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Bohr effect vs. Haldane effect | Human anatomy and physiology | Health & Medicine | Khan Academy

February 15, 2020

So we’ve talked a little bit
about the lungs and the tissue, and how there’s an interesting
relationship between the two where they’re trying to
send little molecules back and forth. The lungs are trying to
send, of course, oxygen out to the tissues. And the tissues are
trying to figure out a way to efficiently
send back carbon dioxide. So these are the
core things that are going on between the two. And remember, in terms
of getting oxygen across, there are two major
ways, we said. The first one, the easy one
is just dissolved oxygen, dissolved oxygen in
the blood itself. But that’s not the major way. The major way is when oxygen
actually binds hemoglobin. In fact, we call that HbO2. And the name of that
molecule is oxyhemoglobin. So this is how the
majority of the oxygen is going to get
delivered to the tissues. And on the other side,
coming back from the tissue to the lungs, you’ve got
dissolved carbon dioxide. A little bit of carbon
dioxide actually, literally comes just right in the plasma. But that’s not the majority of
how carbon dioxide gets back. The more effective ways of
getting carbon dioxide back, remember, we have this
protonated hemoglobin. And actually
remember, when I say there’s a proton
on the hemoglobin, there’s got to be some bicarb
floating around in the plasma. And the reason that works is
because when they get back to the lungs, the proton, that
bicarb, actually meet up again. And they form CO2 and water. And this happens because
there’s an enzyme called carbonic anhydrase inside
of the red blood cells. So this is where the carbon
dioxide actually gets back. And of course,
there’s a third way. Remember, there’s
also some hemoglobin that actually binds
directly to carbon dioxide. And in the process, it forms
a little proton as well. And that proton can
go do this business. It can bind to a
hemoglobin as well. So there’s a little
interplay there. But the important ones I want
you to really kind of focus in on are the fact that
hemoglobin can bind to oxygen. And also on this
side, that hemoglobin actually can bind to protons. Now, the fun part
about all this is that there’s a
little competition, a little game going on here. Because you’ve got,
on the one side, you’ve got hemoglobin
binding oxygen. And let me draw it twice. And let’s say this top one
interacts with a proton. Well, that protons going to want
to snatch away the hemoglobin. And so there’s a little
competition for hemoglobin. And here, the oxygen gets
left out in the cold. And the carbon dioxide does
the same thing, we said. Now, we have little hemoglobin
bound to carbon dioxide. And it makes a proton
in the process. But again, it leave
oxygen out in the cold. So depending on whether
you have a lot of oxygen around, if that’s the kind
of key thing going on, or whether you have a lot
of these kinds of products the proton or the
carbon dioxide. Depending on which one you
have more of floating around in the tissue in the
cell, will determine which way that reaction goes. So keeping this
concept in mind, then I could actually step
back and say, well, I think that oxygen is affected
by carbon dioxide and protons. I could say, well, these two,
carbon dioxide and protons, are actually
affecting, let’s say, are affecting the, let’s say,
the affinity or the willingness of hemoglobin to bind,
of hemoglobin for oxygen. That’s one kind of
statement you could make by looking at that
kind of competition. And another person come
along and they say, well, I think oxygen
actually is affecting, depending on which one,
which perspective you take. You could say, oxygen is
affecting maybe the affinity of hemoglobin for the
carbon dioxide and proton of hemoglobin for
CO2 and protons. So you could say it
from either perspective. And what I want to point
out is that actually, in a sense, both
of these are true. And a lot of times we
think, well, maybe it’s just saying the
same thing twice. But actually, these are
two separate effects. And they have two
separate names. So the first one, talking about
carbon dioxide and protons, their effect is called
the Bohr effect. So you might see that
word or this description. This is the Bohr effect. And the other one, looking at
it from the other prospective, looking at it from
oxygen’s perspective, this would be the
Haldane effect. That’s just the name
of it, Haldane effect. So what is the Bohr effect
and the Haldane effect? Other than simply saying
that the things compete for hemoglobin. Well, let me actually bring
up a little bit of the canvas. And let’s see if I
can’t diagram this out. Because sometimes I think a
little diagram would really go a long way in
explaining these things. So let’s see if I can do that. Let’s use a little graph and see
if we can illustrate the Bohr effect on this graph. So this is the partial
pressure of oxygen, how much is dissolved
in the plasma. And this is oxygen
content, which is to say, how much total oxygen
is there in the blood. And this, of course,
takes into account mostly the amount of oxygen
that’s bound to hemoglobin. So as I slowly increase the
partial pressure of oxygen, see how initially,
not too much is going to be binding
to the hemoglobin. But eventually as a few
of the molecules bind, you get cooperativity. And so then, slowly the
slope starts to rise. And it becomes more steep. And this is all because
of cooperativity. Oxygen likes to bind where other
oxygens have already bound. , And then it’s
going to level off. And the leveling off
is because hemoglobin is starting to get saturated. So there aren’t too many
extra spots available. So you need lots and lots of
oxygen dissolved in the plasma to be able to seek out and
find those extra remaining spots on hemoglobin. So let’s say we
choose two spots. One spot, let’s say,
is a high amount of oxygen dissolved
in the blood. And this, let’s
say, is a low amount of oxygen dissolved
in the blood. I’m just kind of choosing
them arbitrarily. And don’t worry about the units. And if you were to think
of where in the body would be a high
location, that could be something like
the lungs where you have a lot of oxygen
dissolved in blood. And low would be, let’s say,
the thigh muscle where there’s a lot of CO2 but not so much
oxygen dissolved in the blood. So this could be two
parts of our body. And you can see that. Now, if I want to figure
out, looking at this curve how much oxygen is being
delivered to the thigh, then that’s actually
pretty easy. I could just say, well, how much
oxygen was there in the lungs, or in the blood vessels
that are leaving the lungs. And there’s this much
oxygen in the blood vessels leaving the lungs. And there’s this much
oxygen in the blood vessels leaving the thigh. So the difference, whenever
oxygen is between these two points, that’s the amount of
oxygen that got delivered. So if you want to figure out
how much oxygen got delivered to any tissue you can simply
subtract these two values. So that’s the oxygen delivery. But looking at this, you
can see an interesting point which is that if you wanted to
increase the oxygen delivery. Let’s say, you wanted
for some reason to increase it, become more
efficient, then really, the only way to
do that is to have the thigh become more hypoxic. As you move to the
left on here, that’s really becoming hypoxic,
or having less oxygen. So if you become more
hypoxic, then, yes, you’ll have maybe a lower point
here, maybe a point like this. And that would mean a
larger oxygen delivery. But that’s not ideal. You don’t want your
thighs to become hypoxic. That could start
aching and hurting. So is there another way to
have a large oxygen delivery without having any
hypoxic tissue, or tissue that has a low
amount of oxygen in it. And this is where the Bohr
effect comes into play. So remember, the
Bohr effect said that, CO2 and protons
affect the hemoglobin’s affinity for oxygen. So let’s think of a situation. I’ll do it in green. And in this situation, where
you have a lot of carbon dioxide and protons, the
Bohr effect tells us that it’s going to be harder
for oxygen to bind hemoglobin. So if I was to sketch
out another curve, initially, it’s going to
be even less impressive, with less oxygen
bound to hemoglobin. And eventually, once the
concentration of oxygen rises enough, it will
start going up, up, up. And it does bind
hemoglobin eventually. So it’s not like it’ll
never bind hemoglobin in the presence of carbon
dioxide and protons. But it takes longer. And so the entire curve
looks shifted over. These conditions of high
CO2 and high protons, that’s not really
relevant to the lungs. The lungs are thinking,
well, for us, who cares. We don’t really have
these conditions. But for the thigh,
it is relevant because the thigh
has a lot of CO2. And the thigh has
a lot of protons. Again, remember, high
protons means low pH. So you can think
of it either way. So in the thigh, you’re going
to get, then, a different point. It’s going to be on the green
curve not the blue curve. So we can draw it at
the same O2 level, actually being down here. So what is the O2 content in the
blood that’s leaving the thigh? Well, then to do it
properly, I would say, well, it would actually be over here. This is the actual amount. And so O2 deliver is actually
much more impressive. Look at that. So O2 delivery is increased
because of the Bohr effect. And if you want to know exactly
how much it’s increased, I could even show you. I could say, well, this
amount from here down to here. Literally the vertical
distance between the green and the blue lines. So this is the extra oxygen
delivered because of the Bohr effect. So this is how the Bohr effect
is so important at actually helping us deliver
oxygen to our tissues. So let’s do the same thing,
now, but for the Haldane effect. And to do this, we actually
have to switch things around. So our units and our axes
are going to be different. So we’re going to have the
amount of carbon dioxide there. And here, we’ll do carbon
dioxide content in the blood. So let’s think through
this carefully. Let’s first start
out with increasing the amount of carbon
dioxide slowly but surely. And see how the content goes up. And here, as you increase
the amount of carbon dioxide, the content is kind of
goes up as a straight line. And the reason it
doesn’t take that S shape that we had
with the oxygen is that there’s no cooperativity
in binding the hemoglobin. It just goes up straight. So that’s easy enough. Now, let’s take two
points like we did before. Let’s take a point,
let’s say up here. This will be a high amount
of CO2 in the blood. And this will be a low
amount of CO2 in the blood. So you’d have a low amount,
let’s say right here, in what part of the tissue? Well, low CO2, that
sounds like the lungs because there’s not
too much CO2 there. But high CO2, it
probably is the thighs because the thighs like
little CO2 factories. So the thigh has a high
amount and the lungs have a low amount. So if I want to look at the
amount of CO2 delivered, we’d do it the same way. We say, OK, well, the
thighs had a high amount. And this is the amount of
CO2 in the blood, remember. And this is the amount
of CO2 in the blood when it gets to the lungs. So the amount of CO2 that
was delivered from the thigh to the lungs is the difference. And so this is how
much CO2 delivery we’re actually getting. So just like we had O2 delivery,
we have this much CO2 delivery. Now, read over the
Haldane effect. And let’s see if we can actually
sketch out another line. In the presence of high
oxygen, what’s going to happen? Well, if there’s a
lot of oxygen around, then it’s going to change
the affinity of hemoglobin for carbon dioxide and protons. So it’s going to allow less
binding of protons and carbon dioxide directly
to the hemoglobin. And that means that you’re
going to have less CO2 content for any given amount
of dissolved CO2 in the blood. So the line still is a straight
line, but it’s actually, you notice, it’s kind
of slope downwards. So where is this relevant? Where do you have
a lot of oxygen? Well, it’s not really
relevant for the thighs because the thighs don’t
have a lot of oxygen. But it is relevant
for the lungs. It is very relevant there. So now you can actually say,
well, let’s see what happens. Now that you have high
O2, how much CO2 delivery are you getting? And you can already see it. It’s going to be more because
now you’ve got this much. You’ve got going all
the way over here. So this is the new
amount of CO2 delivery. And it’s gone up. And in fact, you can even
show exactly how much it’s gone up by, by simply
taking this difference. So this difference right
here between the two, this is the Haldane effect. This is the visual way
that you can actually see that Haldane effect. So the Bohr effect and
the Haldane effect, these are two important
strategies our body has for increasing the
amount of O2 delivery and CO2 delivery going back and
forth between the lungs and the tissues.


  • Reply xirishluck7 November 9, 2013 at 5:01 pm

    Thank you so much!! Your videos are helping me through my RN program!

  • Reply Anon November 26, 2013 at 3:10 am

    Love your videos! I'm in medical school and every time I see the Khan Academy I instantly get excited because I know you guys will explain it clearly and concisely

  • Reply Jay hawkeye December 14, 2013 at 4:50 pm

    It makes so much sense on a detailed level now. Thank you.

  • Reply Katie January 10, 2014 at 5:36 pm

    your videos are just the best form of revision ever

  • Reply wu ryan January 30, 2014 at 2:52 am

    Thx for this awesome vid! Btw what kind of software did you use to present?

  • Reply Sumaira Nawaz Syed February 22, 2014 at 8:39 pm

    Very good explanation, thanks

  • Reply Stephanie Klein March 5, 2014 at 7:23 pm

    Excellent 🙂

  • Reply Zeeshan Mansuri March 20, 2014 at 11:42 pm

    Brilliant..Hats off

  • Reply Ean Frank March 29, 2014 at 1:06 am

    I thought CO2 and O2 didn't compete for hemoglobin since one binds to the heme, and the other to globin, am I mistaken?

  • Reply Charles Kakilla April 11, 2014 at 2:04 am

    Very helpful, thanks.

  • Reply Charles Kakilla April 11, 2014 at 2:04 am

    Very helpful, thanks.

  • Reply Lorenzo April 14, 2014 at 9:06 pm

    You are a god. I wish all professors were as clear as you. It would save hours of headaches.

  • Reply Lernik Shirvanian May 4, 2014 at 7:29 pm

    I wish my teacher could explain just like you. EXCELLENT!

  • Reply Passion for Medicine May 21, 2014 at 1:51 pm

    Awesome !! 😀 the Best videoooooo everrrr :'D
    THankkkk you

  • Reply James Dubreze June 24, 2014 at 3:00 am

    I thought the Bohr effect resulted in oxygen deprivation due to toxicity. The blood circulation from lungs to our cells. The more toxins in our blood the less oxygen it carries. Therefore the more of our cells that are deprived from oxygen the more …… 

  • Reply Jefferson Hunter August 7, 2014 at 3:35 am

    The difference between the oxygen content delivered (just after leaving the lungs) and the oxygen content after tissue metabolism does not equal O2 delivery, it equals o2 consumption or demand (Vo2).Am I confused?

  • Reply jakir hossain August 18, 2014 at 5:52 pm

    Thank you

  • Reply mrssmallwood404 October 13, 2014 at 1:27 am

    So when hemoglobin has less O2 saturation it is more attractive to CO2 to bind to it & the when there is less CO2 bound to hemoglobin the more attractive it is to O2??

  • Reply Midori Crush October 28, 2014 at 10:45 am

    Thanks! I kind of understand! need to watch this twice though! xD

  • Reply David Guillén November 8, 2014 at 10:42 pm

    Awesome!!! Thank you so much.

  • Reply Darius Garcia November 19, 2014 at 1:55 am

    your voice is made for this! thank you!

  • Reply Mr Dakso November 21, 2014 at 12:28 pm

    Excellent, very helpful

  • Reply Ho Sheung Fung December 5, 2014 at 4:51 pm

    crystal clear

  • Reply Tanvi Maurya February 8, 2015 at 6:57 am

    I have to present a seminar on the same. ..thank you for helping me out…

  • Reply John Michael March 14, 2015 at 7:46 pm

    God bless you. There are only a handful of people who can teach like u can. Mashallah

  • Reply Amerah Magumpara March 20, 2015 at 4:42 am

    Thank you for a very clear discussion…. 🙂

  • Reply Rimel N April 14, 2015 at 7:40 pm

    i love khan academy soooooooo much! like i have sooo much love for you guys!! first  you helped me ace my a levels and now you're making uni soo much more easier and enjoyable for me <3 may God bless you

  • Reply SiberianHuskyF1 April 20, 2015 at 11:33 am

    My lecturer somehow explained it in a way that I thought the two weren't even related. Thanks for clearing things up.

  • Reply Sam Crossley May 7, 2015 at 8:01 pm

    Oxyhameoglobin is HbO8 NOT HbO2

    Hb + 4O2 ——> HbO8

  • Reply Candy G May 28, 2015 at 4:27 am

    Thanks for making the Bohr effect and Haldane effect so easy to understand 🙂

  • Reply Anm Arefin July 29, 2015 at 4:26 am

    very fast be slow

  • Reply Mohanned Alnammi August 18, 2015 at 1:04 am

    Dang Sir….Best explanation Ive seen so far.I Salute sir.

  • Reply stojkotic August 24, 2015 at 10:57 pm

    Why did you disable timebar on Khan academy website? Its silly.

  • Reply Nuwan Bodhinayake October 26, 2015 at 6:54 pm

    So well explained! Thank you!

  • Reply Danielle Abeyta October 28, 2015 at 2:38 am

    Praise God that I found you!!! I can now understand what's going on in class!

  • Reply Inaam Hashim March 5, 2016 at 11:35 am

    Thanks for the video! This video helped me a lot! Although one note for other viewers would be that you need to have some understanding of the mechanisms involved to really grasp this concept. So if you watched this and didn't really understand it yet, go ahead and read up to remind yourselves of the principles of respiration: How gases such as O2 and CO2 move in and out of the body via the lungs.

    Concepts like Cooperativity and the principles of Hb affinity are also important, but this video covers it a bit.

  • Reply khushboo mishra March 14, 2016 at 12:20 pm

    thank u a lot… its really good for understanding… its help me a lot

  • Reply pramitbanerjee March 23, 2016 at 8:28 pm

    does haemoglobin take up carbon di oxide or is it just the plasma?

  • Reply Tamara Musick March 26, 2016 at 8:19 pm

    Omg, omg, omg. lol Thank you soooo much for creating this video. I read my textbooks and was like WHAT???? Found this video and understood every word. Thanks so much for making my learning easier. 🙂

  • Reply Yun Choi April 5, 2016 at 3:52 am

    Wow, this is an excellent explanation!!!! I was very confused about these two concepts but now I totally understand. Thank you very much!

  • Reply antony kolony April 14, 2016 at 11:23 pm

    thanks a lot!

  • Reply Cvs Manas April 22, 2016 at 4:24 pm

    when can we say that the curve turns to left ?

  • Reply TheHadesShade April 26, 2016 at 9:37 am

    this explanation is a lot better than the one my teacher gave me! Thanks for explaining! 😀

  • Reply Alan Allos June 16, 2016 at 1:04 am


  • Reply fayejur ahsan August 30, 2016 at 7:52 am

    Very helpful

  • Reply Ansh Midha September 8, 2016 at 5:45 pm

    cristal clear

  • Reply Ansh Midha September 8, 2016 at 5:45 pm

    cristal clear

  • Reply Nelly Hoffman October 2, 2016 at 2:25 pm

    This is amazing ! 🙂

  • Reply Ing00 October 4, 2016 at 12:49 pm

    at 2:50 why does it make a proton in the process? where does the proton come from?

  • Reply arjun , January 6, 2017 at 3:27 am

    did understand something but not all

  • Reply Angus Wong February 26, 2017 at 5:34 am

    thank you so much:)

  • Reply afra fatimah March 14, 2017 at 3:37 pm

    you saved our lives thank you

  • Reply Shivani Rajpara March 14, 2017 at 7:19 pm

    Thank you Dr. Desai !!

  • Reply Sadaf Khidir March 15, 2017 at 4:02 pm

    I didn't get the actual difference between the two quantities on the two axes

  • Reply Snehal Shelke May 9, 2017 at 11:29 am

    you make the concepts Amazingly simplified! too good..😃👌👌

  • Reply Osmosis Bin Laden May 24, 2017 at 11:12 pm

    very good video

  • Reply Dhruv Kumar June 15, 2017 at 9:02 am

    wat an explanation

  • Reply be bo June 15, 2017 at 12:15 pm

    I hope someone will answer me. For the Bohr effect, how can you raise the CO2 and Proton levels in the muscle?

  • Reply Steven Günther July 14, 2017 at 8:27 pm

    Can't say it in better Words than John Michael. You Sir, are a brilliant teacher and you just helped a medicine student from Germany to pass the first (and most feared) part of his finals. Thank you so much!

  • Reply mix text September 13, 2017 at 5:26 pm

    Why do we have so much of co2 in the thighs .. thank u

  • Reply Lucas Azevedo September 21, 2017 at 2:44 pm

    Coloquem legendas, por favor!!!

  • Reply Borat Sagdiyev October 2, 2017 at 5:37 pm

    Just something that was confusing for the Haldane effect. You kept saying that the x axis is the "amount of CO2 in the blood" and that the y axis is the "content of CO2" in the blood, it didn't seem as though the distinction was as clear as it could have been.

  • Reply justin wong October 22, 2017 at 8:11 am

    thank you so much

  • Reply 張永蔚 October 22, 2017 at 2:58 pm

    always a fraction for co2 to be eliminated

  • Reply srusti dash October 25, 2017 at 4:12 pm

    Eargasm ..😅

  • Reply Justin Oh November 9, 2017 at 10:51 pm

    Thank you so much for the great explanation! I'm not sure if someone has already asked you this but I was wondering what kind of equipment you are using to both record and draw out your explanations? Thank you for your time and I hope to hear from you soon.

  • Reply Angshu Changdar February 23, 2018 at 12:59 pm

    great lecture

  • Reply kriya bing April 7, 2018 at 3:21 am

    Best video ever

  • Reply nicole peterson May 31, 2018 at 7:11 pm

    what if it shifts more to the left or right? Will it get bad or worse?

  • Reply The Mintleaf June 3, 2018 at 11:17 am

    Beautifully explained, thank you so much for making this crystal clear :O, I have tried time and again, to understand this concept, and never really managed to until I bumped into this video 🙂

  • Reply Justin Howlader June 22, 2018 at 1:31 pm

    Why is there co operative effect when co2 is added to haemoglobin?

  • Reply shravan kashyap July 1, 2018 at 4:10 pm

    for Haldane effect how are u taking thigh point on one curve and the lung point on the other curve & then taking the difference of CO2 and then showing that there is a inc in CO2 delivery? i am confused shouldn't both points be on the same curve if ur taking the difference

  • Reply kittendivine1 July 18, 2018 at 3:29 pm

    This is complicated for me. Can anyone tell if Buteyko breathing method's interpretation of Bohr effect is correct? It says that the more oxygen you breathe in, the less oxygen can be delivered to your tissues, because CO2 will be low as well with deep breaths, you expel more of it.

  • Reply kittendivine1 August 4, 2018 at 11:47 am

    This was really insightful. What I don't understand though: how come oxygen saturation is 95-99% in the finger? And why is it the same in my toe? Yes I tested it with an oxymeter!

  • Reply rakesh bagali August 27, 2018 at 10:29 am

    Great explaination👌

  • Reply FadeSkywards August 27, 2018 at 7:27 pm

    Why is the CO2 graph depict a straight line and not a curve? Shouldn't there be a gradual change in affinity of Hb for CO2?

  • Reply Mandeep mehra Kaur September 3, 2018 at 8:23 am


  • Reply Bikrant Khanal September 22, 2018 at 1:49 pm

    Dear human being, i thank you very much!

  • Reply Sanjay Kumar October 16, 2018 at 4:42 am

    Sir very clear concept … hope this will help our budding doctors.

  • Reply kirti phulwani October 30, 2018 at 6:22 am

    Hey can you tell me which app you are using

  • Reply Ken Warden November 20, 2018 at 11:05 am

    Seahawks should of run the ball

  • Reply Ailin Monti November 20, 2018 at 10:23 pm

    Gracias por este vídeo! me ha servido de mucho!

  • Reply Allison Magang Joot Marial December 5, 2018 at 12:28 pm

    love your videos i,will be medical man some year coming and every time i see the khan academy

  • Reply Anastasiia Kolerova December 30, 2018 at 6:03 am

    Thank you so much! Great explanation! Hundred likes

  • Reply fictional writing January 22, 2019 at 5:19 am

    Khanz are best in every field

  • Reply Saiyed Muhammad Tahmeed January 29, 2019 at 4:07 pm

    How do you gys write this stuff on screen??…due you use the mouse or a digital pen??

  • Reply T. F. February 1, 2019 at 10:42 pm

    10:30 haldane effect

  • Reply Carolina Salazar May 16, 2019 at 1:47 pm

    thank you so much!!!! you saved me

  • Reply Eric Erickson June 17, 2019 at 9:50 pm

    In my opinion, just read about both effects; this was a pretty confusing “picture show” and narrative.

  • Reply Social Justice August 9, 2019 at 2:24 am

    Thanks man! That was really well explained. One thing that I'd recommend to make the video just a bit more clear, is explaining the second way that CO2 delivery back to tissues. I was a bit confused at 2:26 until I looked back at First Aid. Other than that, this was really perfect. Not even boards and beyond cleared this up to the extent that you did ^.^

  • Reply Asura August 26, 2019 at 2:38 pm

    what causes the oxygen molecules to behave cooperatively? why don't they just bind linear/proportional to the partial pressure as well?

  • Reply HEAR MEME August 26, 2019 at 8:21 pm

    Good job

  • Reply Chien Hong September 23, 2019 at 1:39 pm

    I want to ask is it some of the H+ ions from H2CO3 will bind with Hb while some of it will diffuse out of RBC? but on the other hand, there is also chloride shift that HCO3- ions moves out of RBC. I'm a bit confused about the change of blood pH..

  • Reply ersad Kaya October 30, 2019 at 8:45 pm

    I’m about to fucking lose, jesus fuck medschols hard

  • Reply Pragya Jha November 1, 2019 at 12:54 pm

    Imaging studying this in 11 grade for your exams

  • Reply Maddie Rose November 6, 2019 at 1:25 am

    Wow bless you! My professor is awful but you explained things perfectly.

  • Reply Kamaljeet Sidhu November 19, 2019 at 4:29 pm

    who is watching in nov. 2019…?

  • Reply Abdo Ly December 17, 2019 at 5:50 pm

    Where is the Arabic translation 😢?

  • Reply UNI FOODs January 20, 2020 at 10:21 pm

    One of the best revision videos I have ever seen. Cheers!

  • Reply jy l February 6, 2020 at 2:41 am

    there is no practice section? I'd like to solve the questions.. Anyway thanks for great explanation!

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