Avian Respiratory Physiology

For his Ph.D., Dr. Hastings' studied avian respiratory physiology with Dr. Frank Powell in the UCSD Physiology/Pharmacology graduate group. Lungs in birds differ dramatically from lungs in mammals. Gas exchange in the avian lung takes place in rigid tubes called parabronchi and fits best with a crosscurrent model instead of the co-current exchange mechanism in operation in mammals. Air flows through the parabronchi in one direction during both inspiration and expiration due to expansion and contraction of cranial and caudal air sacs. Thus, the ventilatory
apparatus in the bird lung is distinct from the gas exchange region.

Dr. Hastings dissertation focused on effective ventilation and dead space in ducks. One manuscript demonstrated that gas exchange in birds could be supported by high frequency ventilation (respiratory rates = 1 Hz) but required tidal volumes greater than anatomic dead space volume, unlike the situation in mammals. Another paper explored single breath CO2 expirograms for measuring anatomic dead space, akin to the Fowler dead space. This work found discrepancies that could be explained by shunting of air from caudal air sacs that bypassed the parabronchial gas exchange region.

A third paper evaluated measures of physiologic dead space based on blood and expired gas oxygen and CO2 tensions. Because of the cross current gas exchange relationships, the Bohr dead space formula, modified by Enghoff, does not accurately measure physiologic dead space and computer modeling is required.

The final paper in this body of work demonstrated that pulmonary vascular resistance in birds was independent of flow. Thus, the avian pulmonary circulation does not appear to employ recruitment or distention to accommodate changes in flow.

References on avian respiratory physiology