1. The Forced Expiratory Volume (FEV) was measured in normal persons and patients with obstructive airways disease at mouth pressures from 0 mmHg to levels high enough to prevent air flow altogether by using a Starling resistor at the mouth.

2. Evidence was obtained in support of the idea that, during forced expiratory flow, airways might function with the properties of a Starling resistor. This is considered to divide the airway into upstream and downstream segments at the site of airway collapse. The technique was simple, capable of being performed on outpatients and provided a means of studying the collapsibility of airways, airways resistance and alveolar pressure.

3. The FEV in normal persons and patients with disease was determined by the properties of the upstream segment. Since this segment contains only a proportion of the total airways resistance it was not surprising to find imprecise correlations between the FEV and airways resistance measured by the body plethysmograph.

4. Preliminary observations of patients during acute exacerbations of chest disease suggest that functional changes can occur throughout the length of the airway (i.e. in both upstream and downstream segments).

5. In the longer term during the natural history of obstructive airways disease, airways resistance and the more ready collapse of airways may develop independently.

6. Impaired alveolar pressure may contribute towards the reduction in air flow during acute exacerbations of chest disease.

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