Extreme hypoxemic tolerance and blood oxygen depletion in diving elephant seals

Abstract

Species that maintain aerobic metabolism when the oxygen (O2) supply is limited represent ideal models to examine the mechanisms underlying tolerance to hypoxia. The repetitive, long dives of northern elephant seals ( Mirounga angustirostris) have remained a physiological enigma as O2stores appear inadequate to maintain aerobic metabolism. We evaluated hypoxemic tolerance and blood O2depletion by 1) measuring arterial and venous O2partial pressure (Po2) during dives with a Po2/temperature recorder on elephant seals, 2) characterizing the O2-hemoglobin (O2-Hb) dissociation curve of this species, 3) applying the dissociation curve to Po2profiles to obtain %Hb saturation (So2), and 4) calculating blood O2store depletion during diving. Optimization of O2stores was achieved by high venous O2loading and almost complete depletion of blood O2stores during dives, with net O2content depletion values up to 91% (arterial) and 100% (venous). In routine dives (>10 min) PvO2and PaO2values reached 2–10 and 12–23 mmHg, respectively. This corresponds to So2of 1–26% and O2contents of 0.3 (venous) and 2.7 ml O2/dl blood (arterial), demonstrating remarkable hypoxemic tolerance as PaO2is nearly equivalent to the arterial hypoxemic threshold of seals. The contribution of the blood O2store alone to metabolic rate was nearly equivalent to resting metabolic rate, and mean temperature remained near 37°C. These data suggest that elephant seals routinely tolerate extreme hypoxemia during dives to completely utilize the blood O2store and maximize aerobic dive duration.