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The mechanics of breathing in the turtle, Pseudemys scripta Vitalis, Timothy Zoltan


Measurements of pulmonary mechanics on anaesthetized specimens of the aquatic turtle Pseudemys scripta, indicate that the static pulmonary mechanics are determined primarily by the mechanics of the body wall of the animal rather than those of the lungs. The mechanics of the body wall are also predominant under the dynamic conditions of pump ventilation, but only at low frequencies. As pump frequency increases, the multicameral lungs of the turtle begin to contribute an ever increasing portion to the total mechanical work required to produce each breath as measured from pressure volume loops. The rise in the work performed on the lungs results from an increase in the nonelastic, flow resistive forces which must be overcome during ventilation. The respiratory system is the major site of nonelastic work. The primary bronchus to each lung branches to seven intrapulmonary chambers, and is the most likely site of flow-resistance. There is also a small elastic component of the work required to ventilate the lung arising from forces in the intrapulmonary septa and the striated muscle surrounding the lungs. The contribution of the body to the total mechanical work required to generate each breath remains relatively unchanged with increasing ventilation frequency indicating that the majority of the forces to be overcome in the body wall are elastic in nature. Since the shell of the animal is rigid these elastic forces must reside in the flanks and pectoral regions. For a constant alveolar ventilation rate (V[sub A]) as frequency increases, the elastic work done per minute decreases while the nonelastic work done per minute begins to rise. This results in a 'U'-shaped curve for the total mechanical work done per minute in ventilating the lungs at a constant V[sub A] as breathing frequency increases. There is then, for any given level of ventilation, a combination of tidal volume and frequency at which the rate of mechanical work is minimized. This occurs at a frequency of 35 breaths per minute. The normal breathing pattern of P. scripta consists-of bursts of continuous breathing separated by variable periods of breath holding. Increases in pulmonary ventilation, upon stimulation by hypercapnia (3% CO₂ in air) or hypoxia (4% O₂ in N₂), are caused by increases in the number of breaths per minute due to the shortening of the breath hold period. Tidal volume and breath duration remain unchanged. The instantaneous breathing frequency (f’=60/T[sub tot]) corresponds to the pump frequency which minimizes the rate of mechanical work as measured in anaesthetized turtles. This indicates that turtles breathe at a combination of tidal volume and f' which minimizes the rate of mechanical work required to ventilate the lungs. To increase ventilation, the breath hold is shortened and more breaths are taken at this optimal combination. Bilateral vagotomy drastically alters the breathing pattern producing an elevation in tidal volume, a slowing of breathing frequency and a prolongation of the breath duration, resulting in an increase in the mechanical cost and thus in the oxidative cost of breathing even though total minute ventilation (V[sub E]) changes very little. These data suggest that periodic breathing in this species may represent an adaptive strategy which is under vagal afferent control and which serves to minimize the cost of breathing.

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