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Let's talk about blood pressure homeostasis

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and what homeostasis means is balance.

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But how is it that our body is able to create balance

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for our blood pressure?

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So this is the heart and we've got branches of the aorta coming off of it.

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I haven't been drawing these braches every single time but this time i think it's quite helpfull to see.

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We've got here the left brachial artery

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going to the left arm.

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And we've got the left carotid artery here.

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And again I'm writing left and right from the perspective of the person who's heart this is.

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And you've got the right carotid artery here

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and the right brachial artery.

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This is blood going to the right arm.

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And we've got blood going to the right neck.

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One interesting thing if you look at the right carotid

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is that it bulges right here

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in fact both sides do

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and they bulge right before they split

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and so that bulge is actually called the

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carotid sinus.

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We call it that because a sinus

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is any cavity.

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And so this is the right carotid sinus

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and this is the left carotid sinus.

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Another spot I am going to talk about is the aortic arch

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which is right there.

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So these 3 spots, the 2 carotid sinuses and the aortic arch

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are really really interesting and actually

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they are very important

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for learning how it is that our body is able to create balance

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in our blood pressure.

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So at the top I drew kind of a blown up version of the carotid sinus.

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And at the bottom is the aortic arch.

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If you'd look closely under a microscope

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you'd see nerve endings at the outer layer of the vessel.

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So these nerve endings

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basically join up and form a nerve

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and these on the carotid sinus do the same thing.

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They basically are going to send 2 large nerves that are going to go off.

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They send information about what's happening in the blood vessel,

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specifically about stretch.

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So as blood is pulsing through this vessel right here,

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this carotid sinus,

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or as it's pulsing through the aorta even,

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that wall is being stretched out

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and as it gets stretched, these nerves

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they're actually called baroreceptors,

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baro- meaning pressure,

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and these receptors for pressure,

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these baroreceptors are feeling the effects

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of stretch. What they do is that they

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send a signal down the nerve

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that tells the brain how much stretch is happening.

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And so if this is the brain

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and here you have your mid brain,

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these nerve endings are actually going here

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and tell the brain, communicate information about

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how much stretch is happening in those vessels.

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Now we know the more pressure is in the vessels the more it's going to stretch.

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So, follow me in a little example

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So let's say we have blood pressure over here.

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And we have our blood pressure at 115 over 75.

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In green we have action potentials per minute.

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What happens is that, as my blood pressure is 115/75,

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those nerves are getting a certain amount of stretch, whatever that amount is.

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And they're gonna send a signal

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not just one, but they're gonna send a handful.

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Let's say they send 10 signals.

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I'm gonna draw them up there.

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5, 6, 7, 8, 9, 10 in one minute.

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Just imagine that both nerves are doing that, right?

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They're doing 10/minute.

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Well, that's a pretty normal number let's say.

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And this over times becomes what my brain regards as my normal setpoint.

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The brain starts to assume that if 10 action potentials are fired per minute,

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then that's pretty normal for me.

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The brain regards this as my normal setpoint.

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Now, if my pressure goes up,

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let's say that I'm running late to an exam

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or something happens that really worries me

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and my pressure goes to 140/90.

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Now I have hypertension.

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And this is my new pressure, 
this would be much higher than normal.

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My body would register this.

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And my nerves would start firing, let's say 30 times per minute.

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So if they're firing 30 times per minute,

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then my body is thinking, or my brain is thinking: 
well, that's higher than normal.

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So, this must be high.

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It regards this as high.

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And on the flipside let's say that, you know, 
I've cut my arm and lose a lot of blood,

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my blood pressure starts to fall.

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My stretching is going to happen less than before,

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so it's gonna send less action potentials per minute.

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Maybe only 7 per minute.

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And again, my mid brain is gonna get 7 little green arrows per minute.

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7 action potentials per minute.

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And it's gonna think: well that's very odd, before it was 10 per minute.

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This represents a fall in blood pressure.

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So now you have high blood pressure in pink 
and a fallen blood pressure in blue.

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So what exactly can the brain do to help normalize or create balance?

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So let me write that over here.

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Let's write 'response?'

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So the body has a couple of strategies.

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And they're basically summed up in the autonomic nervous system.

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There are two major branches of your autonomic nervous system

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or two parts to it, let's say parts.

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One is called the sympathetics, almost like sympathy.

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And the other one is called parasympathics.

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They're very similar words, except the word para- is in front of this one.

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And I want you to remember now, that there's a formula.

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And I'm gonna write that formula down here, 
just to remind us that pressure equals flow times resistance.

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And additionally, I want you to remember that flow, this one right here,

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is going to be related to stroke volume times heart rate.

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So if I can do anything, my body can do anything to raise the stroke volume,

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or the heart rate, or the resistance, then my pressure will go up.

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And vice versa, if I can drop the stroke volume, or heart rate, or resistance, then my pressure will go down.

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Ok.

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So what this sympathetics do, is they have an effect on the heart and the vessels.

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The heart and the blood vessels all over the body,

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not just the carotid sinus or the aortic arch, 
I'm talking about all blood vessels.

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And so the sympathetics are going to, 
for the heart, they're going to increase the heart rate

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and they're going to increase the stroke volume.

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And the parasympathetics do the opposite.

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They actually drop the heart rate and drop the stroke volume.

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And the way that they do that.

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The heart rate is controlled by how many beats you get per minute,

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obviously that's the heart rate.

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The sympathetics are gonna cause the heart cells that control that, to work faster.

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The parasympathetics will slow them down.

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And for the stroke volume, the sympathetics force the heart to contract harder

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and then you have more volume of blood gushing out every beat.

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The parasympathetics make the heart work less forcefully, so you have less blood gushing out with every beat.

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And the sympathetics finally, they actually cause vasoconstriction.

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And, you guessed it, the parasympathetics do the opposite.

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So they cause vasodilation.

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And vasoconstriction and vasodilation basically mean whether the artery stays open or closes down.

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So for the sympathetics, the arteries and arterials primarily, mostly they're arterials, they start getting smaller.

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And as they get smaller, that increases resistance.

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And for the parasympathetics, they will cause the arterials to get bigger, to dilate

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and that will cause the resistance to fall.

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So taking a quick peak at our equation, 
that I wrote out for you on the right

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you can see that the sympathetics basically do everything that will help to increase the pressure.

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So if you have a pressure again of 140/90, 
then what would happen is,

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your body will see that as a high pressure and will try to get the parasympathetics to be active.

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It will activate all the parasympathetic nerves.

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And if your pressure is low, it's 90/60, then the body is going to respond by getting all the sympathetics to react.

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You see how it works?

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And of course if your pressure is, let's say 115/75, and the baroreceptors are firing the usual 10 times per minute,

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then there should be really over all no response.

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Because the body thinks: well everything is already balanced, there is nothing more to do.

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So this is how the body is able to control blood pressure in a rapid way.

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That's the final point I want to make,

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that the input here, the baroreceptors, these are nerves

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and the autonomic nervous system, obviously these are nerves.

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So the information going in is the baroreceptors, 
the information going out is the autonomic nervous system.

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All this is happening rapidly.

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This is all very rapid.

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And when I say rapid, I mean on the order of seconds to minutes.

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So within seconds to minutes this response can happen.

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So this is a fantastic example of how your body can take in information really quickly and really respond quickly,

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to help keep your blood pressure balanced.