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Learn about how the arteries use nerve impulses to help regulate blood pressure. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.
Video transcript
Let's talk about blood pressure homeostasis and what homeostasis means is balance. But how is it that our body is able to create balance for our blood pressure? So this is the heart and we've got branches of the aorta coming off of it. I haven't been drawing these braches every single time but this time i think it's quite helpfull to see. We've got here the left brachial artery going to the left arm. And we've got the left carotid artery here. And again I'm writing left and right from the perspective of the person who's heart this is. And you've got the right carotid artery here and the right brachial artery. This is blood going to the right arm. And we've got blood going to the right neck. One interesting thing if you look at the right carotid is that it bulges right here in fact both sides do and they bulge right before they split and so that bulge is actually called the carotid sinus. We call it that because a sinus is any cavity. And so this is the right carotid sinus and this is the left carotid sinus. Another spot I am going to talk about is the aortic arch which is right there. So these 3 spots, the 2 carotid sinuses and the aortic arch are really really interesting and actually they are very important for learning how it is that our body is able to create balance in our blood pressure. So at the top I drew kind of a blown up version of the carotid sinus. And at the bottom is the aortic arch. If you'd look closely under a microscope you'd see nerve endings at the outer layer of the vessel. So these nerve endings basically join up and form a nerve and these on the carotid sinus do the same thing. They basically are going to send 2 large nerves that are going to go off. They send information about what's happening in the blood vessel, specifically about stretch. So as blood is pulsing through this vessel right here, this carotid sinus, or as it's pulsing through the aorta even, that wall is being stretched out and as it gets stretched, these nerves they're actually called baroreceptors, baro- meaning pressure, and these receptors for pressure, these baroreceptors are feeling the effects of stretch. What they do is that they send a signal down the nerve that tells the brain how much stretch is happening. And so if this is the brain and here you have your mid brain, these nerve endings are actually going here and tell the brain, communicate information about how much stretch is happening in those vessels. Now we know the more pressure is in the vessels the more it's going to stretch. So, follow me in a little example So let's say we have blood pressure over here. And we have our blood pressure at 115 over 75. In green we have action potentials per minute. What happens is that, as my blood pressure is 115/75, those nerves are getting a certain amount of stretch, whatever that amount is. And they're gonna send a signal not just one, but they're gonna send a handful. Let's say they send 10 signals. I'm gonna draw them up there. 5, 6, 7, 8, 9, 10 in one minute. Just imagine that both nerves are doing that, right? They're doing 10/minute. Well, that's a pretty normal number let's say. And this over times becomes what my brain regards as my normal setpoint. The brain starts to assume that if 10 action potentials are fired per minute, then that's pretty normal for me. The brain regards this as my normal setpoint. Now, if my pressure goes up, let's say that I'm running late to an exam or something happens that really worries me and my pressure goes to 140/90. Now I have hypertension. And this is my new pressure, this would be much higher than normal. My body would register this. And my nerves would start firing, let's say 30 times per minute. So if they're firing 30 times per minute, then my body is thinking, or my brain is thinking: well, that's higher than normal. So, this must be high. It regards this as high. And on the flipside let's say that, you know, I've cut my arm and lose a lot of blood, my blood pressure starts to fall. My stretching is going to happen less than before, so it's gonna send less action potentials per minute. Maybe only 7 per minute. And again, my mid brain is gonna get 7 little green arrows per minute. 7 action potentials per minute. And it's gonna think: well that's very odd, before it was 10 per minute. This represents a fall in blood pressure. So now you have high blood pressure in pink and a fallen blood pressure in blue. So what exactly can the brain do to help normalize or create balance? So let me write that over here. Let's write 'response?' So the body has a couple of strategies. And they're basically summed up in the autonomic nervous system. There are two major branches of your autonomic nervous system or two parts to it, let's say parts. One is called the sympathetics, almost like sympathy. And the other one is called parasympathics. They're very similar words, except the word para- is in front of this one. And I want you to remember now, that there's a formula. And I'm gonna write that formula down here, just to remind us that pressure equals flow times resistance. And additionally, I want you to remember that flow, this one right here, is going to be related to stroke volume times heart rate. So if I can do anything, my body can do anything to raise the stroke volume, or the heart rate, or the resistance, then my pressure will go up. And vice versa, if I can drop the stroke volume, or heart rate, or resistance, then my pressure will go down. Ok. So what this sympathetics do, is they have an effect on the heart and the vessels. The heart and the blood vessels all over the body, not just the carotid sinus or the aortic arch, I'm talking about all blood vessels. And so the sympathetics are going to, for the heart, they're going to increase the heart rate and they're going to increase the stroke volume. And the parasympathetics do the opposite. They actually drop the heart rate and drop the stroke volume. And the way that they do that. The heart rate is controlled by how many beats you get per minute, obviously that's the heart rate. The sympathetics are gonna cause the heart cells that control that, to work faster. The parasympathetics will slow them down. And for the stroke volume, the sympathetics force the heart to contract harder and then you have more volume of blood gushing out every beat. The parasympathetics make the heart work less forcefully, so you have less blood gushing out with every beat. And the sympathetics finally, they actually cause vasoconstriction. And, you guessed it, the parasympathetics do the opposite. So they cause vasodilation. And vasoconstriction and vasodilation basically mean whether the artery stays open or closes down. So for the sympathetics, the arteries and arterials primarily, mostly they're arterials, they start getting smaller. And as they get smaller, that increases resistance. And for the parasympathetics, they will cause the arterials to get bigger, to dilate and that will cause the resistance to fall. So taking a quick peak at our equation, that I wrote out for you on the right you can see that the sympathetics basically do everything that will help to increase the pressure. So if you have a pressure again of 140/90, then what would happen is, your body will see that as a high pressure and will try to get the parasympathetics to be active. It will activate all the parasympathetic nerves. And if your pressure is low, it's 90/60, then the body is going to respond by getting all the sympathetics to react. You see how it works? And of course if your pressure is, let's say 115/75, and the baroreceptors are firing the usual 10 times per minute, then there should be really over all no response. Because the body thinks: well everything is already balanced, there is nothing more to do. So this is how the body is able to control blood pressure in a rapid way. That's the final point I want to make, that the input here, the baroreceptors, these are nerves and the autonomic nervous system, obviously these are nerves. So the information going in is the baroreceptors, the information going out is the autonomic nervous system. All this is happening rapidly. This is all very rapid. And when I say rapid, I mean on the order of seconds to minutes. So within seconds to minutes this response can happen. So this is a fantastic example of how your body can take in information really quickly and really respond quickly, to help keep your blood pressure balanced.