absorb greater amounts of gases (N2 and O2) at higher
pressures. As the absolute pressure of the gas mixture increases,
the relative pressure of the various gases also increases.
In turn, the solubility of gas in the tissues increases
as pressure increases, resulting in more gas being absorbed
by the tissues. At the surface, 1 atm of air is composed of
N2 andO2, with relative partial pressures of 0.79 atm forN2
(79% of 1 atm) and 0.21 atm for O2. At 100 m, where the
absolute pressure is 11 atm, the partial pressure of N2
would be 8.69 atm and 2.31 atm of O2. See also: Neural
Development: bHLH Genes
Higher gas solubility at depth means that the animal will
absorb more gas until the tissue becomes saturated at that
pressure. When the animal returns to the surface, the solubility
of the gas in the tissue is lower and itnowflows out of
the tissue into the blood. This is especially problematic for
human divers who breathe air from a scuba tank. As they
descend, the pressure increases and they inhale air at greater
pressure. Their tissues absorb gas that is now more soluble
and dissolved gas accumulates in the tissue. If the diver
returns to the surface without taking sufficient time to allow
the gas to slowly come out of the tissues (decompress), the
gas will come out of solution and form bubbles. This is
called decompression sickness, the bends or caisson disease.
Marine mammals have an advantage in that, unlike human
divers, they exclusively breath-hold dive. That is they
carry only a limited amount of air in their lungs during a
given dive. Some deep-diving marine mammals appear to
avoid problems associated with tissue N2 accumulation by
allowing their lungs to collapse during the initial period of
the dive. As the lung collapses, air is expressed into the
large bronchioles and trachea where gas uptake cannot
occur. Given that N2 and O2 tensions in the blood remain
relatively low during the dive, nitrogen narcosis, decompression
sickness and oxygen toxicity are thought to be
avoided. While this appears to prevent decompression
sickness in animals that undergo lung collapse, there are
many species of diving vertebrates such as penguins, sea
lions and fur seals that make many repetitive dives on a full
lung. It is unclear how these animals avoid the bends as
they make repeated dives to significant depths over a period
of many hours or even days. If, during these dives, the
animals spend more time under water than at the surface, it
is possible they accumulate sufficient N2 in their tissues to
cause decompression sickness. Symptoms consistent with
decompression sickness have been reported in human pearl
divers undergoing prolonged repetitive breath-hold dives.
To date, only one study has examined tissue nitrogen levels
in a repetitively diving animal, the bottlenose dolphin
Tursiops truncatus. While this study found relatively high
levels of N2 in muscle tissue, it was unclear whether the
levels of muscleN2 were sufficiently high as to cause bubble
formation. Further, the researchers were not able to determine
whether there was a mechanism that kept theN2 in
the tissue, or whether there was some capacity to tolerate
N2 bubbles if they formed.
Interestingly, recent histological observations of bone
necrosis in sperm whales are consistent with symptoms associated
with decompression sickness. This suggests that
sperm whales, and possibly other toothed whales, suffer
from periodic decompression sickness in the wild. Similar
observations of fossil mosasaurs and plesiosaurs also suggest
that these ancient diving reptiles had difficulty avoiding
decompression sickness. This is of more than academic
interest, as there has been considerable concern whether
diving mammals are more susceptible to the bends when
they are exposed to military sonars. The issue is whether
sonar stimulates bubble formation and/or elicits aberrant
diving behaviour that results in acute and severe decompression
sickness.