The structure and function of the small airways in pinniped and sea otter lungs

We have looked at the structure of the small airways from the lungs of 58 marine mammals. Six were from sea otters, the others were from 14 of the 21 pinniped genera. In comparison with the terminal airways of terrestrial mammals all showed reinforcement by muscle and/or cartilage. The small airways of phocid seal lungs were reinforced by a moderately slender tube of oblique muscle. Those of otariid seals were strengthened by very much thicker pieces of cartilage. The airways of sea-otter and walrus lungs were of an intermediate pattern. Both types of reinforcement seem to appear late in foetal life rather than by arrest of development. Calculations suggest the reinforcement in otariid lungs may be excessive for limiting nitrogen absorption during deep dives and behavioural observations indicate that strengthened airways may also be required for rapid ventilation at the surface between dives.