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