Control of Breathing in Acrochordus javanicus, an Aquatic Snake

Ventilation and oxygen uptake were measured in the elephant trunk snake, Acrochordus javanicus, during normal, undisturbed breathing. Ventilation is periodic, with ventilatory periods (VP) lasting 2.5-3.5 min, interrupted by nonventilatory periods (NVP) averaging 10 times longer at 25 C. Average tidal volumes ranged from 27.8 to 47.7 ml·kg⁻¹, while breathing frequency during VP ranged from 78 h⁻¹ to 155 h⁻¹. Total ventilation (V̇I) ranged from 250 to 543 ml·kg⁻¹·h⁻¹ BTPS (body temperature and pressure, saturated with water vapor). Oxygen uptake (V̇o2) averaged 16.6 mlO₂· kg⁻¹·h⁻¹ STPD (standard temperature and pressure, dry) at 20 C and 29.3 mlO₂· kg⁻¹·h⁻¹ at 30 C. Corresponding ventilations were 225 ml·kg⁻¹·h⁻¹ and 504 ml·2kg⁻¹· h⁻¹ BTPS. Ventilatory requirements, V̇I/V̇o2, were 13.8 and 17.2 for 20 and 30 C. Both ventilation and oxygen uptake are low in Acrochordus, compared with other reptiles. Also, the V̇I/V̇o2 ratio is much lower for Acrochordus than for most reptiles. A well-developed, richly vascularized lung and the long NVP contribute to unusually high extractions of O₂ from the ventilated air. The mode of breathing of Acrochordus allows it to stay submerged more than 90% of total lapsed time. During hypoxic breathing, ventilation is stimulated when inspired O₂ concentration is less than 10%. Large alveolar-to-blood Po2 gradients indicate that blood and lung O₂ stores are nearly exhausted when the hypoxic stimulus breaks the breath holds. The increased ventilation resulted from a shortening of NVP while tidal volumes and the frequency of breathing during VP remained unchanged. Breathing 100% O₂ significantly reduced ventilation. Hypercapnic breathing showed no ventilatory change when breathing 2% and 4% CO₂. Breathing 6% and 8/% caused an actual depression of ventilation. The results indicate an effective elimination of CO₂ by cutaneous exchange, which may account for the lack of a ventilatory response to CO₂ breathing. Ventilation in Acrochordus appears controlled for maximum utilization of oxygen stores and rapid recovery during breathing.