Pulmonary ventilation-perfusion mismatch: a novel hypothesis for how diving vertebrates may avoid the bends

Hydrostatic lung compression in diving marine mammals, with collapsing alveoli blocking gas exchange at depth, has been the main theoretical basis for limiting N-2 uptake and avoiding gas emboli (GE) as they ascend. However, studies of beached and bycaught cetaceans and sea turtles imply that airbreathing marine vertebrates may, under unusual circumstances, develop GE that result in decompression sickness (DCS) symptoms. Theoretical modelling of tissue and blood gas dynamics of breath-hold divers suggests that changes in perfusion and blood flow distribution may also play a significant role. The results fromthe modellingwork suggest that our current understanding of diving physiology in many species is poor, as the models predict blood and tissue N-2 levels that would result in severe DCS symptoms (chokes, paralysis and death) in a large fraction of natural dive profiles. In this review, we combine published results from marine mammals and turtles to propose alternative mechanisms for how marine vertebrates control gas exchange in the lung, through management of the pulmonary distribution of alveolar ventilation ((V)over dot(A)) and cardiac output/lung perfusion ((Q)over dot), varying the level of (V)over dot(A)/(Q)over dot in different regions of the lung. Man-made disturbances, causing stress, could alter the (V)over dot(A)/(Q)over dot mismatch level in the lung, resulting in an abnormally elevated uptake of N-2, increasing the risk for GE. Our hypothesis provides avenues for new areas of research, offers an explanation for how sonar exposure may alter physiology causing GE and provides a new mechanism for how air-breathing marine vertebrates usually avoid the diving-related problems observed in human divers.