Hemoglobin | Human anatomy and physiology | Health & Medicine | Khan Academy
- Articles, Blog

Hemoglobin | Human anatomy and physiology | Health & Medicine | Khan Academy

I’ve talked a lot about the
importance of hemoglobin in our red blood cells so I thought
I would dedicate an entire video to hemoglobin. One– because it’s important,
but also it explains a lot about how the hemoglobin– or
the red blood cells, depending on what level you want to
operate– know, and I have to use know in quotes. These aren’t sentient beings,
but how do they know when to pick up the oxygen and when
to drop off the oxygen? So this right here, this is
actually a picture of a hemoglobin protein. It’s made up of four
amino acid chains. That’s one of them. Those are the other two. We’re not going to go into the
detail of that, but these look like little curly ribbons. If you imagine them, they’re a
bunch of molecules and amino acids and then they’re curled
around like that. So this on some level
describes its shape. And in each of those groups or
in each of those chains, you have a heme group
here in green. That’s where you get the
hem in hemoglobin from. You have four heme groups and
the globins are essentially describing the rest of it– the
protein structures, the four peptide chains Now, this heme group– this
is pretty interesting. It actually is a porphyrin
structure. And if you watch the video on
chlorophyll, you’d remember a porphyrin structure, but at
the very center of it, in chlorophyll, we had a magnesium
ion, but at the very center of hemoglobin, we have an
iron ion and this is where the oxygen binds. So on this hemoglobin, you have
four major binding sites for oxygen. You have right there, maybe
right there, a little bit behind, right there,
and right there. Now why is hemoglobin– oxygen
will bind very well here, but hemoglobin has a several
properties that one, make it really good at binding oxygen
and then also really good at dumping oxygen when it
needs to dump oxygen. So it exhibits something called
cooperative binding. And this is just the principle
that once it binds to one oxygen molecule– let’s say
one oxygen molecule binds right there– it changes the
shape in such a way that the other sites are more likely
to bind oxygen. So it just makes it– one
binding makes the other bindings more likely. Now you say, OK, that’s fine. That makes it a very good oxygen
acceptor, when it’s traveling through the pulmonary
capillaries and oxygen is diffusing
from the alveoli. That makes it really good at
picking up the oxygen, but how does it know when to
dump the oxygen? This is an interesting
question. It doesn’t have eyes or some
type of GPS system that says, this guy’s running right now and
so he’s generating a lot of carbon dioxide right now in
these capillaries and he needs a lot of oxygen in these
capillaries surrounding his quadriceps. I need to deliver oxygen. It doesn’t know it’s
in the quadraceps. How does the hemoglobin know to
let go of the oxygen there? And that’s a byproduct of what
we call allosteric inhibition, which is a very fancy word,
but the concept’s actually pretty straightforward. When you talk about allosteric
anything– it’s often using the context of enzymes– you’re
talking about the idea that things bind
to other parts. Allo means other. So you’re binding to other parts
of the protein or the enzyme– and enzymes are just
proteins– and it affects the ability of the protein
or the enzyme to do what it normally does. So hemoglobin is allosterically
inhibited by carbon dioxide and by protons. So carbon dioxide can bond
to other parts of the hemoglobin– I don’t
know the exact spots– and so can protons. So remember, acidity
just means a high concentration of protons. So if you’re in an acidic
environment, protons can bond. Maybe I’ll do the protons
in this pink color. Protons– which are just
hydrogen without electrons, right– protons can bond to
certain parts of our protein and it makes it harder for them
to hold onto the oxygen. So when you’re in the presence
of a lot of carbon dioxide or an acidic environment, this
thing is going to let go of its oxygen. And it just happens to be that
that’s a really good time to let go of your oxygen. Let’s go back to this
guy running. There’s a lot of activity in
these cells right here in his quadriceps. They’re releasing a lot of
carbon dioxide into the capillaries. At that point, they’re going
from arteries into veins and they need a lot of oxygen, which
is a great time for the hemoglobin to dump
their oxygen. So it’s really good that
hemoglobin is allosterically inhibited by carbon dioxide. Carbon dioxide joins on
certain parts of it. It starts letting go of its
oxygen, that’s exactly where in the body the oxygen
is needed. Now you’re saying, wait. What about this acidic
environment? How does this come into play? Well, it turns out that most
of the carbon dioxide is actually disassociated. It actually disassociates. It does go into the plasma, but
it actually gets turned into carbonic acid. So I’ll just write a little
formula right here. So if you have some CO2 and you
mix it with the water– I mean, most of our blood, the
plasma– it’s water. So you take some carbon dioxide,
you mix it with water, and you have it in the
presence of an enzyme– and this enzyme exists in
red blood cells. It’s called carbonic
anhydrase. A reaction will occur–
essentially you’ll end up with carbonic acid. We have H2CO3. It’s all balanced. We have three oxygens, two
hydrogens, one carbon. It’s called carbonic acid
because it gives away hydrogen protons very easily. Acids disassociate into their
conjugate base and hydrogen protons very easily. So carbonic acid disassociates
very easily. It’s an acid, although I’ll
write in some type of an equilibrium right there. If any of this notation really
confuses you or you want more detail on it, watch some of the
chemistry videos on acid disassociation and equilibrium
reactions and all of that, but it essentially can give away
one of these hydrogens, but just the proton and it keeps the
electron of that hydrogen so you’re left with a hydrogen
proton plus– well, you gave away one of the hydrogens so
you just have one hydrogen. This is actually a
bicarbonate ion. But it only gave away the
proton, kept the electron so you have a minus sign. So all of the charge adds up to
neutral and that’s neutral over there. So if I’m in a capillary
of the leg– let me see if I can draw this. So let’s say I’m in the
capillary of my leg. Let me do a neutral color. So this is a capillary
of my leg. I’ve zoomed in just one
part of the capillary. It’s always branching off. And over here, I have a bunch
of muscle cells right here that are generating a lot
of carbon dioxide and they need oxygen. Well, what’s going to happen? Well, I have my red blood
cells flowing along. It’s actually interesting–
red blood cells– their diameter’s 25% larger than
the smallest capillaries. So essentially they get squeezed
as they go through the small capillaries, which a
lot of people believe helps them release their contents and
maybe some of the oxygen that they have in them. So you have a red blood cell
that’s coming in here. It’s being squeezed through
this capillary right here. It has a bunch of hemoglobin–
and when I say a bunch, you might as well know right now,
each red blood cell has 270 million hemoglobin proteins. And if you total up the
hemoglobin in the entire body, it’s huge because
we have 20 to 30 trillion red blood cells. And each of those 20 to 30
trillion red blood cells have 270 million hemoglobin
proteins in them. So we have a lot
of hemoglobin. So anyway, that was a little
bit of a– so actually, red blood cells make up roughly
25% of all of the cells in our body. We have about 100 trillion
or a little bit more, give or take. I’ve never sat down
and counted them. But anyway, we have 270 million
hemoglobin particles or proteins in each red blood
cell– explains why the red blood cells had to shed their
nucleuses to make space for all those hemoglobins. They’re carrying oxygen. So right here we’re dealing
with– this is an artery, right? It’s coming from the heart. The red blood cell is going in
that direction and then it’s going to shed its oxygen
and then it’s going to become a vein. Now what’s going to happen is
you have this carbon dioxide. You have a high concentration
of carbon dioxide in the muscle cell. It eventually, just by diffusion
gradient, ends up– let me do that same color– ends
up in the blood plasma just like that and some of it
can make its way across the membrane into the actual
red blood cell. In the red blood cell, you have
this carbonic anhydrase which makes the carbon dioxide
disassociate into– or essentially become carbonic
acid, which then can release protons. Well, those protons, we just
learned, can allosterically inhibit the uptake of oxygen
by hemoglobin. So those protons start bonding
to different parts and even the carbon dioxide that hasn’t
been reacted with– that can also allosterically inhibit
the hemoglobin. So it also bonds
to other parts. And that changes the shape of
the hemoglobin protein just enough that it can’t hold onto
its oxygens that well and it starts letting go. And just as we said we had
cooperative binding, the more oxygens you have on, the better
it is at accepting more– the opposite happens. When you start letting go of
oxygen, it becomes harder to retain the other ones. So then all of the
oxygens let go. So this, at least in my mind,
it’s a brilliant, brilliant mechanism because the oxygen
gets let go just where it needs to let go. It doesn’t just say, I’ve
left an artery and I’m now in a vein. Maybe I’ve gone through some
capillaries right here and I’m going to go back to a vein. Let me release my oxygen–
because then it would just release the oxygen willy-nilly
throughout the body. This system, by being
allosterically inhibited by carbon dioxide and an acidic
environment, it allows it to release it where it is most
needed, where there’s the most carbon dioxide, where
respiration is occurring most vigorously. So it’s a fascinating,
fascinating scheme. And just to get a better
understanding of it, right here I have this little chart
right here that shows the oxygen uptake by hemoglobin or
how saturated it can be. And you might see this in maybe
your biology class so it’s a good thing
to understand. So right here, we have on the
x-axis or the horizontal axis, we have the partial pressure
of oxygen. And if you watched the chemistry
lectures on partial pressure, you know that partial
pressure just means, how frequently are you being
bumped into by oxygen? Pressure is generated by gases
or molecules bumping into you. It doesn’t have to be gas,
but just molecules bumping into you. And then the partial pressure
of oxygen is the amount of that that’s generated
by oxygen molecules bumping into you. So you can imagine as you go
to the right, there’s just more and more oxygen around so
you’re going to get more and more bumped into by oxygen. So this is just essentially
saying, how much oxygen is around as you go to
the right axis? And then the vertical axis tells
you, how saturated are your hemoglobin molecules? This 100% would mean all of the
heme groups on all of the hemoglobin molecules or proteins
have bound to oxygen. Zero means that none have. So
when you have an environment with very little oxygen– and
this actually shows the cooperative binding– so let’s
say we’re just dealing with an environment with very
little oxygen. So once a little bit of oxygen
binds, then it makes it even more likely that more and
more oxygen will bind. As soon as a little– that’s why
the slope is increasing. I don’t want to go into algebra
and calculus here, but as you see, we’re kind
of flattish, and then the slope increases. So as we bind to some oxygen,
it makes it more likely that we’ll bind to more. And at some point, it’s hard for
oxygens to bump just right into the right hemoglobin
molecules, but you can see that it kind of accelerates
right around here. Now, if we have an acidic
environment that has a lot of carbon dioxide so that the
hemoglobin is allosterically inhibited, it’s not going
to be as good at this. So in an acidic environment,
this curve for any level of oxygen partial pressure or any
amount of oxygen, we’re going to have less bound hemoglobin. Let me do that in a
different color. So then the curve would
look like this. The saturation curve will
look like this. So this is an acidic
environment. Maybe there’s some carbon
dioxide right here. So the hemoglobin is being
allosterically inhibited so it’s more likely to dump the
oxygen at this point. So I don’t know. I don’t know how exciting you
found that, but I find it brilliant because it really is
the simplest way for these things to dump their oxygen
where needed. No GPS needed, no robots needed
to say, I’m now in the quadriceps and the
guy is running. Let me dump my oxygen. It just does it naturally
because it’s a more acidic environment with more
carbon dioxide. It gets inhibited and then the
oxygen gets dumped and ready to use for respiration.

About James Carlton

Read All Posts By James Carlton

100 thoughts on “Hemoglobin | Human anatomy and physiology | Health & Medicine | Khan Academy

    I know the chemical formula is C587H1213N218S3Fe if that helps at all ?

  2. More people need to like his videos, seriously we get what we want and give nothing in return so the least thing we could do is like the videos.

  3. really a nice talk about hemoglobin,, its a nice way to explain about things.. best explanation i have ever heard before.. thnks… 🙂

  4. This is excellent! I wish the title were more descriptive, because maybe I would have watched it when I needed to! Still a great lesson on the oxygen-hemoglobin dissociation curve and what drives it. Thanks!

  5. he makes me understand concepts I always tried to understand but never do…………best teacher in the whole universe!!!!! 2 thumbs up!!!!! 🙂

  6. great videos and find them really helpful, but I can't help but notice haemoglobin is spelt wrong and you called it the heme group where as it's Haem.

  7. Haemoglobin and hemoglobin (and heme + haem) are both right, one's the US and and the other is the UK, I forget which way around but that's not the point 🙂

  8. You're such a life-saver. This is a great lecture for my incoming Prelim exam on Hematology 🙂 You explained it way better than my book.

  9. They are most like from Christians who detest science and evolution. hemoglobin in different animals proves evolution.

  10. Wow. Awesome clip clip.

    My dad was once a fatty. He went from 284lbs of pure fat into 201 lbs of complete lean muscle. Everybody was in shock. I just registered myself coz I'm planning to get enormous muscle mass. He made use of the Muscle Building Bible (Google it)…

  11. Sal, why haven't you say down and count the exact number of cells in or body?!
    Oh, all right. Just more videos will do indeed 😀

  12. u did not mention 3rd allosteric inhibitor – bpg and carring on how a fetous gets oxygen from mummy i find it super intresting 😉 but to conclude i love the video

  13. This is most impressive.  It is not just the great info but the great speaking voice that makes watching this video great.  Here's to more of the same.

  14. The Creator is indeed an intelligent being!!! I have never seen a mechanism so accurate and well-coordinated as this one! 

  15. I was ten years old when I first found khan academy and I first learnt about the lungs and pulmonary system from there videos and my science teacher was so impressed and now I'm his favourite student after watching more videos

  16. great video! i studied hemoglobin a lot at a very high level and then watching this video helped connect it all. maybe post a video on hemoglobin and its interaction 2,3- BPG in the blood.

  17. Thank you for this Video.!! 
    I was confused on this section but from watching this video I actually love the subject on how Hemoglobin works..
     😀 yay!! thanks again..

  18. 2,3-Bisphosphoglycerate lowers Hemoglobin's binding affinity for O2- molecules which allows O2- molecules to travel to tissue requiring oxygen for anyone wondering why oxygen is removed.

  19. this is soo helpful! THANKS! You know, I've been studying those facts, but this helps to connect the information, to truly understand the mechanism… and not to lose the overview.

  20. gracias a este vídeo me e dado cuenta que el color rojo de la sangre se debe a la globina y no solo eso es mas también pude aprender que el eritrocito hace un cambio de gases los cuales son: dióxido de carbono y el oxigeno.
    también pude diferenciar e identificar las cadenas que con tiene ya que es importante recalcar que los eritrocitos recorren todo el cuerpo para mantenerlo en un buen estado es decir con el oxigeno suficiente en cada porte de nuestro cuerpo.

  21. May I know how blood transfusion can increase the hemoglobin level in the blood? Any specific enzymes and biochemical pathway involved?

  22. You never explained HOW it dumps the oxygen. What causes the oxygen to detatch from the Iron. And you also never explained how the oxygen leaves the red blood cell and gets to the muscle cell.

  23. 👍this is smart news i used to believe well i thought blood was just made into from jesus christ or maybe just from my heart pumping.. red food we eat and now i know for sure its oxygen?

  24. Thank you so much!!! You are the best teacher ever. Your enthusiasm and joy in teaching has no parallel. I am a teacher myself taking the Boards and I have nothing but good words for you. Thanks a million you are the best

  25. (-_+) Are you trying to tell me a guy was just running down the street with a chemical respirator on for the hell of it no.8?
    (8_+) Yes
    He didn't even make it to the corner house no.13 without struggling!
    (-_+) If that was real, then i'm moving houses asap! This town ain't what it used to be!
    There are a lot of weirdos!
    I want out!
    Eye want a city apartment on docklands with a private car space and a hi tech graphics lab! and a Ferrari for Sundays!
    and enough money in the bank to live off of the interest and pay my bills!
    (-_+) and give 33 his cut! And his lodge a present!
    I'm not greedy but at the same time i'm very pissed off!

  26. I love your videos on LUB DUB and HEART SOUNDS….. I have learned so much from those also… I love everything connected with the heart, blood, and immunity………… Your videos are so interesting and enjoyable… Thank you!!!!

  27. This might be interesting: Scientists estimated the number of human cells in the body, about 84% of which are red blood cells, finding there to be about 30 trillion human cells in the body.

  28. Thanks for keeping the "why" at the forefront! I have watched at least 10 videos on this subject and this one is by far the best. thank you thank you!

  29. a verkeringes segitsegevel müködeskeptelenne teszi az agymüködesünket az agysejtek közötti elektromos atvitelt transzmittert gatolja

  30. Hemoglobin's are larger than red blood cells, and are not good. iRLBT is a vaccine that gets rid of them. You can see hemoglobins with the naked eye without too much trouble. Red blood cells are extremely small. Hemoglobins are separate particles from red blood cells.

  31. I'm here because I want to educate myself because my son has a rare case of korle bu and I want to understand how the blood works and yeah I'm confused as crap but your video was a ton of help.. thanks

  32. Isn't the main reason that O2 gets dumped in the tissues because the partial pressure of O2 is much lower in the tissues than in the lungs?

Leave a Reply

Your email address will not be published. Required fields are marked *