Abstract
Investigation of marine mammal dive-by-dive blood distribution and oxygenation has been limited by a lack of noninvasive technology for use in freely diving animals. Here, we developed a noninvasive near-infrared spectroscopy (NIRS) device to measure relative changes in blood volume and haemoglobin oxygenation continuously in the blubber and brain of voluntarily diving harbour seals. Our results show that seals routinely exhibit preparatory peripheral vasoconstriction accompanied by increased cerebral blood volume approximately 15 s before submersion. These anticipatory adjustments confirm that blood redistribution in seals is under some degree of cognitive control that precedes the mammalian dive response. Seals also routinely increase cerebral oxygenation at a consistent time during each dive, despite a lack of access to ambient air. We suggest that this frequent and reproducible reoxygenation pattern, without access to ambient air, is underpinned by previously unrecognised changes in cerebral drainage. The ability to track blood volume and oxygenation in different tissues using NIRS will facilitate a more accurate understanding of physiological plasticity in diving animals in an increasingly disturbed and exploited environment.
Highlights
near-infrared spectroscopy (NIRS) measurements of [tHb], [Hbdiff], and Tissue saturation index (TSI) showed repeated patterns, in both the brain and blubber, that were consistent between dives, across trials, and between animals
We present the use of a noninvasive waterproof NIRS system to measure continuous, high-resolution, dive-by-dive blood volume and oxygenation changes in the blubber and brain of voluntary diving seals
Highly reproducible patterns of changes in local blood volume and oxygenation that are indicative of cognitive control of circulation in diving seals and the likely use of increased venous drainage as a mechanism for modulation of cerebral reoxygenation during apnoea
Summary
The marked cardiovascular responses to submersion that diving mammals exhibit consist of bradycardia (reduction in heart rate [HR]), with decreased cardiac output (CO) and arterial constriction [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19]. The resulting major redistribution of blood flow conserves oxygen by restricting perfusion of peripheral tissues nonessential to diving [4,10,13]. These physiological changes, collectively referred to as the ‘dive response’, have historically been assumed to be an autonomic response to submersion. Very little is known about the timing or magnitude of blood redistribution associated with the dive response in breath-hold diving animals, such as marine mammals, at the level of individual tissues.
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