I've always heard that the lungs collapse in diving mammals, forcing the air out of the alveoli into trachea where it can no longer be absorbed. This prevents further N2 uptake limiting the N2 supply to that which is already in the bloodstream and tissues
American Scientist had a very good article diving mammals and birds in 1997. Unfortunately only the abstract and a couple of pictures with captions are available on the web. Fortunately one figure,
http://www.amsci.org/amsci/articles/97articles/kooymancap10.html illustrates this concept. The magazine should be available in any university library and hopefully most public libraries.
Ralph
http://www.amsci.org/amsci/articles/97articles/Kooyman.html
American Scientist
November-December 1997, Volume 85, No. 6
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The Challenges of Diving to Depth
Gerald L. Kooyman and Paul J. Ponganis
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Abstract
People have always measured their physical abilities against those of other animals. Ancient mythologies are filled with tales of people who fly like birds or submerge themselves to great depths beneath the oceans like seals and whales. The physical reality, of course, is quite different, and people require complex equipment to do either -- which makes the question of how animals achieve these feats all the more interesting. Marine mammals and birds can dive to depths greater than one kilometer, versus 130 meters for the record-setting human dive. The immediate problem for deep-sea divers is getting enough oxygen and avoiding pressure-related decompression sickness, commonly known as the "bends". Deep-diving marine animals, on the other hand, possess the physiological equipment to deal with these problems. In this article, the authors describe the adaptations that allow animals to achieve what people can still only dream about.