is actually flowing to that area. So for example, if I were to go for a run, I would need lots of blood flowing to the muscles of my legs, and so I would have vasodilation in the vessels going to the muscles of my leg. If I'm going for a run, then I wouldn't necessarily
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need as much blood flowing to my digestive system to digest food, and so I would have vasoconstriction of the blood vessels that are going to the organs of my digestive system. As mentioned, in this way, we can control our blood pressure. But let's take a look at the overall blood pressure within our body. We mentioned systolic pressure and diastolic pressure at the contraction pressure of our heart and the relaxation pressure of our heart. When a heart beats and blood flows out of it,
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the arteries directly after the heart, in this case we're showing the aorta, have the highest amount of blood pressure within them. As the aorta continues to branch down to large arteries and then smaller arteries to arterials and capillaries, the blood pressure within those vessels continues to decrease. We get down to venules, small veins, large veins, and then vena cava, the blood pressure within those vessels is very low. And so one might ask, how does the blood actually
10:20
even get back to the heart if the pressure within those veins is so low? Obviously, the blood is moving within the arteries and capillaries because of the pressure of the heart beating. But within the veins, there are a couple ways that the blood is returned, and it does not deal directly with pressure from the heart. The first factor involved in blood return to the heart is skeletal muscle action. Here, we're showing a leg with the veins of the leg.
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How does the blood actually move up those veins all the way up to the inferior vena cava and then back to the heart? We've zoomed in here on a vein that is ultimately surrounded by muscle, because as much as we've been looking at individual blood vessels here, we need to remember that those blood vessels are embedded within other tissues. So here we have a vein with valves that is surrounded by muscle. As we move around and the muscles contract, as on the right side, what you can see happening
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is the muscles are flexing and contracting and squeezing up against the vein. As the muscles squeeze up against the vein, they squeeze the blood up the vein towards the heart. You can see that the top valve is open, allowing the blood to move up, while the bottom valve is closed, not allowing the backflow of the blood down the vein. When our muscles relax, we move back to the left image, where the vein opens up a little bit. But as the blood tries to flow backwards,
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the top valve is now closed, preventing backflow. As we continue to move around and we contract, again, more blood is squeezed up, pushing the blood we mentioned earlier up the vein even further. And then the valves close when the muscles relax, preventing backflow. And this continues over and over again and sort of milks the blood up the veins back towards our heart. Another factor involved in blood return to the heart are respiratory movements. So here, we can see, again, that the heart
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is within the thoracic cavity within the media steinem between the lungs. We looked at this zoomed in picture earlier on. As we breathe, we expand the size of our thoracic cavity, so our chest gets bigger. As our chest gets bigger, the pressure within our chest actually drops. Because the blood vessels that we saw earlier on are within that chest cavity, the pressure within them drops. So the superior and the inferior vena cava have a drop in pressure.
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The blood that is lower down in the body or higher up in the body is under higher pressure. And so the blood is squeezed from high pressure to low pressure, being forced back to the heart. One final factor that we should look at brings us back to this earlier image. Even though the smooth muscle layer here of the vein is much thinner than the artery, it can contract as well, causing vasoconstriction. And when that smooth muscle contracts causing vasoconstriction, it allows for greater movement
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of blood back to the heart.
13:32
The next learning outcome is related to the artery that we see here. What is the pulse? Ultimately, we have a number of areas on the body that we can actually take the pulse. One of the main ones here is highlighted, the radial artery, the artery that runs along our radius within our forearm. But there are a number of different arteries here where we can take pulses. But part of the reason that we show this is so that it can help us to define pulse. As our heart beats, blood leaves the heart out
14:04
into an artery under high pressure. And then as the heart relaxes, the blood is not under as much pressure. Therefore, the blood that's coming out under high pressure and then lower pressure and then high pressure again causes an alternating expansion and then recoil of the arteries with each beat of the heart. This expansion as the high pressure goes through and then the recoil as the artery goes back down to the original size that it was is what the pulse is.
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In the end, if we were to take the pulse at the radial artery here, what we're ultimately feeling is the beating of the heart. But it is the high pressure and then low pressure of the blood that started originally at the heart and has now moved down through the arteries out to our radial artery. Therefore, we're able to measure the heart rate, but what are the factors that ultimately affect the heart rate?
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We'll start with this equation here. Cardiac output is equal to heart rate times the stroke volume. What cardiac output is referring to is the volume of blood that is put out by the heart per minute. It's equal to the heart rate, which is the number of beats per minute multiplied by the volume that comes out with each beat of the heart. Let's start by taking a look at stroke volume.
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Here we have two situations. On the left side, we have high venous return to the heart, and on the right side, we have decreased venous return to the heart. Why might this be the case? We just finished taking a look at what allows venous blood to return to the heart. One of those factors was the flexing of muscles. If I were to go for a run, as I go for my run, my muscles are flexing quite a bit, and so I have a lot of pumping of my blood back to my heart.
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Therefore, returning a high amount of blood through my veins back to my heart, and my ventricles will stretch more and more as the higher volume of blood is pumped from the atria down into them. Because I'm putting more blood into my ventricles, every time my ventricles beat, I actually have an increased stroke volume. On the right side might be an example of where I'm sitting at home watching TV. I'm not returning as much blood to my heart through my veins,
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because I'm sitting on the couch not really moving. Therefore, my ventricles stretch less, and each time they beat, they're actually pumping out less volume, therefore, my stroke is decreased. In order to calculate cardiac output, as we said in the equation here, it's the heart rate times the stroke volume. Therefore, my heart can actually do one of two things in order to increase the cardiac output. As we just looked at, we can increase the stroke volume.
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But another thing that my heart can do is actually beat faster. And my heart can beat faster because I send signals from my brain along my sympathetic neurons that send signals to my SA node, the pacemaker of my heart, saying to beat faster. Therefore, I can increase the volume per beat, but I can also increase the rate at which my heart is actually beating. This unit is quite large, and there's a number of different concepts that we've gone through.
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Now we want to put some effort into making a connection between the various concepts. So we'll return to defining blood pressure, but then we also want to list factors that ultimately affect our blood pressure. We can measure blood pressure using a blood pressure cuff. What happens is that, we put the blood pressure cuff around the arm. The brachial artery, shown here, is the artery that runs down the arm. After we put the cuff to measure pressure on the arm,
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we continue to inflate the cuff until there's enough pressure that we do not hear a pulse. The brachial artery is now closed by the cuff, and no blood is flowing.
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As we start to release the air from the cuff, the pressure of the cuff decreases until we hear the first sounds of the blood squeezing through the constricted artery. This value that we get is our highest blood pressure value. Under normal conditions, this is somewhere around 120 millimeters of mercury. As we then continue to release air from the cuff, we get to a point where we no longer hear a sound. The sounds typically disappear around 80 millimeters of mercury.
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The blood is freely flowing back through the brachial artery, and so this is the low pressure point. Again, the high pressure point is related to the contraction of the heart, and so it's called the systolic pressure. The lowest pressure is related to the relaxation point of the heart, and so it's called the diastolic pressure. Going back to something we saw earlier in this unit, we can talk about the size of blood vessels and their effect on blood pressure.
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Arterials, as we looked at earlier, can vasodilate, they get bigger. And therefore, there is a decreased resistance to the blood flow moving through them. They can also vasoconstrict, get smaller. And so then there would be an increase to the vascular resistance. An analogy to this would be the flow through a garden hose. On the left side, k have a garden hose that's wide open on the end. We turn on the nozzle, and water droplets come out the end of the garden hose.
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But if we were to put a nozzle on the end of the hose, or if we were to put our thumb over the end of the hose, even though the water is at the same level, we have a smaller size of hole that the water can move through. And now, the water is under higher pressure. This is one of the main factors that affects our blood pressure, because our body can undergo these natural processes to help affect our blood pressure. But how does our body know to control the size of these blood
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vessels or affect other things that ultimately affect our blood pressure? There's a number of different ways. We have special receptors in our body called baroreceptors. And so we can see here, the carotid artery has the carotid barreceptors, and the aorta itself has the aortic baroreceptors. These pressure receptors are located in the walls of these vessels, and they can sense sudden changes in blood pressure and send signals up sensory nerves to our medulla oblongata.
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The medulla oblongata, in the brain stem then, processes the sensory information, and it can then decide what to do. If the blood pressure becomes too low, it can send information both to the heart as well as to blood vessels, telling them what to do in order to affect the pressure. If the blood pressure is low, the medulla oblongata tells the heart and blood vessels to increase the pressure. It sends sympathetic information to the heart, causing it to beat faster and contract harder.
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It also sends signals to arterials to constrict so that the blood pressure goes up. If the medulla oblongata gets information that the blood pressure is too high, it sends signals to the heart and to the blood vessels to do the opposite in order to decrease blood pressure.
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