Measurement of the distensibility of the parabronchi in duck lungs

Air flow resistance in the parabronchial lung of the duck was measured at various pressure differences between the lung and the body surface (Prs) using a body plethysmograph. One lung of the anesthetized animal was ventilated at a steady flow rate, from trachea, through the parabronchial lung, and out via a cannula in the caudal thoracic air sac (Tr leads to CS flow), or vice versa (CS leads to Tr flow), all flow being directed over the parabronchi (Pb) by blocking the main bronchus between the medioventral (MV) and mediodorsal secondary bronchi (MD). Pressure differences were measured between MV and MD (Ptot), and between the clavicular air sac and MD giving the pressure drop along the parabronchial tubes (PPb). The pressure drop along MV, PVb, was derived as Ptot-PPb. Air flow resistances, Rtot, RPb, RVb, were calculated from the ratio of the corresponding pressure difference to the flow rate. Results show: (1) All resistances decreased with increasing distending pressure (Prs) from -20 to +20 cm H2O this change being most pronounced around Prs = 0; (2) The flow resistance of these structures depended on the flow direction, being smaller with Tr leads to CS flow during distension than in the opposite direction; (3) Arterial blood gases did not significantly change with varying distending pressure, suggesting unimpaired gas exchange even when the lung is significantly compressed. The results indicate that the parabronchi and the secondary bronchi of the duck lung have a finite compliance but that changes in intrapulmonary pressure, compression of the lung, do not result in significant collapse of the air capillaries with ensuing impairment of gas exchange.