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Course: Health and medicine > Unit 4
Lesson 3: Breathing controlCentral chemoreceptors
Find out how the your body uses special cells that are central to the brain (inside the brain) to sense levels of CO2 and pH. Rishi is a pediatric infectious disease physician and works at Khan Academy.
These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Rishi Desai.
Want to join the conversation?
- Aren't Chemoreceptors supposed to use Oxygen ?(2 votes)
- The central chemoreceptors are more sensitive to pCO2 because it relays better information about the acid-base balance in the blood. Refer to the equation @5:12.
In certain disease processes, chronic obstructive pulmonary disease (COPD) being an example, these receptors are no longer as sensitive to pCO2 and are more sensitive to pO2 because the receptors are overwhelmed by chronically elevated CO2 levels in the blood.(6 votes)
- Between5:33and5:37, Rishi mentions that if we have high levels of CO2, then we have high levels of H+.
However, I have learned that as soon as H+ ions are formed, they are taken up by heamoglobin to form heamoglobinic acid, often abbreviated as HHb. This acts as a buffer... then how can the pH get low??(3 votes)- Even the binding of H+ to Hb is an equilibrium reaction. It's not to say that every molecule of H+ is instantly mopped up by Hb. Rather, some H+ will always be in solution. Even water is in constant equilibrium with H+ (simplification).(2 votes)
- Rashi says, that central chemoreceptors analyse the concentration of CO2 and H+ in interstitium. Is this really true? What about cerebrospinal fluid?(2 votes)
- Yes, this is 100% true. The cerebrospinal fluid also contribute to stimulate the chemoreceptors when the concentration of protons goes up. Because what this means is that the pH goes down and there is acidification going on.
So, in the CSF CO2 and water reacts to form H2CO3, which again reacts to form HCO3- and H+ (protons). These protons can not cross the blood brain barrier, and therefore build up in the extracellular fluid --> stimulating the chemoreceptors. CO2 can cross the blood brain barrier.(2 votes)
- At1:28, the central chemoreceptors, are they actually regulatory interneurons?(2 votes)
- Based on this video, they appear to be only sensory neurons, not interneurons(2 votes)
- Why can't the central chemoreceptors monitor oxygen levels? but the peripheral chemoreceptors can monitor oxygen? Is this in some way a protective evolutionary mechanism that has evolved?(2 votes)
- Why doesn't the brain detect pO2?(2 votes)
- What neurotransmiters do chemoreceptors secrete, when depolarized?(1 vote)
- Why is it that blood arterial gas analysis tells us that the blood becomes more basic as bicarbonate increases? It is confusing because as bicarbonate forms, hydrogen protons are released, which would make the blood more acidic...(1 vote)
- I think you understand that more carbon dioxide means more H* ions and the person becomes acidic. https://www.khanacademy.org/test-prep/mcat/chemical-processes/acid-base-equilibria/a/chemistry-of-buffers-and-buffers-in-blood
https://www.khanacademy.org/science/biology/water-acids-and-bases/acids-bases-and-ph/v/buffer-system(1 vote)
- the central chemoceptors respond to elavated levels of carbon dioxide?(1 vote)
- If it is a high percentage of protons (hydrogen), why is it called a low pH, as pH stands for percentage of hydrogen?(1 vote)
- It stands for Power of Hydrogen. pH= -log[H3O] in a solution, so as the free H+ concentration goes up, due to the negative logarithm, pH goes down.(1 vote)
Video transcript
So here's a brain I
sketched out ahead of time. And I wanted to label a
couple parts of this brain that we're going to
talk about right now. So the first is the
pons, and the second is what we sometimes
call the medula, or you might even hear the
term medulla oblongata. And it's right next to the
pons, kind of right there. And people have actually
studied these parts of the brains, the pons
and the medulla oblongata. And they found that they are
actually little respiratory centers. And I'm actually
shading them in green. And sometimes you might even
see these areas subdivided. But the idea is that there
are a couple of spots here where the neurons
in these green locations are very, very important
to our breathing. And so they call them
respiratory centers. And even though I'm drawing
it as kind of a green blob, these respiratory
centers are really just a bunch of neurons
packed together. So many, many neurons, I'm going
to draw a couple of them for us just to kind of
illustrate the point. And these neurons are going to
put out their little feelers. And they're going to try
to collect information. That's basically
what they're doing. They're going to
collect information about all sorts of
things like pain. Are you anxious? Are you running
late for an exam? What is the situation
going on right now? And they're going to make a
decision about how fast we should be breathing, how
often we should be breathing, all that kind of stuff. So where do they get the
information from exactly, the information they need? I'm actually going to draw in
a couple of important neurons right here in the medulla,
and they're actually going to be these
neurons we are going to be talking about right now. So these neurons are called
the central chemoreceptors, and that's the focus
of our video today. So central
chemoreceptors are what we're going to be discussing. And these guys, and let me
show them a little bit bigger. They're basically neurons. Are so I'll draw
a couple of them. These neurons are going to be
projecting their little axons all the way over to
the respiratory center, and they're going to
communicate their message through neurotransmitters,
which are basically the language of neurons. Neurotransmitters allow
neurons to talk to one another. So what we're going
to see is that these central chemoreceptors
are going to collect information about a chemical. And that's why they're
called chemoreceptors. And just to back up
a second, they're called central
because they're part of the central nervous system. They are in the medulla
oblongata itself. They're physically right
there in the brain. So that's why we call them
central chemoreceptors. And the first
chemical that they're going to recept, or
receive information about, is going to be carbon dioxide. So like any cell, these neurons
are making carbon dioxide. And where does it usually go? Where would you assume that
this waste product would go? Well of course there's
a blood vessel. This blood vessel is going
to have less carbon dioxide, we presume, maybe just a
couple molecules of it. And so you have this
nice little gradient where the CO2 is going
to go into the blood and get swept away, and
of course eventually it's going to make its
way to the lungs and you might breathe it out. But let's assume for a
second that the levels of CO2 in the blood
are very high. Let's assume that
the partial pressure of carbon dioxide in the
blood is really, really high. What would that mean? Well, let me draw a
bunch of carbon dioxide molecules in this blood. And what that means is that
of course this gradient, this wonderful little
gradient that we had is going to not be
so strong anymore. Now there's no strong
diffusion gradient because of the differences
in pressure are negligible. There's a lot of CO2 in
the blood, a lot of CO2 all around the neurons in
the interstitial space. That's the space right
here, interstitial space. So because that gradient
is not as impressive-- let me write fluid
instead of space-- because the gradient
is not as impressive, you're going to have more carbon
dioxide kind of building up around these central
chemoreceptors. In fact, you might even
have some molecules of carbon dioxide
that are building up within the neurons, the
central chemoreceptor neurons. So these chemoreceptors
are going to notice the extra
carbon dioxide, and they're not going
to like it one bit. So you know what
they're going to do? They're going to start
firing action potentials. Let's say they usually fire
two action potentials let's say per one second. I'm just assuming that number. That's not the true number,
but let's just assume that. Now that the carbon
dioxide levels are high, they're going to fire
off maybe six action potentials in that
same timeframe. So all of a sudden, they're
firing more action potentials, because they don't like the high
CO2 levels that they're facing. So these respiratory
centers are going to get this message
loud and clear. And they're going to say,
wow we need to do something. Maybe we need to do something
in the way of making this person breathe
faster, for example. So this is one of
the many things that you might see happen,
is breathing faster. So you can see how
this signal might work. Now you remember we talked about
the relationship between CO2 and water. We said that CO2 binds to water. And that they form
carbonic anhydrates, H2CO3, and that is actually in turn
going to form bicarbonate. So it's going to form this. So if you have
high levels of CO2, you have high levels
over here, you can also assume high
levels of protons. And that's just another
way of saying a low pH. So the two things that our
central chemoreceptors respond to then are one,
high levels of CO2. And the other thing
they respond to are high levels of
protons, or a low pH. Now what they don't respond
to-- and this is actually very important-- what they don't
respond to is oxygen levels. So they don't respond
to low oxygen levels. And this is actually
a difference between the central
chemoreceptors and the peripheral
chemoreceptors. So this is something
to keep in mind. Now a final point
I want to make is I want to show you
a three dimensional view of the same thing
we just talked about. So let's take this picture
and kind of absorb it. This is a picture I drew
out a little bit earlier. Some of the important
features are going to be-- let's
orient ourselves first to the central chemoreceptor. That's this guy right here. Actually I think
there are two of them. So central chemoreceptors,
and they have of course a starring role in this picture. That's this guy right
here, and of course there is a second fellow right there. We also have our astrocytes. These are kind of important
cells for structural support. And they're also
important in setting up what we know as the
blood brain barrier. So that's right here right. I'm going to focus in
on this right here. And the blood brain
barrier of course allows us to keep what's
going on in the blood separate in many
ways from what's going on in the interstitial
fluid around the brain. So then to quickly recap, if
there's a lot of CO2 in here, in this blood vessel if
you see high levels of CO2, you're not going to get much
diffusion into that blood vessel. So CO2 levels start going
up all around our two central chemoreceptors. They're not going
to be too happy. And so they are going to start
firing more action potentials towards our respiratory
centers down these two axons. I hope you enjoyed that.