47. Why do you feel dizzy after standing up?

Dizzy after workout: 7 causes and what ...

Hey everyone. So in lecture 15, we talked about how sympathetic neurons and parasympathetic neurons can be used to help you regulate blood pressure. And just as a quick review before we jump into anything else, let’s just look at that diagram that we referenced a lot in lecture 15, and it was this one. It includes all the big components that were involved.

And specifically, all the key components of how we regulated blood pressure using homeostasis. So if you guys remember, just as a quick review, if blood pressure changes, so if it goes up, if it goes down, even if it’s normal. These baroreceptors in these key arteries are constantly sensing what the blood pressure is.

They then send that signal, that information, to our integrating center, the medulla in the brain. That will compare this value to our normal homeostatic blood pressure range. And if it decides that the blood pressure’s gotten too high or too low, it can change sympathetic and parasympathetic neuron activity to change blood pressure by affecting heart rate, stroke volume and vessel diameter, or the resistance to blood flow.

Now I said this was really awesome or I guess I said it was really awesome, I hope you think too. Because this happens extremely quickly. And so for example, if you wake up, you feel dizzy. We need a very, very quick change and neurons are the way for us to make a very quick change to signal very quickly. So just as a way to recap what we talked about in lecture 15, let’s try to use all the tools we have, sympathetic neurons, parasympathetic neurons, to help this guy.

He got up too fast and he paid the price. Blood pressure fell too much and he didn’t fix it fast enough before getting dizzy and passing out. So let’s look at that case specifically.

So if you’re sitting down or you’re lying down, it’s pretty easy for blood to flow evenly throughout the entire body. But as we’ve said, if you suddenly stand up very quickly, like maybe you wake up in the morning and you just jump out of bed. What literally happens is due to gravity blood will actually overall start draining downwards towards your lower extremities. But that means that there is less blood in the carotid arteries in the aorta.

Hey everyone. So in lecture 15, we talked about how sympathetic neurons and parasympathetic neurons can be used to help you regulate blood pressure. And just as a quick review before we jump into anything else, let’s just look at that diagram that we referenced a lot in lecture 15, and it was this one. It includes all the big components that were involved.

And specifically, all the key components of how we regulated blood pressure using homeostasis. So if you guys remember, just as a quick review, if blood pressure changes, so if it goes up, if it goes down, even if it’s normal. These baroreceptors in these key arteries are constantly sensing what the blood pressure is.

They then send that signal, that information, to our integrating center, the medulla in the brain. That will compare this value to our normal homeostatic blood pressure range. And if it decides that the blood pressure’s gotten too high or too low, it can change sympathetic and parasympathetic neuron activity to change blood pressure by affecting heart rate, stroke volume and vessel diameter, or the resistance to blood flow.

Now I said this was really awesome or I guess I said it was really awesome, I hope you think too. Because this happens extremely quickly. And so for example, if you wake up, you feel dizzy. We need a very, very quick change and neurons are the way for us to make a very quick change to signal very quickly. So just as a way to recap what we talked about in lecture 15, let’s try to use all the tools we have, sympathetic neurons, parasympathetic neurons, to help this guy.

He got up too fast and he paid the price. Blood pressure fell too much and he didn’t fix it fast enough before getting dizzy and passing out. So let’s look at that case specifically.

So if you’re sitting down or you’re lying down, it’s pretty easy for blood to flow evenly throughout the entire body. But as we’ve said, if you suddenly stand up very quickly, like maybe you wake up in the morning and you just jump out of bed. What literally happens is due to gravity blood will actually overall start draining downwards towards your lower extremities. But that means that there is less blood in the carotid arteries in the aorta.

But if there’s less blood here, that means that the pressure in these arteries has dropped. And these baroreceptors are right there to sense that change. And what they’re gonna do is say hey, we don’t know what happened but there is not much blood in your aorta. There is not much blood in your arteries.

Your blood pressure is low and it’s going to send that signal to the medulla and the medulla is gonna have to decide if that blood pressure is too low and what to do about it. So this is why you feel dizzy when you get up too fast. Blood drains down and blood pressure in the arteries also decreases, making it harder for you to pump blood upwards against gravity. So let’s try to use the tools we have to fix this problem. We saw this diagram in lecture 15 for the case of somebody with high blood pressure.

47 Feeling dizzy after standing? Your ...

But now let’s flip everything to show what would happen for somebody whose blood pressure were too low. So if your blood pressure falls. That is always sensed by the baroreceptors. But if blood pressure is too low, the baroreceptors, as we saw in lecture 15, those sodium channels that we were stretching open with the pressure, they’re gonna close cuz there’s nothing to stretch them anymore. And that’s why here I put that the baroreceptors will actually stop firing action potentials.

Low blood pressure means no action potentials in the baroreceptors. The neurons then send that signal to our integrating center, and let’s say the integrating center decides, hey, the baroreceptor are super silent. They are not having action potentials.

And that means that the blood pressure is incredibly low. In order to increase blood pressure, we can do a couple things. First, we can activate sympathetic neurons. And we know that they‘ll release norepinephrine, so if sympathetic neurons get active, more norepinephrine gets released. But we know that sympathetic neurons can do three things, norepinephrine combining to alpha receptors on the surface of blood vessels.

That will cause them to constrict, increasing the resistance to blood flow, and therefore increasing arterial blood pressure. That will help you when you’re standing up right away. So when you stand up too fast, you can’t feel it, and you can’t see it.

But the majority of your blood vessels and your body are [SOUND] scrunching up, constricting in order to help increase pressure. Parasympathetic neurons can also do something else, that norepinephrine will find its way to beta 1 receptors on the ventricular muscle. And we saw on lecture 15 that these beta 1 receptors when activated by norepinephrine will lead to increased force or increased stroke volume by the ventricles.

Last time we saw it this was because the ventricular muscle was releasing more calcium, it was also relaxing much more quickly. So the muscle is going harder and faster because of sympathetic neurons. But if the ventricles pump more forcefully, they pump more blood per minute, and that means more blood will get into your arteries to replace that blood that is draining down due to gravity.

And we know that more blood in the arteries means more pressure in the arteries. You probably won’t feel that either because it’s hard to feel your heart beating harder. But that’s happening too, when you stand up too fast.

Finally, we remember from lecture 15 that the sympathetic output can also talk to the SA node, also using a beta 1 receptor. Because when norepinephrine binds to this beta 1 receptor at the SA node, that leads to an increase in heart rate. And this was because more sodium was coming in more easily, more calcium was coming, the auto rhytmic cells were getting more positive more easily. And this is the effect that you can actually feel when you stand up too fast.

And if you’re ever curious, I do this actually pretty frequently cuz I think it’s pretty awesome cuz when I stand up too fast, I will very immediately start taking my heart rate. And I’ll notice, that it just jumps through the roof when I’m getting dizzy. And this is it. This is why.

Cuz my sympathetic neurons are going crazy, my beta 1 receptors are getting activated, and my heart rate is just going through the roof trying to help me increase cardiac output to increase blood pressure. So sympathetic neurons are very powerful. A second way that we have to increase blood pressure, and which happens at the same time is we’re gonna decrease the parasympathetic activity.

Sympathetic neurons, they’re kind of like your gas pedal, they help heart rate go up. Force go up. Constriction go up.

Parasympathetic neurons are kinda like your break. They help slow down heart rate. So if you want everything to go faster, and harder, and blood pressure to go up, we don’t want this break pedal to be active.

So we’re gonna turn off parasympathetic output, which means they are not gonna release acetylcholine anymore or less of it. Which means it’s not gonna bind to this muscarinic receptor on the SA node, which means that parasympathetic output will no longer decrease heart rate. So by turning off parasympathetic neurons, that will also help us increase heart rate.

It’s like in order for your car to go faster, yeah, you can push on the gas pedal, that’ll help you go faster. But what you can also do is take your foot off of the brake, activate gas, take your foot off the brake.

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