Blood oxygen depletion during rest-associated apneas of northern elephant seals (Mirounga angustirostris).

SUMMARY Blood gases ( P O 2, P CO 2, pH), oxygen content, hematocrit and hemoglobin concentration were measured during rest-associated apneas of nine juvenile northern elephant seals. In conjunction with blood volume determinations, these data were used to determine total blood oxygen stores, the rate and magnitude of blood O 2 depletion, the contribution of the blood O 2 store to apneic metabolic rate, and the degree of hypoxemia that occurs during these breath-holds. Mean body mass was 66±9.7 kg (± s.d.); blood volume was 196±20 ml kg –1 ; and hemoglobin concentration was 23.5±1.5 g dl –1 . Rest apneas ranged in duration from 3.1 to 10.9 min. Arterial P O 2 declined exponentially during apnea, ranging between a maximum of 108 mmHg and a minimum of 18 mmHg after a 9.1 min breath-hold. Venous P O 2 values were indistinguishable from arterial values after the first minute of apnea; the lowest venous P O 2 recorded was 15 mmHg after a 7.8 min apnea. O 2 contents were also similar between the arterial and venous systems, declining linearly at rates of 2.3 and 2.0 ml O 2 dl –1 min –1 , respectively, from mean initial values of 27.2 and 26.0 ml O 2 dl –1 . These blood O 2 depletion rates are approximately twice the reported values during forced submersion and are consistent with maintenance of previously measured high cardiac outputs during rest-associated breath-holds. During a typical 7-min apnea, seals consumed, on average, 56% of the initial blood O 2 store of 52 ml O 2 kg –1 ; this contributed 4.2 ml O 2 kg –1 min –1 to total body metabolic rate during the breath-hold. Extreme hypoxemic tolerance in these seals was demonstrated by arterial P O 2 values during late apnea that were less than human thresholds for shallow-water blackout. Despite such low P O 2s, there was no evidence of significant anaerobic metabolism, as changes in blood pH were minimal and attributable to increased P CO 2. These findings and the previously reported lack of lactate accumulation during these breath-holds are consistent with the maintenance of aerobic metabolism even at low oxygen tensions during rest-associated apneas. Such hypoxemic tolerance is necessary in order to allow dissociation of O 2 from hemoglobin and provide effective utilization of the blood O 2 store.