Marine mammals have very compliant alveoli and stiff upper airways; an adaptation that allows air to
move fromthe alveoli into the upper airways, during breath-hold diving. Alveolar collapse is thought occur
between 30 and 100m and studies that have attempted to estimate gas exchange at depth have used the
simplifying assumption that gas exchange ceases abruptly at the alveolar collapse depth. Herewe develop
a mathematical model that uses compliance values for the alveoli and upper airspaces, estimated fromthe
literature, to predict volumes of the respiratory system at depth. Any compressibility of the upper airways
decreases the volume to contain alveolar air yielding lung collapse pressures 2× that calculated assuming
an incompressible upper airway. A simple relationship with alveolar volume was used to predict relative
pulmonary shunt at depth. The results from our model agree with empirical data on gas absorption at
depth as well as the degree of tracheal compression in forced and free diving mammals.