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Marine Mammal Physiology: Requisites for Ocean Living
4.4.3 Consideration of alternatives
Finally, perhaps for any science hampered by limited data, there is a tendency toward generalization. In addition to better consideration of phylogenetic variation, we perhaps need
to question the assumptions we often make. In terms of understanding how marine mammals avoid decompression sickness, we have a tendency to consider only the role of N 2 .
However, careful measurement of gas composition of bubbles in stranded cetaceans has
suggested a potential role for CO 2 as the initial instigator to help bubbles form and grow (de
Quiros et al. 2012, 2013a). Carbon dioxide is produced by aerobic metabolism and may accumulate to high levels in metabolically active tissues. Previous work in terrestrial mammals
has shown that rising CO 2 levels were associated with a higher DCS incidence, and it was
suggested that the higher diffusion rate of CO 2 could initiate bubble growth (Behnke 1951).
Since bubble gas composition changes in a predictable manner in animals with decompression stress as compared with those receiving gas emboli from barotrauma or from putrefaction gas, gas composition may therefore be a viable way to assess whether stranded animals
have experienced decompression stress. Gas composition in bubbles from stranded animals
believed to have experienced gas bubble disease had a high level of CO 2 (de Quirós et al.
2012). It was suggested that elevated levels of CO 2 follow a burst of activity, and its much
higher diffusion rate, as compared with N 2 , could initiate the growth of a bubble which
would then grow with diffusion of N 2 into the bubble as supersaturation increases close
to the surface (de Quirós et al. 2012; Fahlman et al. 2014b). This hypothesis could explain
how sonar-induced changes in dive behavior and the resultant burst of activity could cause
increased DCS risk by causing an aversion response that increases anaerobic metabolism
and muscle and vascular CO 2 levels to initiate bubble growth (Fahlman et al. 2014b).
In addition to managing several blood gases, animal are managing several physiological stresses simultaneously, such as dealing with buoyancy, thermoregulation, energy balance and the need for muscle oxygenation (Hooker et al. 2012). Thus, diving physiology
and the regulation of pressure effects cannot be considered in isolation. New work examining the interaction between these stresses (e.g., between exercise and diving) (Williams
et al. 2015) is likely to help illuminate the ways that diving animals manage this balance.
Glossary
Artery/arterial: A blood vessel carrying blood away from the heart. Systemic arteries
carry oxygenated blood from the heart to the body and pulmonary arteries carry
deoxygenated blood from the heart to the lungs.
ATA: Atmospheres absolute, a pressure unit that includes surface pressure.
Atelectasis: Alveolar closure when no gas exchange occurs.
Barotrauma: Trauma caused by pressure.
Compliance: A measure of the ease of expansion of a structure.
Emboli/Embolism: The lodging of an embolus (blood clot, fat globule, or gas bubble) in
a blood vessel.
Hyperbaric: Higher pressure.
Hypocapnia: CO 2 levels lower than normal.
Hypocoagulable: Less prone to coagulate (form clots).
Hypoxemia: Arterial O 2 levels lower than normal.
Hypoxia: Tissue O 2 levels lower than normal.
Lung squeeze: When the chest is exposed to a pressure that reduces the lung volume below
the functional residual capacity and a negative pressure develops inside the lung.
Marine Mammal Physiology: Requisites for Ocean Living
4.4.3 Consideration of alternatives
Finally, perhaps for any science hampered by limited data, there is a tendency toward generalization. In addition to better consideration of phylogenetic variation, we perhaps need
to question the assumptions we often make. In terms of understanding how marine mammals avoid decompression sickness, we have a tendency to consider only the role of N 2 .
However, careful measurement of gas composition of bubbles in stranded cetaceans has
suggested a potential role for CO 2 as the initial instigator to help bubbles form and grow (de
Quiros et al. 2012, 2013a). Carbon dioxide is produced by aerobic metabolism and may accumulate to high levels in metabolically active tissues. Previous work in terrestrial mammals
has shown that rising CO 2 levels were associated with a higher DCS incidence, and it was
suggested that the higher diffusion rate of CO 2 could initiate bubble growth (Behnke 1951).
Since bubble gas composition changes in a predictable manner in animals with decompression stress as compared with those receiving gas emboli from barotrauma or from putrefaction gas, gas composition may therefore be a viable way to assess whether stranded animals
have experienced decompression stress. Gas composition in bubbles from stranded animals
believed to have experienced gas bubble disease had a high level of CO 2 (de Quirós et al.
2012). It was suggested that elevated levels of CO 2 follow a burst of activity, and its much
higher diffusion rate, as compared with N 2 , could initiate the growth of a bubble which
would then grow with diffusion of N 2 into the bubble as supersaturation increases close
to the surface (de Quirós et al. 2012; Fahlman et al. 2014b). This hypothesis could explain
how sonar-induced changes in dive behavior and the resultant burst of activity could cause
increased DCS risk by causing an aversion response that increases anaerobic metabolism
and muscle and vascular CO 2 levels to initiate bubble growth (Fahlman et al. 2014b).
In addition to managing several blood gases, animal are managing several physiological stresses simultaneously, such as dealing with buoyancy, thermoregulation, energy balance and the need for muscle oxygenation (Hooker et al. 2012). Thus, diving physiology
and the regulation of pressure effects cannot be considered in isolation. New work examining the interaction between these stresses (e.g., between exercise and diving) (Williams
et al. 2015) is likely to help illuminate the ways that diving animals manage this balance.
Glossary
Artery/arterial: A blood vessel carrying blood away from the heart. Systemic arteries
carry oxygenated blood from the heart to the body and pulmonary arteries carry
deoxygenated blood from the heart to the lungs.
ATA: Atmospheres absolute, a pressure unit that includes surface pressure.
Atelectasis: Alveolar closure when no gas exchange occurs.
Barotrauma: Trauma caused by pressure.
Compliance: A measure of the ease of expansion of a structure.
Emboli/Embolism: The lodging of an embolus (blood clot, fat globule, or gas bubble) in
a blood vessel.
Hyperbaric: Higher pressure.
Hypocapnia: CO 2 levels lower than normal.
Hypocoagulable: Less prone to coagulate (form clots).
Hypoxemia: Arterial O 2 levels lower than normal.
Hypoxia: Tissue O 2 levels lower than normal.
Lung squeeze: When the chest is exposed to a pressure that reduces the lung volume below
the functional residual capacity and a negative pressure develops inside the lung.