I always worry a bit about Wiki entries because they aren't  
necessarily coming from the professional community. However, I did  
find this further bit on the same page as the link you gave (emphasis  
added):
"For mothers to deliver oxygen to a fetus, it is necessary for the  
fetal hemoglobin to extract oxygen from the maternal oxygenated  
hemoglobin across the placenta. This requires the fetal hemoglobin to  
have a HIGHER oxygen affinity than that of the maternal carrier. This  
is achieved by a fetal hemoglobin subunit γ (gamma), instead of the b  
(beta) subunit. The γ subunit has less positive charges than the  
adult hemoglobin b subunit. This means that 2,3-bisphosphoglycerate  
(2,3-BPG) is LESS electrostaticly bound to fetal hemoglobin as  
compared to adult hemoglobin. This means that 2,3-BPG is less  
effective in lowering the oxygen affinity of the fetal hemoglobin.  
This LOWERED affinity allows for adult hemoglobin (the maternal  
hemoglobin of the mother) to readily transfer its oxygen to the fetal  
hemoglobin".

That's pretty clear about fetal Hb having a higher affinity for  
oxygen and why.

Way too much info on the subunits but I think I understand what they  
are saying. Basically, mom's blood cells have to let go of oxygen  
easily for the fetal blood to be able to get it through the placenta.

 From Lawrence and Lawrence (5th ed) pg 481 which was the only place  
I saw anything close to fetal hemoglobin (HbF) mentioned. They don't  
address the oxygen carrying affinity of HbF.
"The normal full-term infant has a hematocrit in utero of 50% to 60%.  
Because of the low oxygen tension delivered to the fetus via the  
placenta the fetus requires more hemoglobin (Hg) to carry the  
oxygen.  As a soon as the infant is born and begins to breathe room  
air, the need is gone."
I can't remember the exact ratio but I know room air is about 22%  
oxygen and (if I am remembering correctly) the blood stream carries a  
little less than half of that to the baby via the placenta.

Because the fetal environment is low in oxygen there is more  
hemoglobin in the blood, So the blood begins to rid it self of excess  
hemoglobin.. turns into biliverdin then turns into bilirubin. etc...

So it would seem that HbF does have a high affinity for oxygen;  
meaning R&A are incorrect on that point. (I was looking around in the  
3rd ed. of Breastfeeding and Human Lactation and didn't see the  
oxygen affinity paragraph you quoted. Is it possible that the  
information was removed from a later edition than the one you have?)

A higher affinity for oxygen would also explain why pre-term infants  
are so susceptible to oxygen damage (like the eyes for example) from  
comparatively low concentrations of supplemental oxygen.


Can people still write an explanation of why an answer they give on  
the exam differs from the ones given? I remember doing that with a  
few questions on my exams. I knew of one in particular having to do  
with when the stages of Lactogenesis occur, that I was prepared to  
challenge on the exam because I didn't agree with the answer on a  
practice exam.

Thanks for the physiology refresher
Marie Davis RN IBCLC

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