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.