• Compliance equals the change in pressure during a linear increase of 1l in volume above FRC.

• The alveolar-arterial oxygen difference is <2kPa in youth and <3.3kPa in old age.

• The Bohr equation calculates physiological deadspace, VD. The normal value is below 30%.

• The shunt equation estimates the proportion of blood shunted past poorly ventilated alveoli (QS) compared to total lung blood flow (QT).

These equations allow estimation of ventilation/perfusion mismatch:

• V/Q = 1, ventilation and perfusion are well-matched.

• V/Q>1, increased deadspace (where alveoli are poorly perfused but well ventilated).

• V/Q<1, increased venous admixture or shunt (where alveoli are perfused but poorly ventilated).

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