Regulation of blood pressure with baroreceptors 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.
Regulation of blood pressure with baroreceptors
- 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.
- 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.
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