Now if we just think about hemoglobin
for a second and we compare the oxygen
binding properties of hemoglobin, we
see something really interesting.
This plot shows four different measures
of hemoglobin's ability to bind oxygen.
In each case, the only
difference is the pH
of the solution that the
hemoglobin is found in.
The curve farthest on
the left corresponds
to hemoglobin binding
oxygen at a pH of 7.6.
The next curve is at 7.4.
The next curve is at 7.2.
And the last curve, the
farthest to the right is
binding of oxygen at 7.0.
Now if we use that principle that I showed
you of moving the curve to the right,
when that happens, we're
seeing less binding of oxygen,
what this curve is what these
curves are telling us,
is that the lower the pH, the less
hemoglobin will bind to oxygen.
Well, why is that important?
The reason that that's important is
cells that are rapidly metabolizing --
let's say that muscle cell that's out
there while you're running that race.
Cells that ara rapidly
metabolizing release protons.
And protons are what
cause the pH to fall.
So that change in pH occurs around
rapidly metabolizing cells.
And what cells need
the most oxygen?
The ones that are
So as we look at this graph, we see that if
we look at a given oxygen concentration,
we have less bound for any of the
ones that have the lower pH.
And if we look at a given
we see that it takes more
oxygen to get to the same
saturation level for the
one with the lower pH.
Again, consistent with what I
said earlier about the curve
moving to the left having
less affinity for oxygen.
Now we learn something as
a result to this graph.
And that is that protons
are important in
affinity for oxygen.
Well, this effect that I've just described
to you is known as the Bohr Effect
named for the person who discovered
it at the turn of the 20th century.
Protons are important
in this process because
they actually work by
binding to hemoglobin.
And when they bind to
hemoglobin, they cause
hemoglobin to have a
slight change of shape.
We've seen numerous times now
that changes of shape change
the properties of the protein
whose shape is being changed.
The reshape hemoglobin
is losing some of its
oxygen because it's less
able to hold on to it.
And that shape changes happen because
the protons are bound to it.
So since rapidly metabolizing
tissues are releasing protons,
the rapidly metabolizing tissues are
getting more oxygen from hemoglobin.
Well, what I've shown you already is
pretty remarkable about hemoglobin.
We see that it has changing
affinity for oxygen
depending upon the oxygen
concentration and it has
changing affinity for oxygen
based on the concentration
of the protons and the
solution that it's in.
But that's not the only thing that
hemoglobin is also interesting for.
This graph shows an addition to what
I showed you in the last graph.
So let's step through it.
In this graph we see, in
this set of graphs, we see
three different solutions
that hemoglobin is found in.
The first solution
is the blue one.
That blue curve is the
farthest to the left
indicating the greatest
affinity for oxygen.
That solution measures hemoglobin's
affinity for oxygen at pH 7.4.
That’s about the
normal pH of blood.
That's measured in the
absence of carbon dioxide
and I'll mention why that's
important in a second.
The orange curve, the one in the
middle is measured at pH of 7.2.
And consistent with what I
showed you in the last graph,
the affinity has shifted,
meaning that this pH is causing
hemoglobin to have less affinity
for oxygen than the one at 7.4.
That's consistent with
what we said before.
More protons, lower pH, lower
pH, less affinity for oxygen.
But now, what's interesting here is
if we look at the same pH of 7.2
but we add carbon dioxide to it,
we see that the green curve has
shifted further to the right.
What is that mean?
It means that carbon dioxide can also
affect hemoglobin's affinity for oxygen.
It causes hemoglobin to let go
of its oxygen much more readily.
Now, so we now see that
there are three things
that can affect hemoglobin's
affinity for oxygen,
the concentration of oxygen,
the concentration of protons and the
concentration of carbon dioxide.
Why is carbon dioxide important?
Well, carbon dioxide is the end
product of respiration of cells.
cells in addition to
releasing protons are
releasing carbon dioxide.
And carbon dioxide is another
-- carbon dioxide is another
signal that the cell is saying,
"Hey, I need the oxygen."
And the oxygen is being left
in the right place for it.
So we've seen now that acid favors the
release of oxygen from hemoglobin.
Carbon dioxide favors the release
of oxygen from hemoglobin.
And acid and carbon dioxide are released
by rapidly metabolizing tissues.
We're starting to see a pattern.