Abstract
We examined structural properties of the marine mammal respiratory system, and tested Scholander's hypothesis that the chest is highly compliant by measuring the mechanical properties of the respiratory system in five species of pinniped under anesthesia (Pacific harbor seal, Phoca vitulina; northern elephant seal, Mirounga angustirostris; northern fur seal Callorhinus ursinus; California sea lion, Zalophus californianus; and Steller sea lion, Eumetopias jubatus). We found that the chest wall compliance (CCW) of all five species was greater than lung compliance (airways and alveoli, CL) as predicted by Scholander, which suggests that the chest provides little protection against alveolar collapse or lung squeeze. We also found that specific respiratory compliance was significantly greater in wild animals than in animals raised in an aquatic facility. While differences in ages between the two groups may affect this incidental finding, it is also possible that lung conditioning in free-living animals may increase pulmonary compliance and reduce the risk of lung squeeze during diving. Overall, our data indicate that compliance of excised pinniped lungs provide a good estimate of total respiratory compliance.
Highlights
In 1940, Per Scholander described the unusual properties of the marine mammal respiratory system, and suggested that the highly compliant lung and rib cage would compress and shunt air into the rigid upper airway (Scholander, 1940)
Having a compliant chest wall that empties to very low volumes would reduce the chances of extreme negative intra-thoracic pressures, which are known to protect against pulmonary edema, cardiac arrhythmias and caval rupture (Leith, 1989)
Our results suggest that pinnipeds have a highly compliant chest wall that recoils inward to approach the residual volume (RV, Figures 1, 4, 5)
Summary
In 1940, Per Scholander described the unusual properties of the marine mammal respiratory system, and suggested that the highly compliant lung and rib cage would compress and shunt air into the rigid upper airway (Scholander, 1940). Only a few studies have attempted to determine how pressure affects gas exchange in forced diving or freely diving marine mammals (Ridgway and Howard, 1979; Kooyman and Sinnett, 1982; Falke et al, 1985; McDonald and Ponganis, 2012, 2013). These studies revealed considerable differences among species in the estimated depth at which alveolar collapse occurred, but used different methods to quantify alveolar compression. Differences ranged from directly measuring the depth related pulmonary shunt (Kooyman and Sinnett, 1982), to indirectly measuring O2 or N2 tension in arterial and venous blood during a dive (Falke et al, 1985; McDonald and Ponganis, 2012, 2013), or measuring N2 removal from the muscle following a series of repeated dives (Ridgway and Howard, 1979)
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