Experimental and computational biomechanical characterisation of the tracheo-bronchial tree of the bottlenose dolphin (Tursiops truncatus) during diving.

Dolphins have adapted their anatomic structures to survive in the water environment and so far, the behaviour of their respiratory system during diving has not been fully understood, since they being protected species cannot be subjected to invasive analysis. Aim of this work is to model the tracheo-bronchial tree of the bottlenose dolphin Tursiops truncatus to study its behaviour during diving by coupling experimental in vitro mechanical characterisation of airways tissues to finite element computational analyses. Furthermore, a comparison was performed between the mechanical behaviour of tracheo-bronchial trees of dolphins and that of the goat, a terrestrial mammal, whose conformation of the upper airways is similar to that of the human, to determine how different structures respond to pressure in a controlled experimental set-up. The comparison between the goat and dolphin airways' mechanical behaviour highlights a lower collapsibility of the dolphin structure due to higher stiffness, lack of musculature and irregular shape of cartilaginous rings. Our data showed that the air entrapped into the airways plays a key role in avoiding the collapse. This effect is enhanced when accounting for the air flow escaping the alveoli that start to collapse during descent, even at depth as shallow as 10m of sea water. The comparison between airways behaviour of marine and terrestrial mammals may help in shedding a light on the biomechanical behaviour of human airways during breath-holding diving.