Gas Tensions and Blood Distribution in Sea Snakes at Surface Pressure and at Simulated Depth

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Abstract
In the resting, air-breathing sea snake, Laticauda colubrina, the gas tension differences between the alveoli and the pulmonary vein ; appear to result primarily from absolute shunts in the form of arterial-venous (a-v) anastomoses or collapsed alveoli. Pure O₂ breathing suggests that inequalities in the ventilation:perfusion ratio and membrane diffusion resistances are relatively unimportant in determining the differences. In air-breathing Laticauda an average of 28% of the systemic cardiac output effectively bypasses the lung via intrapulmonary (16%) and intraventricular (12%) shunts, whereas in air-breathing Hydrophis belcheri the total shunt averages about 76% of systemic flow. Compression collapses the lung nonuniformly and increases both and , by increasing intrapulmonary and possibly intraventricular shunting. During compression, , remains considerably lower than , by virtue of central shunting and cutaneous N₂ loss. This effect offers protection from N₂ bubble formation during decompression; and the degree of protection is less in the amphibious, shallow diving Laticauda than in the totally aquatic, deep diving Hydrophis, in which maximum , is virtually independent of depth. Increased shunting resulting from pressure-induced lung collapse provides more protection; and, although this mechanism has little significance at shallow depth, it become more important in dives below about 50 m.