A new study offers a hypothesis of how marine mammals generally avoid getting the bends. The key is the unusual lung architecture of whales, dolphins and porpoises, and possibly other breath-holding diving vertebrates.

When air-breathing mammals dive, their lungs compress. The ultra-deep-diving feats of some marine mammals go beyond our current understanding of respiratory physiology and lung mechanics. But historically, researchers assumed the chest structure of marine mammals meant their lungs compressed automatically at great depths, an adaptation that prevented them from taking up excess nitrogen and getting the bends.

In a new study just published by the Royal Society a team of scientists found clues in the lungs of marine mammals that hint at why the creatures are able to traverse the depths of the ocean without getting the bends. There is a distinction between the marine and land animals’ lungs. In the marine mammals, their lung architecture leads to the formation of two distinct regions under pressure – one air-filled and the other collapsed.

The researchers believe that blood flows mainly through the collapsed region of the lungs. That causes what is called a ventilation-perfusion mismatch, which allows some oxygen and carbon dioxide to be absorbed by the animal's bloodstream, while minimizing or preventing the exchange of nitrogen. According to the study’s lead author, Dr Daniel García-Parraga of the Fundacion Oceanografic, this mechanism could prevent marine animals from taking up excess levels of nitrogen and therefore minimise risk of the bends.