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Nitrogen absorption avoided in sea-lion diving

By Dave Armstrong - 19 Sep 2012 8:43:17 GMT
Nitrogen absorption avoided in sea-lion diving

California sea lions instrumented with a partial pressure of oxygen (PO2) data logger and time depth recorder; © Birgitte McDonald

The sea lion lives in colonies, often close to human activity, and dives to catch squid, octopus, cod, herring mackerel and quite a few other dwellers of the deep. The significant feature is the external pinnae or ears, along with their light or dark brown fur. Surprisingly, one population survived until WW2 in the Sea of Japan, but the furthest they occur from California now is the Galapagos Islands.

Marine mammals contrive to collapse their lungs when diving deep. With a deep dive, sea lions (Zalophus californianus) increase their air intake volume and collapse their lungs when they are far below the surface at around 180m. Enough oxygen has to be in the lungs to maintain a normal ascent.

As exchange of CO2 (carbon dioxide), O2 (oxygen) and N2 (nitrogen) is prevented by the collapse of the lungs, then no more nitrogen can be taken into the bloodstream to cause decompression problems on the ascent to the surface. Whales have been known to suffer from this decompression sickness, just like divers, when they are exposed to naval sonar.

Alveoli are the air sacs that can no longer transfer the gases to the blood capillaries in their walls when they collapse. As the pressure of oxygen gas in the arteries declines steeply at great depth, scientists believe this can only show the closure of the alveoli. This is the first ever evidence that free-diving marine mammals can collapse these single-cell-thick membranous sacs.

This enables oxygen to be retained in the upper airways for 88% saturation of the haemoglobin during the ascent, after the dive. Without this, the sea lion would be liable to shallow water black-outs as the brain failed to take in the oxygen it needs. Antarctic fur seals for example don't achieve this re-oxygenation, so they have to exhale. This works for them because the oxygen in the blood doesn't then diffuse back into the lung and cause blackout.

With these easily accessible, intelligent and healthy sea lions, we have an opportunity to study nitrogen uptake or its avoidance in the future. The study of all marine mammals and human diving capabilities will be vastly improved by this paper, Lung collapse in the diving sea lion: hold the nitrogen and save the oxygen, published in Biology Letters. Birgitte I. McDonald and Paul J. Ponganis of the Center for Marine Biotechnology and Biomedicine, La Jolla, California performed the careful study using data loggers and a radio transmitter on a single female animal.

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Topics: Mammals