Effects of hypoxia on lung volume in the garter snake

Intrapulmonary CO2 inhibits inspiration in garter snakes

This study was undertaken to assess the role of intrapulmonary chemoreceptors in the control of breathing in reptiles. Garter snakes were tracheostomized to remove the influence of upper airway receptors. After recovery from anesthesia, the animals were unidirectionally ventilated or allowed to ventilate normally, but without glottal control of the breath-hold. Breathing movements were recorded by means of a pneumotachograph. When a bidirectionally breathing snake was presented with CO2 at the tracheal tube, inspiratory duration and volume decreased. When CO2 was removed from the inspired air between breaths, inspiratory duration and volume increased. Removing CO2 from the airstream of a unidirectionally ventilated snake during a breath-hold produced apnea, but removing CO2 at the beginning of inspiration caused the duration and volume of the inspiration to increase. This reflex appears to represent a positive feedback mechanism for enhancing inspiration once a breath has been initiated.

Respiratory mechanics and control of the ventilatory cycle in the garter snake

We measured respiratory airflow and intrapulmonary pressure in spontaneously breathing garter snakes (Thamnophis sirtalis) before and after bilateral vagotomy. Each breathing cycle consisted of expiration, inspiration, and a breath-holding period with the glottis closed, as previously reported. Respiratory system compliance was very high compared with that of mammals (0.042 ml/cm H2O per g), but was considerably reduced when the animal was coiled. Resistance to airflow averaged 6.35 cm H2O per ml/sec. Lung volume during the breath-hold was usually above the relaxed volume of the system. Analysis of dynamic pressure-volume and flow-volume loops showed that both expiration and inspiration were active, and that little use was made of passive recoil pressure in expiration. Evidence that vagally mediated, volume-related feedback influenced the breathing pattern was found in some snakes, but this influence was weaker than in most mammals and other reptiles. The results suggest that breathing in snakes is mainly a centrally programmed process, not markedly dependent on respiratory system mechanics or neuromechanical feedback. This arrangement is advantageous in view of the high, posturally variable compliance and the frequent regional distortions of the respiratory system that result from the unique structure of these animals.

Ventilatory response to inspired CO2 in the lizard, Tupinambis nigropunctatus

Elevating inspired levels of CO2 (1-4%) in Tupinambis nigropunctatus leads to an increase in tidal volume, mean expiratory flow, mean inspiratory flow, duration of the non-ventilatory period, inspiratory duration, expiratory duration and end inspiratory lung volume. Minute ventilation is variable and end expiratory volume decreases. An increase or decrease in CO2 concentration surrounding the head affects the duration of the non-ventilatory period before the altered CO2 concentration is inspired into the lungs. The change in duration of the non-ventilatory period before altered CO2 concentration is inspired into the lungs is probably mediated by CO2 sensitive receptors located in the mouth, nose or on the head surface.