PhD Defense – Rhea Storlund
Mechanisms that facilitate and control the dive response in pinnipeds
Location: Room 200 of the Graduate Student Centre (6371 Crescent Road)
Cardiovascular adaptations enable marine mammals to breath-hold dive for long durations. However, the mechanisms that maintain blood flow and control heart rate in diving mammals are not fully understood. To address this, I used ultrasound imaging, implantable cardiac monitors, and comparative methods in pinnipeds to determine 1) the adaptive function of the aortic bulb, 2) the implications of heart rate fluctuations during diving, and 3) the autonomic regulation of the circulatory system. Imaging the ascending aorta of anaesthetized Steller sea lions and northern fur seals supported the hypothesis that the aortic bulb maintains blood flow throughout the cardiac cycle and also indicated that stroke volume did not change with heart rate. These results suggest that the increased filling of the aortic bulb needed to support continual blood flow in the face of low diving heart rates is primarily driven by increased vascular resistance rather than increased stroke volume. Comparing my aortic bulb measurements of Steller sea lions and northern fur seals with five other pinniped species revealed that aortic bulb diameter scales allometrically and that species with relatively wider aortic bulbs dive for longer durations. These results indicate that the aortic bulb is a critical vascular adaptation to diving that supports the low heart rates that occur as part of the dive response. Heart rate monitoring in diving Steller sea lions demonstrated that heart rate slowly decreases and then oscillates throughout the dive. The rate at which heart rate initially declines appears to be related to the conditions of the dive, while the oscillations may reflect blood pressure regulation by the baroreflex. Measuring heart rate and heart rate variability in resting, free-moving, and diving Steller sea lions showed that diving resulted in the highest parasympathetic and baroreflex activity, affirming that heart rate is vagally controlled and blood pressure is actively managed during dives. Overall, these findings provide new insights into some of the cardiovascular adaptations that enhance the diving capabilities of marine mammals.
Examining committee:
Dr. David Rosen (Supervisor)
Dr. Andrew Trites (Supervisor)
Dr. William Milsom (Committee member)
Dr. Bill Sheel (University examiner)
Dr. Patricia Schulte (University examiner)
Wednesday, December 4, 2024 at 4:00 PM in Room 200 of the Graduate Student Centre (6371 Crescent Road).