Oxygenated blood is delivered to the fetus via the umbilical vein from the placenta, which is anchored to the wall of the mother's uterus. As blood courses through the mother, oxygen is delivered to capillary beds for gas exchange, and by the time blood reaches the capillaries of the placenta, its oxygen saturation has decreased considerably. In order to recover enough oxygen to sustain itself, the fetus must be able to bind oxygen with a greater affinity than the mother.<unk> 's affinity for oxygen is substantially greater than that of adult<unk> . Notably, the P50 value for<unk>  (i.e., the partial pressure of oxygen at which the protein is 50% saturated; lower values indicate greater affinity) is roughly 19 mmHg, whereas adult<unk>  has a value of approximately 26.8 mmHg. As a result, the so-called "oxygen saturation curve", which plots percent saturation vs. pO2, is left-shifted for<unk>  in comparison to the same curve in adult<unk> .
This greater affinity for oxygen is explained by<unk> 's interaction with 2,3-bisphosphoglycerate (2,3-BPG). In adult red blood cells, this substance decreases the affinity of<unk>  for oxygen. It is also present in<unk>  red blood cells, but does not interact with<unk> , leaving its affinity for oxygen unchanged. Adult<unk>  alone actually has a higher affinity for oxygen than its<unk>  equivalent, but the levels of 2,3-BPG reduce it.
For mothers to deliver oxygen to a fetus, it is necessary for the<unk>  to extract oxygen from the maternal oxygenated<unk>  across the placenta. This requires the<unk>  to have a higher oxygen affinity than that of the maternal carrier. This is achieved by a<unk>  subunit γ (gamma), instead of the b (beta) subunit. The γ subunit has less positive charges than the adult<unk>  b subunit. This means that 2,3-bisphosphoglycerate (2,3-BPG) is less electrostaticly bound to<unk>  as compared to adult<unk> . This means that 2,3-BPG is less effective in lowering the oxygen affinity of the<unk> . This lowered affinity of<unk>  for 2,3-BPG allows for adult<unk>  (the maternal<unk>  of the mother) to readily transfer its oxygen to the<unk> .
Fetal hemoglobin
Hemoglobin
Hemoglobin C
Methaemoglobin
Beta globulins
Hemoglobin E
Beta-2 transferrin
Mean corpuscular hemoglobin