THE AVIAN LUNG - IS THERE AN AERODYNAMIC EXPIRATORY VALVE

The unidirectional gas-flow pattern through the avian lung is thought to result from 'aerodynamic valves'; support for this hypothesis lies mainly in the failure to find any evidence for anatomical valves. Duri ng expiration, air hows from the caudal air sacs through the major exc hange area of the lung, the paleopulmonic parabronchi, instead of bypa ssing the lungs via the intrapulmonary bronchus. We tested whether the effectiveness of this expiratory flow control mechanism depends on ae rodynamic factors, especially convective inertial forces that depend o n gas density and flow velocity. In pump-ventilated, anaesthetized gee se, a bolus of tracer gas was introduced into both the right and left caudal thoracic air sacs during an end-inspiratory pause. During the f irst expiration, the rise of tracer levels within the caudal trachea w as measured. Valve efficacy was positively correlated with the rate of expiratory gas flow, Vao (range 8-200 ml s(-1)), At flows assumed to occur during exercise in geese (VAO>100 ml s(-1)), the expiratory valv e efficacy was approximately 95%; it was less effective at lower hows. Surprisingly, the density (p) of the background gas (p of He/O-2=0.43 gl(-1), Ar/O-2=1.72 gl(-1) or SF6O2=5.50 gl(-1)) had no effect on exp iratory valving. We suggest two possible mechanisms that might explain this unusual combination of how dependence without density dependence . (1) If airway geometry changes occurred between experiments with dif ferent gases, flow in the vicinity of the expiratory valve may have va ried independently from flow measured at the airway opening. (2) Alter natively, valving may depend on dynamic compression of the intrapulmon ary bronchus, which could depend mainly on viscous resistance and thus on flow velocity hut not gas density.