4. Discussion
In 1940 Scholander proposed that marine mammals have stiffened
upper airways that would stay open and receive air from
highlycompressible alveoli during diving. Thismovement of airwas
thought to collapse the alveoli and prevent gas exchange. However,
this idealized system would tend to underestimate collapse depth
if the trachea is compressible and the alveoli not infinitely compliant.
This paper presents a mathematical model based on structural
properties of the upper airways and alveoli that describes how a
closed respiratory system behaves under pressure. Furthermore,
we present a simple gradual shunt model and use it qualitatively
to describe diffusion rate data from the literature. Our aim was to
create a framework that could be used to describe how the closed
respiratory system behaves while exposed to increased pressures
during breath-hold diving. Using this model we have highlighted
physiological variables that are important and that need further
study to understand gas exchange during breath-hold diving.