To characterize the stresses which determine bronchial diameter in the lung, we estimated peribronchial pressure (Px) relative to intrabronchial pressure (Pbr) and to alveolar pressure (PA) for the main lobar bronchus of excised dog lobes using the technique of Takishima et al. (J. Appl. Physiol. 38: 875--881, 1975). The recoil of the bronchial wall, Pbr---Px, when smooth muscle was relaxed varied primarily with bronchial diameter. The recoil of the parenchyma around the bronchus, Px---Pa, varied with lung volume but was also diameter-dependent and served to double approximately the effective elastance of the bronchus in situ. We estimated recoils during slow deflations from TLC with the bronchus untreated, or pharmacologically contracted or relaxed. In untreated and relaxed states, local parenchymal and bronchial recoils were of similar magnitude to overall lung recoil (i.e., Px congruent to Ppl) except at high inflating pressure (PA -- Ppl = 30 cmH2O) where they were about half as great. With contraction, bronchial and local parenchymal recoils increased to as much as twice overall lung recoil. Contracted smooth muscle exerted a radial stress of 36+/-14 cmH2O at full lung inflation but much less during stepwise deflation.
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