Past Papers · SAQ
Placental Gas Exchange
2009B Q24
Exam questionDescribe how gas exchange is influenced across the placenta near the term of pregnancy.
CICMWrecks answer
Master answer
Diffusion via Fick’s Law
- Flow of gas (Rate of movement of solute) across semi-permiable membrane J is
where
C = concentration (or partial pressure for gasses)
A = cross-sectional area
T = thickness of the membrane or distance over which diffusion takes place.
- These factors alter the rate of transfer as per the equation above.
- Valid for Most medium – small substances
- Water soluble (H2O, electrolytes, glucose, urea) via intercelular clefts
- Lipid soluble substances (O2, CO2) via endothelial cells themselves.
- Flow = 1.2ml of O2 mmHg-1minute-1 across entire placenta
- Surface area of placenta for diffusion = 16m2
- Thickness of placental membrane falls as pregnancy progresses
- Minimum thickness is 3.5 micrometres
Oxygen
- Mean partial pressure of oxygen (PO2) in maternal blood is considerably higher than in fetal blood
- As a consequence, oxygen readily diffuses across the placenta from maternal to fetal blood
- Despite its low PO2, fetal blood is able to transport essentially the same quantity of oxygen to tissues as maternal blood
| O2 Partial Pressure | Oxygen Content | |
|---|---|---|
| Maternal | PaO2 = 100mmHg (80-110) | CaO2 = 16.1ml/100ml |
| Fetal Umbilical Vein | PvO2 = 30mmHg (30-50) | CvO2 = 16ml/100ml |
- Foetal Hb has a higher affinity for O2 than maternal Hb
- oxygen carrying capacity is improved, despite lower PaO2
- P50 is 20mmHg vs 26 in maternal
- Haemoglobin concentration in blood is higher in the foetus, increasing oxygen carriage
- 120g/L in mother
- 170g/L in foetus
- Double Bohr effect:
- Movement of CO2 from the foetal to the maternal circulation results in a simultaneous rise in maternal CO2 and fall in foetal CO2
- Rise in foetal Hb oxygen affinity, fall in maternal Hb oxygen affinity by the Bohr effect
Carbon dioxide
- Carbon dioxide is produced abundantly in the fetus
- PCO2 of fetal blood is higher than maternal blood
- Carbon dioxide therefore diffuses from fetal blood, through the placenta, into the maternal circulation, and is disposed of by expiration from the mother’s lungs.
| O2 Partial Pressure | |
|---|---|
| Maternal | PaCO2 = 32mmHg |
| Fetal Umbilical Artery | PvCO2 = 55mmHg |
- Hyperventilation in pregnancy (increased sensitivity of chemoreceptors to CO2) → Lower PaCO2
- Smaller gradient than oxygen, but oxygen has 20 times higher solubility
- Double Haldane effect:
- Simultaneous rise in foetal Oxy-Hb and fall of maternal Oxy-Hb
- Increased CO2 carrying capacity of maternal Hb, decreased in foetal Hb
Mooney / JC 2020
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