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If static compliance is change in volume with respect to change in transpulmonary pressure, when there is no gas flow. My question is; if there is no gas flow, how would volume Change?

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First of all, the definition that you have stated, is absolutely true in all sense.

Static compliance, theoretically, is indeed the change in volume with respect to change in transpulmonary pressure, when there is no gas flow. Here gas flow means air should remain static at the time of measurement. Hence it is literally about taking snapshots at varying volumes and noting the transpulmonary pressure.

Here is a diagram (source: https://derangedphysiology.com/main/cicm-primary-exam/required-reading/respiratory-system/Chapter%20031/static-dynamic-and-specific-compliance) i.e. identical to the original diagram from -

Rahn, H., Otis, A.B., & Fenn, W.O. (1946). The pressure-volume diagram of the thorax and lung. Federation proceedings, 5 1 Pt 2, 82 .

enter image description here

For the above study, they acquired normal men, occluded their nostrils with cork stoppers, and measured their airway pressures at different fractions of their vital capacity (the subjects exhaled fully and then inspired a known volume of gas from the spirometer before performing a breath hold). With these manoeuvres, the above relationship was demonstrated.

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