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Placental Gas Exchange

Current · V5 (2025) → P2.v Historical · V4 (2023) → V1.iii 2 exam appearances

2009B Q24

Exam question

Describe 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

J\;={{D\;A\;\Delta C}\over{T}}

where

D=Diffusion\;constant\;\propto\;{{Solubility}\over{\sqrt{Molecular\;Weight}}}

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	P_aO₂ = 100mmHg (80-110)	C_aO₂ = 16.1ml/100ml
Fetal Umbilical Vein	P_vO₂ = 30mmHg (30-50)	C_vO2 = 16ml/100ml

Source: Diagram used in Placental transfer — Master Answer · Source/permission verification pending.

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	P_aCO₂ = 32mmHg
Fetal Umbilical Artery	P_vCO₂ = 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

Reusable content

Formulae used in this answer

Diffusion coefficient — solubility and molecular weight

Diffusion coefficient

CWF-0046

D=Diffusion\;constant\;\propto\;{{Solubility}\over{\sqrt{Molecular\;Weight}}}

Fick diffusion flux

CWF-0047

J\;={{D\;A\;\Delta C}\over{T}}

Answer by JC, Mooney

Past papers

Exam appearances

2 appearances

Exam	Exact exam wording	Candidate success
2009B Q24	Describe how gas exchange is influenced across the placenta near the term of pregnancy.	11%
2008B Q22	Describe how gas exchange is facilitated across the placenta near the end of pregnancy.	—