4.2. Collapse of the dead space
The structural properties of the dead space in a marine mammal
are important in estimating Pcoll. The more collapsible the dead
space, the more the Pcoll will vary from the idealized balloon/pipe
model. As the compliant trachea compresses concurrently with the
alveolar space, it reduces the volume for the alveolar air. Kooyman
et al. (1970) measured compression of the trachea in both Weddell
and elephant seals in a pressure chamber using radiography.
At 650 kPa the diameter of the trachea decreased ∼20% in both
species. Obviously, the trachea must have experienced compression
at depths shallower than 650 kPa.Upon further compression to
3200 kPa both species showed a decrease in diameter of 54% from
surface values. Fig. 4b shows the model predicted and observed
tracheal volumes. There is a close agreement between predicted
and observed data for VA,o =6VD,o. Kooyman et al. (1970) found that
Weddell seals end voluntary dives with around 55% of their forced
inspiratory lung volume. Therefore, by assuming that lungs and trachea
collapse together when VA