00:01
How do you do or how do
you carry the blood?
It’s bound to hemoglobin.
00:06
So hemoglobin is a molecule in which
there are four binding sites for oxygen
and these are denoted here as first
binding site, second binding site,
third binding site and finally
having all four bound.
00:19
And what’s unique about these
particular binding sites
is that the affinity for O2 increases
as the number of molecules
of oxygen are bound.
00:28
So having three molecules bound,
you have a higher propensity
to get a fourth molecule bound
than you had with if
you only have one.
00:40
And this is a unique property
and what it does is it makes
for a non-linear relationship
between the amount of oxygen available
and the amount bound to hemoglobin.
00:50
And I can show you that relationship
in the next few slides.
00:55
This sets up the sigmoidal curve which is
the oxygen-hemoglobin dissociation curve.
01:02
And this is probably one thing that I know
you probably seen before in other classes
that you've attended and
other information.
01:08
But we’re going to add a few
things to this particular curve
that you may not
have had before.
01:15
So you’ll notice that
there are two Y axes.
01:16
So let’s start off
with the first Y-axis.
01:20
This is going to be the
percent of hemoglobin
or percent of the
hemoglobin that is bound.
01:27
So you can have up to 100 percent of your
hemoglobin bound all the way down to 0.
01:33
On the second Y axis, we
have the amount of O2
in terms of a concentration or some
people like to call it a content.
01:41
And that’s going to be the millilitres
of O2 per 100 mLs of blood.
01:45
And so this is a content value and
the other is a percent value.
01:51
On the X-axis here,
we have the PO2,
which is going to be the
partial pressure of oxygen.
01:58
So let’s kind of walk through
this oxygen hemoglobin binding
and we’ll be able to understand
better how changes might occur.
02:08
Hopefully, you’re able to see that
you can bind upwards to about 19,
maybe even close to 20 milliliters
of O2 per 100 mLs of blood.
02:19
So in that case, that is compatible with
life to be able to carry that much O2.
02:23
Because remember in the dissolved
form, it was only around
0.25 milliliters of O2
per 100 mLs of blood.
02:30
So this is carrying a vast
majority or, in fact,
about 98 percent of the total oxygen
carried is in the bound form.
02:43
So,
hopefully, you also can see
on this particular graph
that there is a kind of a large
area in which PO2 can change
without much changing in O2 sat
or in the O2 concentration.
02:59
And that kind of flat area of the
curve is usually where people
will think about what’s on the
arterial side of the circulation.
03:08
It’s fairly flat
up on that side.
03:11
On the venous side of the circulation
is just when it starts to fall.
03:16
So usually, you start off with
close to a 100% saturation,
you return on the venous side
with maybe 75% saturation.
03:25
So you’re only utilizing that top portion of
the oxygen-hemoglobin dissociation curve.
03:31
But if you look at PO2 numbers on the
arterial side of the circulation,
you might’ve started out at 95 PaO2
and returned at PVO2 of only 40.
03:44
So there is about there 55
millimeters of mercury of O2
that were changed and you only had about
25% change in your O2 saturation.
03:55
So why is this so important?
What it does is it allows you
to have quite a bit of reserve,
so you could extract more O2
in certain conditions in where
you needed more metabolism.
04:09
So in those cases where you
were going to extract more O2,
PVO2 would be at a
much lower level,
maybe O2 saturation
would drop to 50%
and you would still be able to garner
enough oxygen to your peripheral tissues.
04:28
Besides this binding, there is another
component that we need to add
and this component
is known as a P50.
04:35
This P50 is the 50% saturation point of the
oxygen hemoglobin dissociation curve.
04:44
And we talk about P50 a lot,
because it’s a good index to telling
us which way a curve might shift,
like a rightward shift in the PO2 curve
is denoted by an increase in the P50.
04:57
While a leftward shift in the curve
is denoted by a decrease in the P50.
05:02
So let’s go through what those
P50 changes might look like.
05:08
Okay, when we look at a
leftward shift in the curve,
we’re actually moving
in this direction.
05:12
So the leftward shift involves
a decrease in the P50.
05:20
This allows for a little
bit greater loading
of oxygen on the flat
portion of the curve,
but what it does is allows for a steeper
portion on the left hand side of the curve.
05:36
Why this is so helpful is it--
what I’d think of it as is it’s
helping the lung to bind O2
but the problem is it’s not letting oxygen
off at the peripheral tissue level.
05:50
So this can be problematic.
05:52
If you’re not letting
oxygen be released,
you’re holding onto it,
peripheral tissues are not
getting as much oxygen
with a leftward shift in the
oxygen-hemoglobin dissociation curve.
06:04
A rightward shift though
does the opposite.
06:08
In this case, you’re moving
the curve in this direction,
which causes an increase in the P50.
06:15
The lung size changes
drop slightly,
but the tissue effect
is very great.
06:22
And that tissue effect allows
you to offload more O to it
any given partial
pressure of O2.
06:30
So this is a very handy way to deliver
more oxygen to peripheral tissues.
06:35
So when might you need more delivery
of oxygen to peripheral tissues?
There are four primary conditions
in which this happens.
06:44
The first is as you increase temperature,
you cause a rightward shift in the
oxygen hemoglobin dissociation curve.
06:51
As you decrease temperature,
you cause a leftward shift.
06:55
CO2 as you increase
the amount of CO2,
you cause a rightward shift in the
oxygen-hemoglobin dissociation curve.
07:06
As you decrease pH or increase
the amount of hydrogen ions,
you also cause a rightward shift in the
oxygen hemoglobin dissociation curve.
07:16
And the final way is if you increase
a metabolite of red blood cells
which is 2,3 DPG or some
people refer to it as BPG.
07:26
This will also cause a rightward shift in
the oxygen hemoglobin dissociation curve.
07:31
So that’s associated with metabolism
in the red blood cell itself.
07:34
This usually only occurs as you ascend
to higher and higher altitudes.
07:40
The first three occur
during sea level,
which is the changes in
temperature, CO2 and hydrogen ions.
07:49
Now, the last aspect of the oxygen
hemoglobin dissociation curve
that we need to deal with is if you have
changes in the amount of hemoglobin
that two or more people
might have have.
08:03
And this is clinically important
especially when someone has either
carbon monoxide poisoning
or has an anemia.
08:12
So let’s go through with the
normal responses first.
08:14
So normally, a person might
have a hemoglobin level
of around 15 grams per 100
mLs of blood per decilitre.
08:23
And this shows a nice oxygen
hemoglobin dissociation curve.
08:26
You’ll notice we have our two
Y axes set up on the graph.
08:30
The percent O2 is fully
bound at a 100%.
08:36
And notice that that corresponds to around
20 milliliters of O2 per 100 mLs of blood.
08:43
Let’s contrast that to someone who
has a disease such as polycythemia.
08:47
This is an increase
amount of hemoglobin.
08:50
The O2 sat goes still at 100%.
08:54
But now, it’s carrying more O2.
08:58
Because now, maybe that is 25
mLs of O2 per 100 mLs of blood.
09:05
Let’s contrast that to
someone who has anemia.
09:07
You can still have a 100% percent saturation
of the amount of hemoglobin that you have,
but that, well, maybe only accounts to 12
milliliters of oxygen per 100 mLs of blood.
09:18
This looks a lot like someone who
has carbon monoxide poisoning.
09:22
You can still have a 100% saturation
and not carry that much O2
and the reason for that is carbon
monoxide binds to those oxygen
binding sites on hemoglobin with a
greater affinity than what oxygen does.
09:39
And therefore, they will preferentially
bind to those binding sites
and prevent O2 from
binding to them.
09:46
So it looks a lot like the
person has an anemia
in terms of the plateau portion of the
oxygen-hemoglobin dissociation curve.
09:55
The one aspect that’s a little
bit different is it causes a
leftward shift in the oxygen
hemoglobin dissociation curve,
which means that peripheral
tissue have harder time
being able to get oxygen from
that particular red blood cell
as it travels through
this circulation.