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go even further. Shortly after starting a dive, the seal collapses its
lungs, squeezing air out of the alveoli and into the bronchial tubes
in an effort to move the air away from its blood. Shortly before
surfacing, the seal’s lungs begin
to reinflate from the throat-full of
air in preparation for the next full
breath at the surface.
Collapsing its lungs also helps
the seal get down to depth. With
a lower volume of gas, the seal
sinks faster and will glide for up
to 10 minutes, using very little
energy on the trip down. Once
down, however, the seal must
contend with a second, and
obvious, problem: lack of oxygen. Several adaptations work in
concert to overcome this basic hurdle. First, Weddells have a
lot of blood — three times as much as humans per kilogram of
muscle — and they have nearly twice the number of red blood
cells per liter. Thus, a seal can store six times more oxygen
than a human in its blood alone. The seal also stores oxygen
in its muscles and has 20 times a human’s concentration of
myoglobin, the muscle protein
that absorbs oxygen. But an
increased capacity for storing
oxygen is just the start of the
seal’s adaptations.
Oxygen keeps the cellular
fires burning, enabling the
transformation of food into
energy. Aerobic metabolism
consumes oxygen and releases
mainly carbon dioxide. Without
oxygen, cells can still function,
but they must produce energy in a different way, resorting
to anaerobic metabolism. In this oxygen-deprived state, cells
start to consume their own organic acids, which ferment,
creating energy at the cost of lactic acid buildup, which can
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In stark contrast to the Antarctic Peninsula and to the high Arctic on
the other end of the globe, where sea ice has been disappearing at an
extreme rate in all locales, the extent of sea ice around the Antarctic
as a whole has actually been increasing recently. Scientists believe
that this counterintuitive trend is due to heat loss from the upper
atmosphere through the Antarctic ozone hole.