Now, hemoglobin is also an important
consideration for the fetus and a mother.
Mom gets oxygen to the
fetus by breathing
and it gets transported through
the blood to the fetus.
But the fetus has to really compete
with mom for getting that oxygen.
And unless the fetus has an advantage,
it's going to lose that battle.
Well, fortunately, the fetus
has something on its side;
it's known as fetal hemoglobin.
And this is a form of hemoglobin that's a
little bit different than mom's hemoglobin.
We remember first of all that
mom has adult hemoglobin,
two units of alpha and
two units of beta.
The fetus does not have the
beta to any significant extent
And you can see on this graph,
that plots the relative amounts
of various forms of hemoglobin
that are present in the various globins
that are present in hemoglobin
at various stages
of a person's life.
So the fetus prior to birth has
very little of the beta unit
and instead has a fairly high quantity
shown in green of the gamma subunit.
So, fetal hemoglobin is mostly
alpha 2, gamma 2, all right?
Now, that's significant because there
is very little alpha 2 beta 2.
These variations change in about birth
and by a year and a half to two years
of age, the fetus has adult hemoglobin.
And that's okay because the fetus
wants to be breathing then.
The alpha 2 gamma 2 of the
fetus inside of mom, however,
has a higher affinity for oxygen than the
alpha 2 beta 2 that mom is carrying.
Now, instead of competing
on an equal footing basis,
the fetus is actually taking
oxygen away from mom very readily.
That's pretty necessary because the fetus
is quite away from the oxygen supply.
The fetal hemoglobin is therefore
able to provide the oxygen
that the fetus needs to do that.
And you might wonder, "Well, how
does the fetal hemoglobin differ
and what's the different
about the gamma unit?"
Well, what's different about the gamma
unit is that the alpha 2 gamma 2
does not have the same hole in the
donut that the alpha 2 beta 2 does.
And the fetus therefore
cannot bind 2,3 BPG.
That means that fetal hemoglobin
stays mostly in the R-state
It doesn’t get converted to the T-state.
The fetus therefore is able to
grab oxygen much more likely
because the R-state that you recall
holds on to oxygen more tightly
and grabs oxygen away.
Now, if we go back to the curves
that we started with earlier,
we can actually see what's
happening with fetal hemoglobin,
as I have described to you in words.
The fetal hemoglobin can't bind 2,3 BPG
and therefore it's mostly in the R-state.
And if we measure the binding of fetal
hemoglobin compared to adult hemoglobin
and compared it to myoglobin,
we see that the fetal hemoglobin
remaining mostly in the R-state
stays in the middle curve
as we can see here.
The middle curve is
the fetal hemoglobin.
As we move to the left, we see
greater affinity for oxygen.
So, we see that the fetal
hemoglobin has a greater affinity
than the mom's adult hemoglobin but
not as much as present in the myoglobin.
And that’s important because you
remember the fetus has to deliver
oxygen to its tissues.
Now, one of the questions
students frequently ask is,
"Well, if it mostly in the R-state, does
it give up oxygen for its tissues?"
And the answer is this graph
shows, of course, that it does.
It gives it up more
readily than myoglobin
but not as readily as
mom's hemoglobin does.
Is that a problem?
No, it's actually not a problem
because mom is out running and exercising
and working and doing all these things.
And all the fetus is doing is
sitting or kicking its legs.
It doesn't have that wildly varying
need for oxygen that mom has.