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Lung-Air-Sac Anatomy and Respiratory Pressures in the Bird
1 Sub-Department of Veterinary Anatomy, University of Cambridge
1. Regardless of its tidal volume an individual air sac shows a respiratory pressure-wave which is similar to that of every other sac. These is a process of pressure equilibration within the lung-air-sac system involving very short-lived streams of air between air sacs, whose significance becomes larger as pressure accelerations become bigger; and when a steady state has been achieved in any respiratory phase the pressure wave becomes normalized in all parts of the system.
2. Small pressure differentials between sacs are part of the equilibrium distribution of pressure within the lung-air-sac system. They result from differences in the resistance path through the lung to each sac, and differences in their respective tidal volumes. Their wave-form closely resembles that of the parent pressure waves and has a maximum value of one-tenth their value.
3. In general, the bronchial pathways to the posterior sacs have greater resistances to air flow than those to the anterior sacs.
4. During vocalization pressures in the coelom and air sacs exceed normal respiratory pressures by about 40 times. Airway resistance vastly increases as the syringeal membranes begin to vibrate.
Submitted on March 10, 1972
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