Sabrinah asked: How marine mammals avoid 'the bends

by Lisa Cardy

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SABRINAH�wanted to know why marine mammals,�such as whales, don't get the 'bends'. Here The AnswerBank provides a detailed explanation:

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Marine mammals, which evolved on land and then moved back to the oceans, have had to adapt to living in an aquatic habitat. One of the biggest adaptations they made is being able to cope with pressure changes when swimming from the depths of the ocean to the surface for air.

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When other, non marine, mammals - humans - make deep and prolonged dives they wear diving tanks containing pressurised air. Pressure is built up in the body and nitrogen in the pressurised air dissolves into the blood stream and tissues. Too much nitrogen is absorbed if a diver stays too deep for too long. Any sudden change in external pressure, like rising to the surface too quickly, causes the nitrogen to form bubbles in the blood and 'the bends' occur.

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Symptoms of the bends range from pain in the joints to death.

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Marine mammals don't suffer from this condition�for two reasons: because they do not breathe pressurised air and their more flexible rib cages ensures no gases,�such as nitrogen, from air can be absorbed when in deep water.

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Whales, for example, hold their breath under water, breathing only at the surface, so there isn't a constant supply of new pressurised air. It breathes out as it dives storing oxygen in its blood and muscle tissue, rather than its lungs, as humans do.

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Additionally, the relatively small amount of air they take down as they dive is forced into nasal passages. These have a thick lining, which prevents any gas from escaping into the blood stream, so the whale can dive to great depths and resurface without any of problems humans' experience.

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Whales have very weak and flexible rib cages. When diving, the rib cage is totally collapsed and the lungs deflated, removing all air from the lungs and expelling it into the water. Therefore, the blood can't absorb any nitrogen.

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Such physiological specialisms mean that the sperm whale can dive to 2000 metres, where it experiences atmospheric pressure 200 times that felt at the ocean surface without any ill effects.

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