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What’s inside of blood? | Lab values and concentrations | Health & Medicine | Khan Academy

December 20, 2019

Let’s say that I go to
the doctor’s office, and I hate when this happens,
but I, once in a while, have to have my blood drawn. And the reason I hate it is
that I’m kind of a chicken when it comes to getting
my blood drawn. I don’t like needles. But of course, I do as I’m
told, and I get my blood drawn. I just try to distract myself
when the blood is actually filling that needle. And I usually look away,
and before I know it, it’s done, right? So it’s out of my
mind, and I walk out of the office pretty
happy, because now I don’t have to think
about it anymore. But here’s what I
want to do now is kind of follow the path that
this blood takes and think about what happens next,
after they draw the blood. So the first step
is they usually put that blood into a tube. And usually that’s done
directly, actually. These days, usually
that tube is already kind of sitting and
waiting, and it’s collecting blood immediately. So this is my cap for my tube. And inside my tube,
I’ve got blood. This is my blood
filling up this tube. And this is kind
of a special tube. And the thing to
know about this tube is that on the walls of the tube
is a chemical that basically prevents the blood
from clotting. You don’t want
the blood to clot, because it’s hard to do
any sort of lab work on it. And so this tube is very
special in that way. It won’t actually clot. And so to make sure it’s working
properly, sometimes people kind of gently shake the
tube up a little bit, just to make sure that
there’s good mixing and so that the
blood doesn’t clot. Now from there, the blood
goes over to the lab. So there’s a machine in the
lab that takes blood from me. This is my blood. But it also takes blood
from other people, let’s say a few other patients
in the hospital that day, or in the clinic. And all of our blood is
kind of labeled and put into this machine. And what the machine
does is it spins. It basically spins
really quickly. So all these tubes,
they’re attached so they don’t fly away. But they basically spin as well. And if all these tubes are
spinning, then what it creates is a force called the
centrifugal force. So this process is
called centrifugation. Let me write it out here. Centrifugation. And the machine is
called a centrifuge. So it’s basically going to
spin really quickly, let’s say, in one direction or the other. And as a result, what
happens is that the blood starts separating out. And the heavy parts
of blood kind of go to the tip of the tube. And the less dense part of blood
actually rises towards the lid. So after you’ve
centrifuged– let’s say you’ve actually gone
through this process, and you centrifuge the blood. Now you have the
same tube, but I’m going to show you kind
of an after picture. So let’s say this was before
I actually spun the tube, and now I’ve got an after. This is my after picture. So after I spin the tube,
what does it look like? Let me draw the tube. And the biggest
key difference here is that instead of
having one similar looking homogeneous liquid, like
we had before, now it actually starts looking really different. You’ve got three
different layers, in fact. I’m going to draw all
three layers for you. So this is the first layer. And this is the most
impressive layer. The largest volume
of our blood is going to be in this top layer. So remember, this is
the least dense, right? It’s not very dense, and that’s
why it stayed near the lid. And it’s actually going
to make up about 55% of our total volume. And we call it plasma. So if you’ve ever
heard that word plasma, now you know what it means. So if I was to take a
drop of this stuff– let’s say I took a little
drop of this plasma, and I wanted to take
a good hard look at what was in my
drop– 90% of plasma is going to be nothing
more than water. So that’s interesting, right,
because the major part of blood is plasma, and the major
part of plasma is water. So now you’re seeing why it
is that we always say, well, make sure you drink
a lot of water. Make sure you’re hydrated. Because a big part of your
blood itself is water. And in fact, that’s true for
the rest of your body as well. But I want to stress that
it’s true for blood as well. So that leaves the rest, right? We’ve got 90%, we
have to get to 100%. So what is 8% of this
plasma made up of? It’s protein. And let me give you some
examples of this protein. So one would be, for
example, albumin. And albumin, if you’re
not familiar with it, it’s an important
protein in your plasma that keeps the liquid
from kind of leaking away out of the blood vessels. Another important
protein, the antibody. And this, I’m sure
you’ve heard of, but antibodies are basically
involved in your immune system, making sure that you
stay nice and healthy and don’t get sick
with infections. And another part of the protein,
another type of protein, to kind of keep in mind,
would be fibrinogen. And this is one important
protein involved in clotting. And there are actually many
other clotting factors, we call them, as well. So I’m just going to put
clotting factors here. So these are proteins. Things like albumin,
antibody, fibrinogen, these are all proteins. Now we’ve still got
2% to account for. And this is going to be things
like hormones, for example. And that could be
something like insulin. We’ve got electrolytes. That could be
something like sodium. And we’ve got, also, nutrients. So nutrients, that could
be something, for example, like glucose. So these things all
make up your plasma. So a lot of the things that
we kind of think about, talk about, are
all in your plasma. including vitamins
and things like that. So now another layer we
have right below the plasma, is here in white. And if I was to kind
of zoom in on it, it would be a very, very tiny
part of blood– less than 1%. And this is actually
white blood cells. This layer contains white
blood cells and platelets. So these are cellular
parts of our blood. And they make up
a very tiny bit, but they’re a very important
part of our blood, of course. Below this layer, so now the
most dense layer of blood, would be the red blood cells. So this is this last bit. And this, just to make it
add up, would be about 45%. And these red blood
cells, of course, contain within them hemoglobin. So sometimes it gets
tricky because you forget, you think, oh, protein,
so that must be plasma. Remember, red blood cells
and white blood cells, within them, they
have proteins as well. So just remember, they
contain lots of protein as well, within them. So, for example, hemoglobin. So this is an example, right? Now one word you may
have heard of is serum. So what is serum, exactly? Well, serum, this word,
is very, very similar to plasma in terms
of what it’s made of. In fact, if I was to
circle what is in serum, I would circle this bit. Basically everything within
my blue line would be circled. This is serum. And so the only thing
I’ve left out of serum is fibrinogen and
the clotting factors. So plasma and serum you can just
remember being very similar. And the exception is
that the serum does not include the fibrinogen
and clotting factors. Now, looking down at the
red blood cells, what can we learn from that? Well, you may have heard
this term hematocrit. And if this was my
blood, if I actually had drawn my blood, as
I drew in this picture, and this was my blood, my
hematocrit would have been 45%. And all that means is hematocrit
equals volume taken up by red blood cells divided
by the total volume. So if, in this case,
my total volume is 100, my percent, I already
told you, is 45%. So that’s why I knew
my hematocrit was 45%. It’s just the percent taken
up by your red blood cells. And that’s an important
percent to know, because the red blood cells
are the part of blood that are actually carrying
the oxygen around. Now to kind of stress this point
of hematocrit further and maybe even kind of introduce
a couple of new words, let me draw out three
little vials of blood. So let’s say I have three
vials here– one, two, three. And these are going to be three
different people, let’s say. But they’re all very similar,
same age, same gender. Because hematocrit,
what is normal, is actually going to change
depending on whether you’re talking about a certain
age, a certain gender, even depending on where you
live in terms of altitude. Because let’s say you live
at the top of a mountain, that is going to affect
your hematocrit as well. So a lot of things
affect hematocrit. But let’s say we have
three people kind of very similar in those ways. Now the first person, I’m going
to draw out their blood here. Their plasma, let’s
say, is taking up this much of their total volume. The second person, their
plasma is taking up this much of their total volume. And the third person,
their plasma is taking up, let’s say, a lot, of
their total volume. Let’s say all the
way down to here. So you spun all three, and
this is what you’ve gotten. Of course, all three still
have white blood cells. Got to draw that in. And they have
platelets, of course, that’s this tiny little
layer– less than 1%, we said. And the remainder then
has to be red blood cells. So this is the red blood
cell layer right here. This is the red
blood cell layer. And it’s really large here
for this second individual. And this third
individual, it’s actually kind of on the smaller side. Not too much of the volume is
taken up by red blood cells. So here, if I was to kind of go
through and label these folks, I would say, well,
this first person is what I would call normal. The second person has a
lot of red blood cells. It’s so, so predominant. This is a very high
percentage, right? I know this is taking up a high
amount of the total volume. So this person has what I
would call polycythemia. It’s just a medical word to say
that the volume of red blood cells over the total
volume is very high. Or you could say their
hematocrit is very high. And this person,
this third person, has a very low amount
of red blood cell volume relative to
the total volume. This is actually pretty low. And so this person, I
would say, has anemia. So if you’ve ever heard these
terms anemia, or I’m anemic, sometimes people say, or even
the word polycythemia, now you know it’s just referring to
what volume of their blood is taken up by red blood cells.


  • Reply sham syr January 29, 2014 at 10:22 am


  • Reply Tate David Lim February 3, 2014 at 1:52 am


  • Reply MAYDAYES February 26, 2014 at 2:58 pm

    Thank you very much! I need to watch this for my phlebotomy state exams

  • Reply Sydney Soleil March 17, 2014 at 2:43 am

    This is awesome 😀  Thank you!

  • Reply Frine Santiago March 17, 2014 at 7:43 pm

    love all the videos from khan academy , keep posting 🙂

  • Reply Yaron Zaret March 18, 2014 at 7:57 pm

    Great video!

  • Reply Mackenzie Miller March 29, 2014 at 11:11 pm

    So I learned that 55+1+45=100? I love Khan Academy though. Great video.

  • Reply Elizabeth Keme May 18, 2014 at 6:31 pm

    Beautiful! (^.^)/ 
    Thank youuu ~~~ 

  • Reply THANH NGUYEN May 30, 2014 at 4:19 am

    I really really like your hand writing mate. 

  • Reply Shirin K June 3, 2014 at 11:49 pm


  • Reply ceoleo June 14, 2014 at 4:44 pm

    Thank you so much for the great intro lesson. I really enjoyed it.

  • Reply Elton Olluri July 4, 2014 at 8:03 pm

    first of all thank you for the video,
    but I'd like to ask one more question:
    1. Why there is no fibrinogen and clotting factors in serum, why do they take it out ?

  • Reply Lester Sanchez August 1, 2014 at 7:58 pm

    Khan School of Medicine. That is where I want to go.

  • Reply Lubna Ahmed August 6, 2014 at 12:12 pm

    Perfect well done

  • Reply divyanshu dembi August 10, 2014 at 5:59 am

    Very good video but if a person has anemia, then it would have more protiens etc. and as a resut may be more healthy than other two or is it that the small percentage of the protiens etc. do not affect the health?

  • Reply Il Saophorn August 25, 2014 at 2:48 pm


  • Reply yacchan1210 September 8, 2014 at 4:50 am

    thank you for the video. im japanese trying to enter a japanese medical school and that was really heipful!

  • Reply kwame kofi September 14, 2014 at 11:58 pm

    great video

  • Reply Cameron Seider September 19, 2014 at 8:29 am

    Great instructions – clear, concise, and organized. You have a gift for teaching.

  • Reply Nirmal Agarwal October 11, 2014 at 2:36 am

    very good video thanks 

  • Reply 1tightline fishing October 22, 2014 at 6:59 pm

    your not bad with a mouse thumbs up

  • Reply Kings lynn October 24, 2014 at 9:43 pm

    I have 0 neg blood and I've noticed that my blood is not bright red, but sort of a maroon or purplish color. I noticed this before I became aware of all the 0 neg hype. Also I'm wondering if you would suggest a copper supplement?

  • Reply ItachiOfTheLeaf October 28, 2014 at 5:49 pm

    Thank you that was very helpful !!! 😉

  • Reply Milka December 18, 2014 at 7:28 am

    Dude that was awesome thank you!!!!

  • Reply morad quderi January 14, 2015 at 3:09 pm


  • Reply Darius Dagys January 20, 2015 at 9:49 pm

    Thank you very much, i liked your practicality and sense of humor

  • Reply Vanessa C February 8, 2015 at 4:05 pm

    So since plasma is the liquid part of blood, does that mean that when it is separated from plasma it will be solid?

  • Reply asha2393 February 11, 2015 at 3:52 am

    Great video, but one minor problem. The color of the tube is actually lavender, not green. This may seem insignificant initially, but the color of the tubes correlate with different additives that are not only used to prevent clotting, but also to preserve the analyte trying to be measured. The color you used in the video would not be a hematology specimen, but most likely a chemistry. In reality if a tube was sent down to the lab in a mint green/green tube this would not be accurate, the specimen would be rejected and require a recollect. 

  • Reply TheLeonSilver February 23, 2015 at 11:48 am

    you're the love, you're the life <333333 clear and precise explanation thankssss

  • Reply Tranquility May 14, 2015 at 8:03 pm

    Great brake down explanation, had me so….. intrigued I Love science!

  • Reply Taskeen Mather May 14, 2015 at 10:01 pm


  • Reply Ekrem Bulut May 17, 2015 at 6:56 pm


  • Reply FL8H May 19, 2015 at 12:18 pm

    i love khan channel

  • Reply Lucas V. May 23, 2015 at 6:53 pm

    Astonishing! Love the visual representation of it!!

  • Reply HamdaSaeed 11s3 June 5, 2015 at 6:53 am

    please someone tell me the truth
    Dose the serum contain fibrinogen -clotting factor – because I searched and found that serum is the blood after clotting and the plasma is after centrifuging
    so what I understand from him that a serum is part from the plasma !! what is right
    Is the fibrin a natural component of the blood in the plasma/ removed when collecting serum or something is added beside the natural clotting factor ?
    sorry if I span your head

  • Reply exploreyourweb June 5, 2015 at 6:55 am

    Wow, what an AWESOME video. Loved the bit about anemia and polycythemia. I suffer from anemia, so I'm gunna donate my plasma more often. Thank you



  • Reply Karen Dhillon June 23, 2015 at 11:23 pm

    You actually want some of the tubes to clot. A lot of them will only separate into plasma and the rbcs if it is already clotted

  • Reply Charlee Dilley August 12, 2015 at 6:41 pm

    Serum=clot, Plasma=not clot!!!!!!!! Three tubes make serum, the rest make plasma or whole blood

  • Reply H August 28, 2015 at 7:01 am

    I make most of my test notes from your videos. So helpful. You truly are a gift, thank you. <3

  • Reply I'mBored777 September 4, 2015 at 1:33 am

    Am i the only one that actually LIKES to watch my blood fill the syringe/tube?

  • Reply Eagle Eagle October 8, 2015 at 11:01 pm

    Love the visual! what software did you use to make the animation?

  • Reply Mina Mina November 4, 2015 at 7:04 pm

    Very nice !!

  • Reply NoOoNy Queen November 7, 2015 at 12:18 pm

    your drawing is amazing ???????

  • Reply Niaz Khan November 26, 2015 at 1:14 am

    What program do you use?

  • Reply Joe Conville December 11, 2015 at 2:15 pm

    You spelt HAEMOGLOBIN wrong!!!

  • Reply ThePublixBandit December 31, 2015 at 3:09 am

    You draw like a boss

  • Reply mike villa February 11, 2016 at 4:43 pm

    Khan i have a request. For halloween could you do teratoma? how it's formed, what may cause it, what's REALLY inside and a break down on what the liquid portion is

  • Reply Richard L. Currier February 13, 2016 at 3:32 am

    rishi is much better than sal at teaching in my opinion. maybe because i am a visual learner and visuals appeal to me better than words.

  • Reply Radhika March 25, 2016 at 1:20 am

    I really enjoyed the video and the explanation. Clean simple explanation.

  • Reply Brandon Watson March 26, 2016 at 6:17 pm

    hi guys

  • Reply Malisa Johnson April 5, 2016 at 7:12 pm


  • Reply ibrahim ali April 10, 2016 at 4:22 pm


  • Reply I Superme April 16, 2016 at 7:50 pm

    Thank you man , that is very nice work .. And you can draw fast and accurate lol ..

  • Reply ishita b May 5, 2016 at 8:26 am

    The lecture was so clear and crisp!! I absolutely loved it !

  • Reply Dat Nguyen May 16, 2016 at 5:04 pm

    you should have somethings like ending so viewer will know that the video going to end

  • Reply Paulette Penning June 20, 2016 at 7:54 pm


  • Reply Agent Soap June 23, 2016 at 6:04 am

    You're good at drawing bro and nice simple explanation

  • Reply Shamir ZZ June 30, 2016 at 1:51 pm

    wow dude thanks ! you have actually summarised literally half the videos on youtube

  • Reply Varun Tanwar August 9, 2016 at 10:07 am

    thankyou 🙂

  • Reply Adrianna Marie August 14, 2016 at 4:30 am

    Nice drawing. Thanks for sharing this video

  • Reply Frank A August 27, 2016 at 8:52 pm

    Excellent, informative descriptive yet simplistic.

  • Reply Tacoma Washington August 28, 2016 at 2:53 am

    great video !

  • Reply LauraJane424 September 14, 2016 at 4:24 pm

    Is fibrinogen a fiber?

  • Reply ジェネレーションニュー September 16, 2016 at 2:08 am

    Thank you for being descriptive

  • Reply Co S September 17, 2016 at 8:04 am

    Shoulllllddddddd be a purple top tube for EDTA.

  • Reply viet ha Nguyen October 4, 2016 at 9:01 am


  • Reply lsaddatova m January 13, 2017 at 9:42 pm

    thank you finally i get to understand ..

  • Reply احمد العنزي February 8, 2017 at 10:37 pm

    great explanation

  • Reply Edward Payne February 12, 2017 at 12:04 am

    So we evolved, don't think so.

  • Reply Sara Mizzi March 26, 2017 at 9:31 pm

    I discovered your videos last year and started watching to help me pass my entrance exam to medical school and I here I am now watching these phlebotomy videos! Thank you for your diagrams and explaining this in a simpler way than my textbooks do 🙂

  • Reply Ali Aljibari April 4, 2017 at 12:46 pm

    I recognize this voice from a video done with Doc Ossareh. I then read the description and it says 'pediatric infectious disease physician' and now I knew it was him. pretty cool!

  • Reply christ's failure 000 April 6, 2017 at 3:54 am

    Sounds like Anxiety War.

  • Reply yvanka semp April 18, 2017 at 7:23 pm

    thank you!!

  • Reply sebastian cuello June 11, 2017 at 1:59 am

    holy shit this video was fucking awesome

  • Reply Shuvhashish Saha June 24, 2017 at 6:26 pm

    according to my textbook, it states that it has wastes. What kind of wastes does it have? and plus, if there is waste, how can it provide nutrients to the cells since wastes and nutrients are mixed into that same liquid concentration.

  • Reply Larry Johnson August 27, 2017 at 5:33 pm

    Very informative video!

  • Reply Ivy Ngai October 11, 2017 at 6:58 am

    most impressive layer …hahhaha

  • Reply Angie W October 18, 2017 at 4:53 am

    Abrupt ending, but great presentation nonetheless.

  • Reply Unbreakable Pickaxe November 9, 2017 at 5:22 pm

    Really helped. Thank you

  • Reply Nazmul Hossain December 29, 2017 at 8:18 pm

    Drawing and handwriting on point.

  • Reply RS_ ALPHA February 26, 2018 at 11:16 am

    91%is h2o

  • Reply RS_ ALPHA February 26, 2018 at 11:17 am

    7%is protein
    2%are rest solutes

  • Reply Aaron Devries March 28, 2018 at 3:37 pm

    dude, how the heq do you write and draw like that with a mouse.

  • Reply ACE_stealth251 [ACE] May 13, 2018 at 1:47 am

    Omg this my 5th time trying to watch the first few seconds omg!! I hate it I can't look eww, I hate blood and injections! I can not even get passed 12 seconds. Argh OMG !!

  • Reply devid Roy May 25, 2018 at 8:15 pm

    sal the super teacher?

  • Reply Lanh Tran June 5, 2018 at 4:04 pm


  • Reply Daniella Sinan June 8, 2018 at 6:20 am

    Iam studying phlebotomy and its my second language its hard to understand the teacher buttt buttt youu save my life you make me to understand everything clearly i love you don’t stop teaching you are the best thank youuj soo much

  • Reply Amalia G July 19, 2018 at 11:14 pm

    Great video.. very informative and easy to follow along with

  • Reply francios marchendie August 8, 2018 at 2:37 pm

    Love you!love you! love you!???????? I understood everything ,loud and clear

  • Reply محمد علي November 11, 2018 at 4:17 pm

    Thank you very much , it is a pleasure to watch the video.

  • Reply Ray Ibo November 26, 2018 at 5:11 am

    Plasma and serum is really confusing me

  • Reply Anil Johny April 3, 2019 at 3:17 pm

    You teach more than that of my leacturer do for 1 hour with 8 min

  • Reply Jhush Smith May 4, 2019 at 3:36 am

    Can the elements of the periodic table create real blood?

  • Reply Narmin Salimova May 21, 2019 at 10:55 pm

    Thank u so much, great explanation. Many things I was confused with, are brilliant clear now. I wasn't able to grasp the concept of centrifugal process, though using that machine is the part of my new experiment, but now I know what I am doing <3

  • Reply Hanan Qaraman November 21, 2019 at 5:49 pm

    You did a mistake

  • Reply Hanan Qaraman November 21, 2019 at 5:50 pm

    I calculated and found that you wrote that the blood is 101/

  • Reply Hanan Qaraman November 21, 2019 at 5:51 pm

    In your video

  • Reply Youssef November 22, 2019 at 11:10 am

    Ayyyy yooooo

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