Describe how gas exchange is facilitated across the placenta near the end of pregnancy.

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College Answer

Candidates were expected to cover the basic principles of gaseous diffusion across the
placenta, with reference to: Both oxygen and carbon dioxide; Fick’s Law, including placental
area and thickness, and relative gas tensions and solubilities; Changes in maternal and foetal
blood flow approaching term; Approximate values for maternal and foetal gas tensions and
content; The differences between foetal and maternal haemoglobin, quantitative and
qualitative; The double-Bohr and double-Haldane effects; Relative maternal hyperventilation
and it’s effects on arterial gas tensions and how these influence foetal transfer.
A good answer would also include a description of the physical arrangement of maternal
sinuses and foetal capillaries; labelled dissociation curves for O2 and CO2 detailing the
differences between maternal and foetal Hb; a placental gas exchange diagram, showing
foetal and maternal arterial and venous gas tensions and content values.
Syllabus ref: O1 2d
Suggested Reading: Review of Medical Physiology / W F Ganong – 22nd ed Chapter 32.
Nunn’s Applied Respiratory Physiology / A B Lumb & J F Lunn - 6th ed