We show, from simulated gas wash-out of an asymmetrical, two-compartment model of the lung, that a third dead-space ('wash-out dead-space', VDW) enables this distinction to be made: VDV-VDW reveals convective, and VDW-VDF diffusive, mixing inefficiency. This expression of mixing inefficiency does not distinguish between convective and diffusive mechanisms. Such non-uniformity represents 'mixing inefficiency' which may be expressed by the difference between the effective dead-space ('ventilatory dead-space', VDV) and the actual airway dead-space ('Fowler dead-space', VDF). Non-uniform composition of gas in the lungs may arise because of uneven dilution of alveolar with inspired gas by convection (bulk flow) or because of the finite rates of molecular diffusion in the small air-spaces.
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