Transport in Mammals: Question 8
Syllabus 8.2
In the placenta, oxygen diffuses from the mother's blood into the fetus's blood across a barrier of tissue, without the two blood supplies mixing directly. At a partial pressure of oxygen of 3 kPa, typical of the placenta, percentage saturation of maternal (adult) haemoglobin is approximately 30%, while percentage saturation of fetal haemoglobin at this same partial pressure of oxygen is approximately 55%.
(a) Describe, in terms of oxygen affinity, how the oxygen dissociation curve for fetal haemoglobin differs from the oxygen dissociation curve for adult haemoglobin, and use the data above to support your answer. [3]
(b) Explain how this difference in oxygen affinity allows oxygen to diffuse from the mother's blood into the fetus's blood within the placenta. [3]
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Worked solution
Part (a): Comparing the two dissociation curves
Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin. That is, at any given partial pressure of oxygen, fetal haemoglobin binds a greater proportion of the oxygen it is exposed to than adult haemoglobin does. On a graph of percentage saturation of haemoglobin against partial pressure of oxygen, this higher affinity shows up as the oxygen dissociation curve for fetal haemoglobin lying to the left of the curve for adult haemoglobin: at any given partial pressure of oxygen, the fetal curve reads a higher percentage saturation.
The data given illustrate this directly: at a partial pressure of oxygen of 3 kPa, adult (maternal) haemoglobin is only about 30% saturated, whereas fetal haemoglobin is about 55% saturated at this same partial pressure. Nearly twice as saturated, confirming that fetal haemoglobin has the higher oxygen affinity of the two.
Part (b): Oxygen transfer across the placenta
Maternal blood and fetal blood flow close together within the placenta but never mix directly; they remain separated throughout by a thin layer of tissue, across which oxygen must diffuse from the mother’s blood supply into the fetus’s blood supply.
The partial pressure of oxygen within the placenta is relatively low (as in the data above, around 3 kPa). At this partial pressure:
- Adult (maternal) haemoglobin, having the lower affinity for oxygen of the two, does not hold onto oxygen very strongly and so readily releases oxygen into solution in the maternal blood plasma close to the placental barrier.
- Fetal haemoglobin, having the higher affinity for oxygen, binds this available oxygen more readily and more completely than adult haemoglobin would at the same partial pressure.
Because fetal haemoglobin keeps taking up and binding oxygen so effectively even at this low partial pressure, the concentration of free (dissolved, unbound) oxygen on the fetal side of the placental barrier is kept lower than on the maternal side. This maintains a partial pressure (concentration) gradient for oxygen across the barrier, so that oxygen continues to diffuse from the mother’s blood into the fetus’s blood for as long as the two blood supplies remain in contact within the placenta.
Final answers
- (a) Fetal haemoglobin has a higher oxygen affinity than adult haemoglobin, so its dissociation curve lies to the left of the adult curve; at 3 kPa, fetal haemoglobin is about 55% saturated compared with about 30% for adult haemoglobin.
- (b) At the low partial pressure of oxygen in the placenta, adult haemoglobin (lower affinity) readily releases oxygen while fetal haemoglobin (higher affinity) readily binds it, keeping free oxygen concentration lower on the fetal side and maintaining a diffusion gradient from mother to fetus across the placental barrier.