During breathing, the inspiratory muscles must create enough pressure to overcome two forces: the friction associated with air flow and also the increase in elastic recoil of the lung as it inflates to a larger volume. Before we consider air flow, we must understand the elastic properties of the lung and chest wall.
These are best studied in static situations; that is, while one holds one's breath (either by tensing one's respiratory muscles or by closing one's glottis).
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