S2-Diapositiva1 from Pulmonary ventilation–perfusion mismatch: a novel hypothesis for how diving vertebrates may avoid the bends

Proposed terrestrial and cetacean model of ventilation (VA) and perfusion (Q) matching in the lung during diving. On the left, the terrestrial lung shows a gradual change in VA/Q ratio, causing regional differences in gas exchange but there is an extensive central area of the lung where VA/Q ratio is close to 1 (where O2 and CO2 exchange is maximized). In the cetacean lung during a dive, there is a distinct, very narrow boundary where the VA/Q would be close to unity, above which it rapidly approaches infinity and below it is virtually zero (see Fig. 2). Thus the cetacean lung during a dive receives proportionally less blood than the terrestrial mammal lung: collateral ventilation can facilitate air escaping to the upper most areas due to gravity and mainly hypoxic pulmonary vasodilation could favor blood into the lower most areas. That would lead to a mismatch avoiding blood gas exchange even at shallow depths when lung collapse is not complete. We hypothesize that based on these mechanisms there is a minimal interface of blood-gas in the cetacean lung compared to the terrestrial lung where high perfusion and mainly hypoxic pulmonary vasoconstriction promotes blood flow to be redirected to most ventilated areas (increased matching). These differences will have a major impact on gas exchange and when properly matched provide selective exchange of gases.