Meeting Abstract
Aerodynamic valves in the lungs of birds cause unidirectional flow of gases through a circuit in the paleopulmo, a circuit that consists of dorsobronchi, parabronchi, and ventrobronchi (dpv). These fluid valves have fascinated biologists for many years, but remain incompletely understood. On inspiration, why doesn’t air flow directly through the ventrobronchi into the cranial set of air sacs as they expand, rather than bypassing these airways to flow through the d-p-v circuit before entering the cranial air sacs? Furthermore, during exhalation, why do the gases in the caudal air sacs travel through the d-p-v system, rather than passing directly out of the body by way of the intrapulmonary bronchus? A guiding dam, located in the caudal region of the dpv circuit, was proposed by Hazelhoff as a mechanism that could create unidirectional flow, and physical glass models proved that such a dam is capable of converting tidal flow into unidirectional flow. In this model, the topography of the cranial region of the d-p-v circuit is not a critical component of the valves. However, experiments debilitating parts of the respiratory system, through either cannulations or by filling these parts with physical plugs indicate that the topography of in the cranial region of the lung is indeed critical to proper valve function. The discovery of unidirectional flow in crocodilians and lizards has enabled a comparative approach to be used to help throw into sharp relief features of this complex anatomy that are evolutionary conserved, and therefore possibly important to proper valve function, from features that are highly derived and may therefore play no, or a minimum, role. This comparative approach corroborates the importance of the cranial aspect of the topography to the aerodynamic valves, and may explain why this region of the lung is generally more cartilaginous, preserving the shape of the airways to ensure the valves function properly.
