Kooyman and Sinnett (1982) measured pulmonary shunts in
harbour seals and California sea lions in a pressure chamber, in
an attempt to define the degree of alveolar collapse with pressure.
Pulmonary shunt is a measure of incomplete gas exchange
with a pulmonary shunt of 100% equivalent to complete alveolar
To improve our understanding of how a closed respiratory system
compresses under pressure,we derived a mathematical model
to predict alveolar volume at depth based on estimated compliances
of the alveoli and trachea of marine mammals. Furthermore,
we made a simple approximation to predict changes in relative pulmonary
shunt as a function of depth.We suggest that the behavior
of a marine mammal’s respiratory system at depth is predictable
with adequate knowledge of the physical properties of the upper
and lower airways as well as the volume of air in the alveoli when
an animal dives. Our results predict that alveoli remain open at
a greater pressure than previously estimated, although, due to a
decrease in VA, the pulmonary shunt increases with depth.