Red blood cells | Human anatomy and physiology | Health & Medicine | Khan Academy

In the last video on the lungs
or the gas exchange in our bodies or on the pulmonary
system, we left off with the alveolar sacs. Let me draw one right here. So we have these alveolar sacs
that I talked about and they’re in these little
clumps like this. Let me draw a couple of them
just so you get the idea. And if you remember from the
last video, these are kind of where air goes in through our
trachea, then that splits up into our bronchi, and then
those split into the bronchioles, and
the bronchioles terminate at these alveoli. So that’s the alveoli. These are these super-small sacs
that we talked about in the last video on the
pulmonary system. You might want to watch
that video if none of this sounds familiar. And then of course we have our
bronchiole that feeds into this, and then that might have
branched off from another one that feeds into another set of
alveolar sacs, but I don’t want to get too focused
on that. I covered that in
the last video. In the last video, we saw that
air, when we breathe in, when our diaphragm contracts and
makes our lungs expand and fill up that space,
air comes in. Air comes in and that air that
comes in is going to be– as we’re breathing atmospheric
air– it’s going to be 21% oxygen and it’s going
to be 78% nitrogen. And actually, in our atmosphere,
carbon dioxide is actually almost a trace gas. It’s less than 1%. So any time you breathe in on
Earth, this is what you’re going to get. And we said in the last video
that you have these capillaries, these pulmonary
capillaries that are running all along the side
of these alveoli. So let me draw those pulmonary
capillaries– and so when they are de-oxygenated– so they come
here to be oxygenated. So when they’re de-oxygenated,
they might look a little purplish. And then they pick up the
oxygen from inside the alveoli– or the oxygen diffuses
across the membrane of the alveoli, into these
capillaries, into these super small tubes. And then once they do, that
makes the blood red. I’m going to talk in a little
bit about why it becomes red. So then it becomes red, and now
that the blood is red, it has its oxygen. The whole point is to
get the oxygen. It’s ready to go back
to the heart. So that’s just one little
part of it. And we learned in the last video
that something that goes away from the heart– so this is
going away from the heart– that is an artery. “A
for Away” – Artery. And something that’s going
towards the heart is a vein. So this right here is a vein. Now one question– and
this actually came up in the last video. Someone asked– which I think is
a very good question– is, gee, when we breathe in, most
of the air is nitrogen. Only 21% is oxygen. What happens to all that
nitrogen there? How come that doesn’t
go into our blood? And that’s actually an
excellent question. So to answer that, I think that
actually helps explain what’s going on here. Let’s draw a little
bit bigger. This is the inside of
of an alveolus. This is its membrane right here,
super thin, almost one cell thick. And then you have a capillary
running right next to it. Let me do that in
a neutral color. So you have a capillary
that’s maybe running right along the surface. And this is porous to gases
like oxygen, and nitrogen, carbon dioxide. And what we have here– let’s
say that this is– so the heart is over here. So this is blood coming from
the heart and then this is going to go back to the heart. Well, the heart’s
on both sides. So let me write it this way. From the heart and
to the heart. And what you have here is–
when we’re coming from the heart, this is de-oxygenated
blood and it’s actually going to have a high concentration
of carbon dioxide. I already did nitrogen
as green. Let me do carbon dioxide
as orange. There’s a lot of carbon dioxide
and actually carbon dioxide actually gets diffused
in the blood. It actually is carried in
the plasma of the blood. It’s not carried by red blood
cells that we’re going to talk about in a second. So that’s a bunch of carbon
dioxide here. And the concentration of
carbon dioxide in the de-oxygenated blood is going
to be higher than the concentration of carbon dioxide
in the alveolus. so if this is porous to carbon
dioxide, this membrane– and it is, these carbon dioxide
molecules are going to diffuse into the alveolus. Now on the other side of that–
we have oxygen here. We’re breathing it in. The air is 21% oxygen so you’re
actually going to have a lot more oxygen than
carbon dioxide. And this is de-oxygenated
blood. We used all of the oxygen in our
body and we’ll talk more about that either at the end of
this video or in a future video on how we use it or where
it goes in our body, but there’s no oxygen here so the
oxygen is going to be taken– it’s going to diffuse across
this membrane because the concentration of
oxygen is low. Now the question is– so
immediately you see that as the oxygen diffuses across
this membrane, all of a sudden, this is oxygenated
blood ready to go back to the heart. So this transition between
artery and vein is a very subtle thing. Very clearly here, you
say that, OK, this is going from the heart. This is our vein. This is going to the
heart– sorry. I always get confused. This is going away from the
heart– and I was looking for an A and I wrote from. This is away from the heart
so this is an artery. And this is going to the heart
so this is a vein. So you could make
the division. You could say, OK, once it’s
oxygenated, maybe we’re going back to the heart, but it’s kind
of an arbitrary– sorry. I spelled artery wrong. These are my flaws. Spelling was never
my strong suit. So it’s hard to say where
the artery ends and the vein begins. A good demarcation is when the
carbon dioxide concentration goes low and that the oxygen
concentration goes high. That’s a good time, where
we start from the pulmonary artery. Probably in the next video, I
will a make a very– you’ll see why the pulmonary arteries
are special, because pulmonary arteries coming away from the
heart have no oxygen or very little oxygen and they have
a lot of carbon dioxide. So pulmonary veins, which is–
it’s arbitrary where the artery turns into a vein. Once it gets oxygenated, it’s
ready to go back to the heart. It’s a pulmonary vein and
it is oxygenated. So it has oxygenated– and we
could write de-oxygenated. Now the reason why I say it’s
special besides the fact that pulmonary arteries and veins go
to and from the lungs, is that they’re kind
of the opposite. Because in the rest of the body
when we’re going away from the heart or we’re talking
about arteries, you’re going to see that that’s
oxygenated blood, while when we’re going away from the heart
to the lungs, that’s de-oxygenated blood. Similarly in the rest of the
body, when we’re going to the heart, where you’re to see
that that’s de-oxygenated blood, but in the pulmonary
vein, when we’re going to the heart, it’s oxygenated because
the lungs are what take up the carbon dioxide and give
us the oxygen. Now I still haven’t answered
that interesting question that rose on the message board
on the last video. What happens to the 78% of
nitrogen that’s sitting here? There’s just a ton of nitrogen
over here, more than the oxygen, a lot more than
the carbon dioxide. What happens to all of these
nitrogen molecules? And the answer is, nitrogen can
diffuse and does diffuse into the blood, but the blood’s
ability to take in nitrogen isn’t that high. And you might say, well,
why is oxygen special? Why can the blood take
up oxygen so much easier than nitrogen? And that’s where the red blood
cells come into play. Let me write this down. I’ll write it in red. Red blood cells, which
are fascinating on a whole set of levels. What red blood cells– these
are these cells that are sitting in– they’re flowing
through our circulatory system and they look kind of
like lozenges, if I were to draw one. They’re kind of like a flattened
sphere with a little divot on either side of it–
a lot like a lozenge. So if I were to draw it from
the side, it might look something like– well, from the
side, it would look like that and if you could see
through it, there’d be a little divot on each side. If I were to draw it at an
angle, it would look something like this. There’d be a little divot on
that side and there’d be a similar divot on
the other side. And red blood cells– and I
could do a whole set of videos just on red blood cells–
they contain hemoglobin. Maybe we’ll do a whole
video on hemoglobin. The hemoglobin are these
small proteins that contain four hem groups. So inside of red blood cells,
you have millions of hemoglobin proteins. And the hemoglobin proteins–
I’ll just draw them as this– they have these four
heme groups. And heme groups, the main
component is iron. And that’s why iron
is so important. If you don’t have enough iron,
you’re going to have trouble processing oxygen in your blood
and your hemoglobin won’t be functional enough. But it has iron on it. It has four of these
heme groups. And each of these heme groups
can bond to oxygen molecules. They’re very good binders
of oxygen. And we’re going to see in a
little bit– probably the next video– how they release the
oxygen, but this has tons, this has millions of heme groups
in it and the oxygen diffuses across the membrane
of the red blood cells and bonds to to the heme groups
on your hemoglobin. So because the red blood cells
have the hemoglobin inside of them, they’re like these sponges
for oxygen because hemoglobin is so good
at taking in oxygen. So the red blood cells are able
to essentially suck up all of the oxygen out
of the plasma. The plasma we can view as just
the general fluid of the blood, not including the
red blood cells. So the red blood cell
here isn’t so red. And the reason– and this is the
key point– the reason why it’s not so red– maybe we had
a red blood cell over here– let me make it clear. Carbon dioxide for
the most part is traveling within the plasma. It gets absorbed into the actual
fluid and I’ll talk about it in a future video. It’s actually in a slightly
different form. It’s as carbonic acid and that’s
actually a key point for how the plasma knows where
to dump the oxygen, but I’ll talk about that in
a future video. But over here, this red blood
cell has a bunch of hemoglobin proteins in it, but those
hemoglobin proteins have dumped their oxygen. And it actually turns out it’s
the hemoglobin– so with oxygen, hemoglobin looks red. It reflects red light. When it doesn’t have oxygen,
hemoglobin does not look red. It looks kind of purplish,
bluish, darkish– something. And that’s why in most of your
body, your veins that have de-oxygenated red blood cells
look kind of bluish. And the reason why it changes
color is that when the oxygen bonds to the hem sites on the
hemoglobin, it actually changes the entire confirmation,
the entire structure of the protein. We’ve see that multiple times. The whole protein folds in
such a way that all of a sudden, instead of purplish or
dark light being reflected, now red light is reflected. And that’s why red blood cells
will become red once they take the oxygen. But I’m going on a tangent. The whole point here is saying,
why we taking up so much more oxygen than nitrogen,
given that there’s less oxygen in the atmosphere
than nitrogen? And the key is these
red blood cells. These red blood cells have these
millions of hemoglobin proteins inside of them and they
take them up and they sop up all of the oxygen
out of the plasma. Actually, they sop about
98.5% of the oxygen. So these red blood cells are
just traveling and they’re going to go back to the heart. They are what make
our blood red. So you have this thing,
hemoglobin, that’s sitting in red blood cells. It’s sopping up all
the oxygen. So it keeps the oxygen
concentration and the actual plasma low. You have nothing like
that for nitrogen. There is no cell that’s sopping
up the nitrogen. Nitrogen does not bond
to hemoglobin. So that’s why oxygen
is taken up so much better than nitrogen. It’s a very interesting question
because if you just think about how much nitrogen
is, it’s kind of a very natural idea. Now I want to focus a little
bit on the red blood cell itself because it’s
fascinating. In the video on the structure
of the cell, I start off saying, all cells have
a membrane and they all have DNA. Now, the fascinating thing about
a red blood cell– I already said it has millions
of hemoglobin molecules or proteins inside of it. The fascinating thing
about a red blood cell– it has no nucleus. And no DNA. This is mind boggling when
I first found out. I was like, well, why
is it a cell? Is it really even
a living thing? And it turns out when
it’s growing, it does have a nucleus. All cells need a nucleus with
DNA in order to generate the proteins that build it up,
in order to exist and structurally make itself the way
it needs to be made, but the whole point of a red blood
cell is to contain as much hemoglobin as possible. And so you can imagine, this
is actually a favorable evolutionary trait, that as red
blood cells are ready to go into business, you’ve built
the whole structure, they actually get rid of
their nucleus. They actually push their nucleus
out of the cell and the whole reason why that’s
beneficial is, that’s more space for hemoglobin. Because the more hemoglobin you
have, the more oxygen you can take up. And I can do a ton of videos
on hemoglobin and all of that– and actually, I’m going
to do a lot more on the circulatory system so don’t
worry about that, but I want to go over one other really
interesting thing about hemoglobin. We already talked about
red blood cells. I think it’s fascinating that
they actually don’t have a nucleus in their mature form. They actually have
very short lives. They live maybe 80, 120 days
so they’re not these long lived cells– so it’s almost
a philosophical question. Are are they still alive once
they’ve lost their DNA or are they just vessels for oxygen
that aren’t really alive because they aren’t regenerating
and producing their own DNA? So actually, instead of going
into the hemoglobin discussion right now, I’ll leave you
there in this video. I realize I’ve been making
20-minute videos where my goal is really to make
ten-minute ones. So I’ll leave you here and in
the next video, we’ll talk more about hemoglobin and
the circulatory system.


  1. Fantastic videos. I really wish I had found you this summer while I was taking Anatomy 1 & 2 and Micro all at the same time! Still glad I know about these now that I'm in nursing school… Super helpful with Pathophysiology!

  2. How long does it take for a blood cell at the heart to go to the lungs, back to the heart, to the brain, and back to the heart?

  3. But khan, I've read somewhere deoxygenated blood is actually a darker shade of red compared with oxygenated blood?

  4. i remembered this children's story where the library woman had lozenges that tasted like war when you said "lozenges" but i just cant remember anything else about the story or what it was called…. its bugging me now :S

  5. a large misconception is that blood turns from blue -> red. it actually goes from light red to a slightly darker red. veins are blue to the eyes because of the light coming through the skin, take away your skin and you'll see your veins are red. you are not alien, your blood is always red lol

  6. "Carbon dioxide actually gets diffused in the blood, it is actually carried in the plasma in the blood; it's not carried by the red blood cells."

    To clarify: ~5% CO2 *dissolves* into the plasma under the partial pressure of venous CO2 at 46 mmHg. Another 5-10% is transported on the globin chain of hemoglobin as "carbaminohemoglobins". CO2 and water form carbonic acid in the RBC; the conjugate base (bicarbonate) leaves the RBC into the plasma via the chloride shift forming the other 85-90%.

  7. but i have been told that hemoglobin is found on RBC membrane…. you said they are found inside the cell….can you clarify please

  8. Kind sir, thank you so much for your videos. I was about to smash by laptop and burn my textbooks and check myself into the crazy house and then i found your videos! You make science fun too learn and easy to understand. Your like a modern Bill Nye.

  9. To be honest the video's head title was about red blood cells and you only talked 4 minuts about it out of the 16 minuts 🙂

  10. I read that nitrogen doesn't get into the blood because at normal pressure it doesn't dissolve into water. That's a reason why we can't stay to long under high pressure, because are blood gets intoxicated with nitrogen.

  11. sir i want to ask does all of the nitrogen goes back in to the atmosphere because our veins have nitrous oxide

  12. @reddogpremier He meant like the shape of cough drops. That's what they're called! Lozenages or something like that haha. Just look up what Ricola is shaped like, or Halls Cough Drops.

  13. pretty sure i have learned that the vein is actually blue and the blood does not give it that color. deoxygenated blood is dark maroon and the material of the vein is blue.

  14. around 12:53 he is saying that because the light of the wavelength of red light is being reflected by oxygenated hemoglobin we perceivably see the RBCs reddish
    This contradicts with what I believe to know:
    If we see the color red, it's not because red is being reflected, but absorbed and all the other wavelengths of the spectrum of the visible light are being reflected; our brain receives the signal that 'red' is missing and interprets us the color as being red; please correct me if Im wrong

  15. You are indeed wrong about sight. When light comes down every wavelength of light but that of what you see is absorbed. The light, this time from red blood cells, is red so thats is what is reflected and seen by you.

  16. Wow. Very good film. My bro was formerly obese. He revolutionized himself from 284 lbs of pure fat into 206lbs of total lean muscle mass. We thought it was outstanding! I just registered myself as I'm working to get more impressive muscles. He made use of the Muscle Building Bible (Look in Google)…

  17. Good video but you lost me at favorable evolutionary trait.IMO there's no way that all that happened by chance.

  18. Veins are not blue because of the low amounts of oxygen!

  19. So it's plasma that gets to carry carbon dioxide to the heart and not rbcs?….what about allosteric inhibition? Where you mentioned co2 binding to hemoglobin

  20. doesn't really answer what happened to nitrogen. Does it diffuse into the capillaries and enter the circulation or there is a mechanism to filter nitrogen gas out before blood returned to the heart?

  21. reallly apppreciate the hard work..keeep it up khans aca…👈👏👏👏👏👏👏👏👏👏

  22. When diffusion occurs between alveoli and capillaries, if there is air on both the sides, then how do the different types of air know that there is a deficiency of that particular kind of air on the other side?

  23. so how does the process of Apoptosis take place with the cell. they die daily and every 7 years we have a renewed cell structure

  24. May I ask happens if the concentration of CO2 in the air is going to increase till 2% caused by global warming?
    How this is going to affect exchanges of CO2 in the alveoli system?

  25. ok, nitrogen difusses too – and I get hemoglobin does not bind it, so it doest stay there in that area between vein/artery at some concentration?
    and if yes why doesnt it spread further to our bloodflow by the law of diffusion?

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