Arterial O₂ Homeostasis during Diving in the Turtle Chelodina longicollis

Periodically breathing animals that store O₂ primarily in the lungs must efficiently transfer O₂ to arterial blood during apnea. Previous experiments on diving freshwater turtles indicate that such transferral is continuous during most dives, but there have been occasional observations of much more periodic O₂ transfer between lung gas and blood. This study investigates the dynamics of lung O₂ utilization and blood transfer during voluntary diving in the Australian long-necked turtle, Chelodina longicollis. Pulmonary arterial blood flow was measured by a relatively noninvasive impedance technique. The PO₂ of pulmonary gas and systemic arterial blood was measured continuously with extracorporeal catheter loops, which minimize sampling disturbances. Lung gas PO₂ declined relatively constantly during apnea at a rate of about 3 mmHg/min. Changes in arterial blood PO₂ showed one of two very distinctive patterns during apnea. In the first pattern, evident in about two-thirds of 87 dives monitored in seven turtles, the PO₂ of arterial blood decreased from levels at the start of the dive at a rate of about 1.0-1.5 mmHg/min. In a second pattern observed in the remaining one-third of the dives, arterial PO₂ actually showed periods of transient increase of at least 4 mmHg at some point during the dive, while about 8% showed a transient PO₂ increase of 10 mmHg or more. This second pattern of arterial blood oxygenation was also quite distinctive in that arterial O₂ saturation was maintained constant at around 85%-95%, even when dives lasted for 20 or more min. Qualitative measurements of pulmonary blood flow during voluntary dives indicate that the transient increases in arterial blood PO₂ are closely correlated with large and equally transient increases in pulmonary perfusion. We suggest that C. longicollis can maintain constant arterial O₂ saturation during long periods of diving by periodically increasing pulmonary blood flow to transfer O₂ stored in lung gas into blood perfusing the lungs. Furthermore, we suggest that this may be a general phenomenon among diving reptiles but that its observation requires animals unstressed by sampling techniques.