Northern elephant seals (Mirounga angustirostris) are most at home in the water, plumbing the depths during their extensive dives and holding their breath for an hour or more, tolerating conditions that would damage humans irreversibly. The seals only emerge onto land for 2-4 months of the year to breed and moult. When asleep on land they show long natural breath-holds called apneas, which last up to 25 min. These apneas probably occur to conserve energy and water since the seals fast while they are on land, and studying them might give scientists clues as to how seals manage their oxygen stores when they hold their breath. To find out more, Torre Stockard, Paul Ponganis and colleagues monitored nine juvenile elephant seals as they slept (p. 2607).
To measure oxygen levels in the blood, the team inserted catheters into the arteries and veins supplying the major internal organs and took blood samples during normal breathing and at regular intervals during apnea. To find out how much of a blood oxygen store the seals had available before apnea, they measured the percentage of the blood that is made up of red blood cells, blood volume, haemoglobin content and its saturation with oxygen. They found that the seals could store around 52 ml O2 in their blood per kilo of body mass, whereas humans can only hold around 10 ml O2 per kilo in their blood, showing that large oxygen stores contribute to seals' diving prowess.
When the seals nodded off and started showing apneas, which lasted between 3 and 11 min, they used up oxygen at a rate of 2-2.3 ml oxygen per 100 ml blood each minute, which is a similar rate to normal conditions. By the end of 7-9 min apneas, the seals had significantly depleted their blood O2store from around 26 ml per 100 ml blood, to around 7 ml per 100 ml blood, a very low level that humans couldn't tolerate.
To find out how much oxygen can be transferred from blood to tissue during apnea, the team measured oxygen partial pressure, which is the pressure exerted by an individual gas in a gas mixture, or in solution. The lower the partial pressure gradient, the slower the transfer to tissues. Although the oxygen partial pressure varied greatly during normal breathing, between 34 and 108 mmHg, partial pressure dropped in all the seals during apnea, and the lowest recorded arterial value was 18 mmHg. Humans black out at an oxygen partial pressure of around 25-30 mmHg, showing that the seals are able to squeeze the most out of their oxygen store to get the gas to the tissues that need it, and tolerate very low oxygen levels without any ill-effects.
The seals used up around 56% of their oxygen stores during a typical 7 min apnea, but one animal pushed the limit and used up nearly 88% of its store during an 11 min apnea. Despite the oxygen stores getting very low in some cases, none of the seals showed evidence of anaerobic metabolism, since there wasn't a build up of anaerobic by-product lactate in the blood. A small decrease in blood pH was accounted for by carbon dioxide accumulating in the blood. `Their breath-holding is very efficient', says Stockard. `Diving is the norm for these seals, so next we're going to find out what's really happening in diving animals'.