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Video transcript
I'm going to quickly sketch out the human heart. We're also going to label some vessels coming off of it. So the big vessel, of course, is the aorta. This is the giant aortic arch. And the aortic arch has a couple of key branches that go, for example, up to the head and neck. It has other branches as well that go out to the arms. But these branches that are going up are the ones I'm going to focus on. So out here on this right side, we have the right common carotid artery. And it's called the common because, eventually, what's going to happen is it's going to bulge here, and then it's going to split. And it's going to split into the internal branch-- this is going inside-- and the external branch over here. So this would be called, for example, the right external carotid artery. And the same thing is happening on the other side, and we name it kind of the same way. We say, OK, there's an internal branch and an external branch. This would be the internal, and this might be the external branch of the left common carotid artery. So I think you're getting the idea now. These are named exactly the same way. And these are the ones we're going to focus on. Now, previously, we had talked about how, in these particular locations, in the internal side and then this bulgy side, we have what are called the carotid sinus. Or sinuses, I suppose. But the carotid sinus is right there. And the sinus refers to any open area or open space. And there's also an area over here in the aortic arch. And these two areas, they are the home for our baroreceptors. Our baroreceptors are basically little nerves that are going to detect pressure. So they're going to detect stretch, or pressure, that is in the vessels. And they're going to give information back to the brain. And that's going to help regulate our blood pressure. Now, in this video, we're actually going to focus on chemoreceptors. Chemoreceptors are also important in giving us information, but they're going to give us information about things like oxygen levels, carbon dioxide levels, pH of the blood, things like that. So these chemoreceptors-- and this gets confusing-- they're located in a similar region, but not exactly the same region. I'm actually going to shade in where our chemoreceptors might be, and then also you might get some over here. So these three areas are where chemoreceptors are. And they're very, of course, closely related to where the baroreceptors are, but they're actually in slightly different locations. And we call them the aortic body and the carotid body. And the reason we use the term "body" is that it's a body of tissue. So that's why that word gets used. And this is actually-- you can see now a slightly different location, and certainly a different job. So let me blow up some of these regions and show you, close up, what this might look like. So let me draw for you the carotid body on this side, and on the other side, we'll do the aortic body. And I'm basically just zooming in on it, so you can see up close what this might look like, so you can visualize it. So for the carotid body, you might have the external artery, the internal artery. And coming off of the external artery, you might have little branches, little branches serving this tissue that's in the middle. And these branches, of course, are going to branch some more. And you're going to get all the way down to the capillary level. And once you have little capillaries in here, there's going to be a bunch of little cells. And these cells are, of course, going to get the nutrition from the capillary. And taken together, all these cells-- if you zoom out of this picture, this would be a little body of cells or body of tissue. And that's why we call this the carotid body. And really, the same thing is going on on the aorta side. So on the aorta side, you've got little branches coming off of the aorta, of course. And these branches are going to branch again, and again, and again, and again. And eventually, you're going to get lots and lots of little capillaries. And these capillaries are going to serve all these little blue cells that I'm drawing here, and these are the chemoreceptors that we're talking about. So these blue cells together make up a body of tissue, and that's where we get the term "aortic body" and "carotid body." Now, on the carotid side, one interesting fact is that this body of tissue gets a lot of blood flow, in fact, some of the highest blood flow in the entire human body. It's about 2 liters per minute for 100 grams. And just to put that in perspective for the carotid body, imagine that you have a little 2-liter bottle of soda. I was thinking of something that would be about 2 liters, and soda came to mind. And you can imagine pouring this soda out over something that's about 100 grams-- maybe a tomato. That's about a 100-gram tomato. And if you could do this in one minute, if you could pour out this bottle in one minute, imagine how wet this tomato's going to get, how much profusion, in a sense, this tomato is going to get. That is how much profusion your carotid body gets. So it really puts it in perspective how much blood flow's going into that area. So let's now zoom in a little bit further. Let's say I have a capillary. And inside my capillary, I've got a little red blood cell here, floating around. And my red blood cell, of course, has some hemoglobin in it, which is a protein. And this protein has got some oxygen bound to it. I'm going to draw little blue oxygen molecules. And of course, there's some oxygen out here in the plasma itself as well. And if we're in our carotid body or aortic body, you might have these special little blue cells that I've been drawing, our peripheral chemoreceptor cells. And specifically they have a name. These things are called glomus cells. I had initially misstated it as a globus cell. But actually it's an M-- glomus. And these oxygen molecules-- these are oxygen molecules over here-- are going to diffuse down into the tissue and get into our glomus cell. It's going to look something like that. And if you have a lot of oxygen in the blood, of course, a lot of molecules are going to diffuse in. But if you don't have too much in here, then not too much is going to make its way into the cell. And that's actually the key point. Because what our cell is going to be able to do is start to detect low oxygen levels. Low oxygen levels in the glomus cell tells this cell that, actually, there are probably low levels in the blood. And when the levels are low, this cell is going to depolarize. Its membrane is going to depolarize. And what it has on the other side are little vesicles that are full of neurotransmitter. And so when these vesicles detect that, hey, there's a depolarization going on, these vesicles are going to dump their neurotransmitter out. And what you have waiting for them is this nice little neuron. So there's a nice little neuron waiting patiently for a signal, and that signal is going to come in the form of a neurotransmitter. So this is how the communication works. There's going to be a depolarization, the vessels release their neurotransmitter, and that is going to send an action potential down to our neuron. And if the oxygen levels fall really low, let's say they get dangerously low, where the cell is very unhappy, then you're going to get much more neurotransmitter getting dumped out, and you're going to get many more action potentials. So this is how the glomus cell helps to detect oxygen. And in fact, it also detects carbon dioxide. Because, remember, this cell is going to be making carbon dioxide. Let's say this is a little molecule of CO2, and that CO2 is going to diffuse out and into the blood. Well, let's say that the blood has a lot of carbon dioxide already. Let's say that it's loaded with carbon dioxide, lots and lots of it. In this situation, it's going to be very difficult for carbon dioxide to make its way from the glomus cell all the way out into the plasma. And as a result, carbon dioxide starts building up. The tissue starts gathering more and more CO2, because it can't go anywhere. And this glomus cell is going to say, hey, wait a second. Our CO2 levels are starting to rise. There are high CO2 levels. And again, that's going to make the cell unhappy, and it's going to send out more neurotransmitter, and it's going to, of course, send out more action potentials. So two different reasons why you might get action potentials coming out of this glomus cell. And now I want to remind you that there's this little formula. There's this formula where carbon dioxide binds with water, and it forms H2CO3. And that's going to break down into bicarbonate and a proton. So this is our formula. So if CO2 levels are rising, like the example I just offered, then the proton level must be high as well. So a high proton concentration-- I'm going to put it in brackets, to indicate concentration. Or another way of saying that would be a low pH. So these are the things that are going to make our glomus cell send off more action potentials. So if you're like me, you're thinking, well, wait a second. This is really interesting. Our cell is depolarizing. It can depolarize. It also has this neurotransmitter that I mentioned. Our glomus cell, then, right here in blue, is basically sounding a little bit like it has properties of a nerve cell. This is a nerve cell. And the reason for that is that, if you actually take a look, these two cells have a common ancestor cell. And so in development, when the fetus is developing, there is a type of tissue called the neuroectoderm. And both of these cells, this nerve cell and this glomus cell, both are derived from this neuroectoderm. So it makes sense that they would have a lot of common features. So we know the glomus cell is not a neuron, but it's going to be talking to neurons. In fact, you're going to have many neurons working together in this area. And they're going to join up, both in the aortic body and the carotid body. And these neurons, going back to the original picture, are going to meet up into a big nerve. And this nerve is going to be called the vagus nerve. The vagus nerve is going to be the one for our aortic body, sometimes also called cranial nerve number 10. And up here with the carotid body, we have a nerve as well. This is another nerve. This one, we call the glossopharyngeal nerve. So these two nerves, the vagus nerve and the glossopharyngeal nerve-- this one, this glossopharyngeal nerve, by the way, is cranial nerve number 9-- these two nerves are not part of the brain. They're headed to the brain, right? So these two nerves are fundamentally taking information from chemoreceptors that are outside of the brain. They're not located in the brain, right? They're peripheral, and they're taking information about chemicals and taking that information to the brain. That's why we call them-- these blue areas, the carotid body and the aortic body-- we call them peripheral chemoreceptors.