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

In the resting, air-breathing sea snake, Laticauda colubrina, the gas tension differences between the alveoli and the pulmonary vein [(A-pv){aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}P_O₂{document} = 38 torr; (pv-A){aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}P_CO₂{document} = 5 torr] 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 (A-pv){aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}P_O₂{document} and (A-a){aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}P_O₂{document}, by increasing intrapulmonary and possibly intraventricular shunting. During compression, {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}Pa_N₂{document}, remains considerably lower than {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}Pa_N₂{document}, 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 {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}PA_N₂{document}, 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.