Hemoglobin moves O2 and CO2 | Human anatomy and physiology | Health & Medicine | Khan Academy

Let’s talk about exactly how
oxygen and carbon dioxide come into and out of the lungs. So you know this is our
alveolus in the lungs. This is the last
little chamber of air where the lungs are going to
interface with blood vessels. So this is our blood
vessel down here. And oxygen is going to make
its way from this alveolus. It’s going to go into
the blood vessel. And it’s going to go
from the blood vessel into a little red blood cell. This is my red blood cell here. He’s headed out for the first
delivery of oxygen that day. And he’s going to
pick up some oxygen. And it’s going to get
inside of the red blood cell through diffusion. That’s how it gets inside. So the oxygen has made its
way into the red blood cell. And where do you
think it goes first? Well, this red blood
cell is, we sometimes think of it as a
bag of hemoglobin. It’s got millions and
millions and millions of hemoglobin proteins. So this is our
hemoglobin protein. It’s got four parts to it. And each part can
bind an oxygen. So hemoglobin, I can
shorten this to Hb. Now, oxygen is going to bump
into, quite literally bump into one of these hemoglobins. And it’s going to bind,
let’s say, right here. And initially,
it’s kind of tricky because oxygen doesn’t
feel very comfortable sitting on the hemoglobin
or binding to hemoglobin. But once a single
oxygen is bound, a second one will
come and bind as well. And then a third will
find it much easier. Because what’s happening is
that as each oxygen binds, it actually changes the
conformation or shape of hemoglobin. And so each subsequent oxygen
has an easier time binding. We call that cooperativity. Has the word, almost
like cooperation in it. And an easy way to think of
cooperativity, the way I think of it, is that if you’re
at a dinner party, you are much more
likely to sit where two or three of your friends are
already sitting, if you think of this as a table
with four chairs, rather than just
sitting at a table by yourself being the
first one to sit there. So we kind of like sitting
with our friends and oxygen is kind of a friendly molecule. And so it also
likes to sit where or bind where other
oxygens have already bound. What are the two,
then, major ways, based on this diagram,
how I’ve drawn it. What are the two
major ways that oxygen is going to be
transported in the blood? One is hemoglobin
binding oxygen. And we call that HbO2. Just Hb for hemoglobin,
O2 for oxygen. And this molecule,
or this enzyme, then, is not really
called hemoglobin anymore. Technically, it’s
called oxyhemoglobin. That’s the name for it. And another way that you can
actually transport oxygen around is, that some
of this oxygen– I actually underlined
it there– is dissolved, O2 is dissolved in plasma. So some of the
oxygen actually just gets dissolved right
into the plasma. And that’s how it
gets moved around. Now, the majority, the
vast majority of it is actually going to be moved
through binding to hemoglobin. So just a little bit is
dissolved in the plasma. The majority is
bound to hemoglobin. So this red blood cell goes
off to do its delivery. Let’s say, it’s delivering
some oxygen out here. And there is a tissue cell. And, of course, it
doesn’t know where it’s going to go that day. But it’s going to go wherever
its blood flow takes it. So let’s say, it
takes a pass over to this thigh cell in your,
let’s say, upper thigh. So this thigh cell
has been making CO2. And remember, sometimes
we think of CO2 as being made only when the
muscle has been working. But you could be napping. You could be doing whatever. And this CO2 is still being made
because cellular respiration is always happening. So this red blood cell has
moved into the capillary right by this thigh cell. So you’ve got a situation like
this where now some of the CO2 is going to diffuse into the
red blood cell like that. And what happens once
it gets down there? So let me draw out, now, a large
version of the red blood cell. Just so you get a closer
view of what’s going on. And we’re in the thigh and the
two big conditions in the thigh that we have to keep in mind. One is that you have
a high amount of CO2 or partial pressure of CO2. And this is dissolved
in the blood. And the other is that you
have a low amount of oxygen, not too much oxygen
in those tissues. So let’s focus on
that second point. If there’s not too much
oxygen in the tissues, and we know that the hemoglobin
is kind of constantly bumping into oxygen molecules
and binding them. And they fall off
and new ones bind. So it’s kind of a
dynamic process. Now, when there’s not
too much oxygen around, these oxygen molecules
are going to fall off as they always do in
a dynamic situation. Except new ones are
not going to bind. Because there’s so little
oxygen around in the area, that less and less
oxygen is free and is available to bump into
hemoglobin and bind to it. So you’re going
to literally start getting some oxygen that falls
off the hemoglobin simply because the partial
pressure of oxygen is low. So one reason for oxygen
to come into the cells is going to be a low pO2. That’s one reason. So these are reasons–
and I’m going to give you another
one, that’s why I’m writing reasons–
for O2 delivery. So one of them is
going to be simply not having too much
oxygen in that area. A second reason has
to do with CO2 itself. So let’s actually
follow what happens once CO2 starts getting
into the red blood cell. Now, this first
CO2 molecule, it’s going to meet up
with a little water. Remember, there’s a lot of
water in the red blood cell. In fact, there’s water
all over the blood. In fact, it’s made
of mostly water. And so it’s not
too hard to imagine that a water molecule
might bump into this CO2. And there’s an enzyme
called carbonic anhydrase. And what it does is, it
combines the water and the CO2 into what we call
H2CO3, or carbonic acid. Now, if it’s an acid, try to
keep in mind what acids do. Acids are going to
kick off a proton. So this becomes HCO3 minus. And it kicks off a proton. And notice that now you’ve got
bicarb and proton on this side. And this bicarb
is actually going to just make its way outside. So the bicarb goes
outside the cell. And the proton, what
it does is, it meets up with one of these
oxyhemoglobins. It kind of finds
an oxyhemoglobin. Remember, there are
millions of them around. And it literally
binds to hemoglobin. And it boots off the oxygen. So it binds to hemoglobin
and oxygen falls away. So this is interesting
because now this is a second reason
for why oxygen gets delivered to the tissues. And that is that,
protons compete with oxygen for– what are they
competing for– for binding with hemoglobin. So they’re competing
for hemoglobin. Now I said there is
another thing that happens to the carbon dioxide. So what’s the other thing? Turns out that carbon
dioxide actually sometimes independently
seeks out oxyhemoglobin. Remember, again, there
are millions of them. So it’ll find one. And it’ll do the same thing. It’ll say, well, hey,
hemoglobin, why don’t you just come bind with me and
get rid of that oxygen? So it also competes with oxygen. So you’ve got some competition
from protons, some competition from carbon dioxide. And when carbon
dioxide actually binds, interesting thing is
that it makes a proton. So guess what happens? That proton can go and
compete again by itself. It can compete
with oxyhemoglobin and try to kick off another
one, kick off another oxygen. So this system is
really interesting because now you’ve
got a few reasons why you have oxygen delivery. You’ve got protons competing. You’ve got now CO2
competing with oxygen. So you’ve got a couple of
sources of competition. And you’ve got, of
course, just simply the fact that there’s just
not too much oxygen around. So these are reasons
for oxygen delivery. So at this point,
you’ve got oxygen that’s delivered to the cells. And these hemoglobin
molecules, they’re still our cell, of course,
inside of a red blood cell. And these hemoglobin
molecules have now been bound by different things. So they’re no longer
bound by oxygen. So you can’t really call
them oxyhemoglobin anymore. Instead they have protons
on them like this. And they might have
some COO minus on them. So they might
have– actually, let me do that in the original
kind of orangey color. So they basically have different
things binding to them. And as a result, the
oxygen is now gone. And our system, so
far, looks good. But let me actually
now turn it around. And let’s ask the
question, how do we carry carbon dioxide from
the thigh back to the lung? Let me start out by
actually replacing the word thigh with lung. So now, our blood has
traveled back to the lung. And the question is,
how much carbon dioxide did it bring with it? And in what different forms
did that carbon dioxide come? So we’ve got a
couple of situations. We’ve got a high
amount of oxygen here. And we’ve got a
low amount of CO2. So really quite
different than what was happening in the thigh. So when the blood is leaving the
thigh headed back to the lung, what’s it got with it? Well, it’s got a few things. One is that it’s
got hemoglobin that is bound to carbon dioxide. And this is actually
called carbaminohemoglobin. And then, it’s also
got some protons that are bound to hemoglobin. So the protons themselves
are attached to hemoglobin. And just keep in mind that
for every proton that’s attached to
hemoglobin, you’ve also got a bicarb dissolved
in the plasma. Because it’s a one-to-one
ratio of these things. So you’ve got a bunch of
bicarb in the plasma as well. And I’m writing in
parentheses just so we don’t forget that point. And finally, what
else is in the blood? We’ve got some CO2 that
just gets dissolved right into the plasma. So this is sounding a
little bit like what happened with the
oxygen situation, where you had some CO2
in the plasma itself. And this is what’s headed back
from the thigh to the lung. So now in the
lung, what happens? You’ve got all this
stuff with you. And the first thing
that happens is that, you’ve got a lot of
oxygen, now, in the area. A lot of oxygen in the
tissue of the lung. And it diffuses into the
cell, goes into the cell. And the oxygen is, because
there’s so much of it, it’s going to go and try to
sit in these hemoglobins. It’s going to try
to find its spot. And if it does, what it
does in terms of equations is kind of the reverse
of what happened before. Now you’ve got a
lot of oxygen here. You’ve got a lot of oxygen here. And because these are
reversible reactions, you basically push this
entire reaction to the left. So now, you’ve got
a lot of oxygen. And it basically competes
for that hemoglobin again. So remember, before the
protons actually ended up snatching hemoglobin
away from oxygen, and now oxygen
returns the favor. It says, well, I’m going to
snatch that hemoglobin right back. And you’ve got
this proton that’s kind of the left out by itself. And on this side,
you’ve got this CO2 that’s kind of
left out by itself. So a couple of interesting
things are happening. Let me actually make sure I
keep track of them up here. So what are some
reasons, now, what are some reasons
for CO2 delivery? How is it getting delivered
back to the lungs? And the first one, probably
the most obvious one, is that we said that the
lungs have a low CO2 content. So simply having very
little CO2 around means that whatever
is there is going to diffuse into the alveolus. So you’re going
to get whatever’s in the red blood cells
going to diffuse in here. Simply because there’s
not a lot of CO2 around. So instead of diffusing
into the red blood cell, now it’s going to
want to diffuse out. A second reason, though this
is the more interesting reason, is that you actually
have oxygen competing, oxygen competes with
protons and CO2. So it’s competing with protons
and CO2 for hemoglobin. And that’s what we drew in
our equation down there. So what it does is it
basically gets you back to the oxyhemoglobin. That’s the first thing. And that’s what
we’ve already drawn there is that, we’ve drawn
oxygen bound to hemoglobin. But it means that these
little CO2s fall off. They fall off. These little protons fall off. And they’re back in
the side of the cell, back in the inside of the cell. So if you’re CO2
you can, again, you can just diffuse
into the alveolus. But if you’re a proton,
let’s say you’re a proton and you just fell
off of the hemoglobin because it got snatched
away by oxygen. Well then, this little bicarb
is going to come back inside. This bicarb comes back inside. And it combines with a proton. And these two form,
you guessed it, H2CO3. So they, remember, this
is reversible as well. So they go back. And they form H2CO3. And it turns out
that you can actually go from H2CO3 over here also
using carbonic anhydrase. So you can basically just do
this whole reaction backwards. And now, you can see that
you’ve got more CO2 formed. So by having bicarb dissolved
in the blood, or in the plasma, it’s kind of just staying there
and kind of waiting it out. And as soon as those protons are
bumped off of the hemoglobin, they go and combine with
them and form the CO2. So you’ve got CO2 coming
from here, from the bicarb. You’ve got CO2 coming from
the carbaminohemoglobin. And you’ve also got
the CO2– remember, we said that some CO2
dissolved in the plasma. So three different ways that
CO2 is actually coming back. And once all that
CO2 is in the lungs, it’s going to diffuse
right into the alveolus because the amount
of CO2 in there is so darn low that the
diffusion gradient gets it going towards the alveolus. And of these
different strategies, the most important
one, the one that gets us most of our carbon
dioxide transportation, is this one. This middle one where
the protons are actually binding hemoglobin and
all that bicarbonate is dissolved in plasma. So of the three different ways
that carbon dioxide comes back, that’s the one you should pay
most particular attention to.


  1. when the O2 competes for hB and forces the release of HCo3 and H+ in the RBC is that the Bohr effect? and when the H+ competes for hB, releasing O2, is that the Haldane effect?

  2. I didn't think H+ competed with oxygen because they bind at different sites on the Hemoglobin. Where does the H+ ions bind to on the hemoglobin? to the histidine? lysine? propionate group? I believe it may be the c-terminal histidine group but I am not sure. Could someone clarify?

  3. Rishi, I have a question: My anatomy textbook says that C02 doesn't bind to hemoglobin in the same area as O2 (the heme group). So, is it correct to say that CO2 "bumps off" O2 when it encounters oxyhemoglobin in systemic circulation?

  4. very well explained ! 🙂
    Eventhough English isnt my mothertounge I could follow you easily.
    Helped me a lot

  5. Great video!
    Does a increased overall CO2(and decreased pH) level from lung desease from e.g smoking, mean the affinity of the O2 in the lungs?

  6. Very nice videos. Hav u guys made any videos on ventilation perfusion mismatch in lungs especially discussing increased dead space due to thrombus in pulmonary artery. Can't understand.

  7. Khan, how can I give you feedback?  Many videos have minor errors, e.g.
      H+ and CO2 do not bind to the same site as O2, so it doesn’t kick O2 off, but it binds somewhere else and lowers the O2 affinity of Hb.  
    Also equations reverse in the lung because CO2 is being lowered, i.e. as it comes off of Hb, it lowers Hbs affinity to O2.   Not that O2 coming aboard kicks off CO2.  

  8. lola pa said it before me: you have no idea how helpful your videos are!!! I wish somebody could help you sometime, how you helped so many of us!

  9. Im having trouble understanding the bicarb, pCO2 system for clin chem can someone please explain?
    In respiratory acidosis, pCO2 is increased. So the compensatory effect is increased HCO3-. All of my notes say that kidneys retain bicarb. How does retention of bicarb increase bicarb? 

  10. I have a slightly off-track question for you about SpO2 vs SaO2 cuz i'm having trouble finding an answer: apparently both measurements can be read by a pulse oxymeter. However, is it true that SaO2 can only be read by blood sampling through an i-STAT reading. I'm a little confused.

  11. Thank you, but just one problem, you said : CO2 + HbO2-> Hb.coo- + O2 + (H+) from where is coming this  H pro to?? and as well the negative charge on Hb.COO- ?? 

  12. In my text book it says that O2 binds with the heme group of the  hemoglobin molecule and CO2 binds with the amino acids in the globin part of the hemoglobin molecule, therefore o2 and co2 do NOT compete.

  13. Another awesome video! I love the way you explain things…way more clear to me than in my textbook. Thanks!

  14. I think you are mistaken when CO2 binds with oxyhemoglobin, it forms just carboxylhemoglobin and O2, no H+ protons

  15. really helpful specially when i am going to sit my as exam very soon and my teachers are still watching youtube during lesson

  16. This video is very helpful! Thank you very much, I appreciate all the help you have given me with your extensive library of video learning!

  17. why oxygen binds to iron if iron 2+ and already binded to four nitrogens and one globin. How oxygen finds place in this complex substance?

  18. I don't know who did this video but, his presentation on this topic is one of the best I've heard. Good job.

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