The summed cross‐sectional area of the airway network as a function of distance from the airway opening can be inferred from impulse reflection data measured at the airway opening [Jackson et al., J. Appl. Physiol. 43 (3) (1977)]. We have critically examined this method. This was accomplished by direct comparison of the area inference with anatomic data for a computer model of the complete tracheo‐bronchial tree [Fredberg and Moore, J. Acoust. Soc. Am. 63 (3) (1978)]. The results show that the serial distribution of airway geometry is well approximated when the branching is symmetric, airway walls are rigid, and the gas is inviscid. Branching asymmetry causes small differences between the inferred area and the anatomic area mouthward of the most central segmental bronchus, but larger and growing differences beyond that. Non‐rigidity of the airway walls causes the area‐inference to exceed the anatomic area by 20%–50% at the level of the carina, and more so distally. [Supported by NHLBI contract N01‐HR‐6‐2901].

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