Abstract

Deep-diving California sea lions (Zalophus californianus) can maintain arterial hemoglobin saturation (S_O2) above 90% despite lung collapse (lack of gas exchange) and extremely low posterior vena caval S_O2 in the middle of the dive. We investigated anterior vena caval P_O2 and S_O2 during dives of an adult female sea lion to investigate two hypotheses: a) posterior vena caval S_O2 is not representative of the entire venous oxygen store and b) a well-oxygenated (arterialized) central venous oxygen reservoir might account for maintenance of arterial S_O2 during lung collapse. During deep dives, initial anterior vena caval S_O2 was elevated at 83.6 + 8.4 % (n=102), presumably due to arteriovenous shunting. It remained high until the bottom phase of the dive and then decreased during ascent, whereas previously determined posterior vena caval S_O2 declined during descent and then often increased during ascent. These divergent patterns confirmed that posterior vena caval S_O2 was not representative of the entire venous oxygen store. Prior to, and early during descent of deep-dives, the high S_O2s of both the anterior and posterior venae cavae may enhance arterialization of a central venous oxygen store. However, anterior vena caval S_O2 values at depths beyond lung collapse reached levels as low as 40%, making it unlikely that even a completely arterialized central venous oxygen store could account for maintenance of high arterial S_O2. These findings suggest that maintenance of high arterial S_O2 during deep dives is due to persistence of some gas exchange at depths beyond presumed lung collapse.