The bends (also known as decompression sickness) is a condition by which nitrogen gas builds up in bodily tissues during underwater descent and can expand to form bubbles during ascent, with painful and even deadly consequences. Diving marine mammals are thought to avoid the bends through physiological processes related to the passive compression of their lungs as pressure increases with depth. However, theoretical models predict that a mechanism this simple would result in blood and tissue nitrogen levels high enough to cause severe decompression sickness. A new study offers a more complex hypothesis how marine mammals’ lung gases are managed during a dive and suggests how these mechanisms might fail when an animal is exposed to stress.
Daniel Garcia Párraga and Andreas Fahlman, of Fundación Oceanogràphic in Valencia, Spain, along with colleague Michael Moore, of Woods Hole Oceanographic Institution on Cape Cod, MA, developed their hypothesis by combining the results of previous research with theoretical modeling. In one study, they placed a deceased seal, dolphin, and domestic pig in a hyperbaric chamber that increased pressure to simulate dive conditions and used computed tomography (CT) images to observe the animals’ lung architecture. In these simulations, only part of the marine mammals’ lungs were compressed, while the rest remained inflated. The pig’s lungs, in contrast, did not share this distribution mechanism and were evenly ventilated.
The authors believe that in live diving mammals, blood flow is directed to the compressed region of the lungs, during which some oxygen and carbon dioxide can be taken up by the blood, but nitrogen uptake is limited. This variation in gas exchange, or “ventilation-perfusion mismatch,” is not all passive, says Fahlman. “These animals can specifically manage which gas they take up, and when.”
A detailed understanding of this physiological mechanism is important if we want to understand how external forces might disrupt it, says Fahlman. Studies of some beached and unintentionally caught marine mammals and sea turtles found gas embolisms in their blood that are associated with the bends. Some of these cases have been linked to underwater human-made noise, or stressful situations, such as net entanglement. “By understanding the physiological mechanisms involved in diving” says Fahlman, “we have a better chance of doing simple and effective mitigation” when conditions go awry. (Proceedings of the Royal Society B)