As a marine mammal dives to depth the trachea is thought to
act as an incompressible reservoir that receives air forced out of
the alveoli. However, compression of the trachea has been measured
by radiography during forced dives in both the Weddell and
elephant seals (Kooyman et al., 1970). At 650 kPa, the diameter of
the trachea decreased ∼20% in both species. Qualitatively, tracheal
compressionwould reduce the volume available for the alveolar air