Human vs. Cetacean Divers Following a deep dive in water, humans can experience severe pains upon returning to the surface too quickly, and the resulting body injury may cause unconsciousness and death. Aches are so severe in some cases, commonly in the joints, that divers bend over in agony. Hence, the name, the bends. However, the more common name for this condition is decompression sickness, meaning that the sickness results from decreasing pressure. Divers experience decreasing pressure upon ascent to the surface of the water.

This is expected because pressure gradually increases with increasing depth of the water. Increases in pressure with depth are a result of increasing weight of the water. For example, at 150 feet, because of this weight, pressures on all surfaces of the body are four and one-half times greater than they are at the surface. At this depth, pressure on the diver's chest cavity, lungs, and air within the lungs causes gases such as nitrogen in that air to dissolve in the blood of lung tissue. Dissolved nitrogen at high pressure and deep water causes no problems as long as the diver does not ascend. When the diver ascends, pressure decreases (decompression), and nitrogen is released from the blood into body tissues as bubbles.

This process is similar to the appearance of bubbles in a soda after the cap is removed. Bubbles appear because the pressure that kept them dissolved was removed with release of the cap. These nitrogen bubbles in tissues are the factor which cause damage, sickness, and death. However, some mammals do not experience decompression sickness.

These are the whales, porpoises, and dolphins, known collectively as cetaceans. Cetaceans do not experience complications during dives as humans do, even though nitrogen dissolves in the blood of cetaceans to the depth o 200 feet. Beyond that level, cetacean lungs collapse and air within them is shifted to various body spaces where gases fail to dissolve in the blood. Why don't cetaceans display decompression sickness when they ascend from a 200-foot depth Authorities give this answer. Because cetacean heart functions are very smooth and quiet, shock waves are minimal in cetacean body systems. Therefore, gases escape from the blood slowly without creating bubbles.

Contrary to cetaceans, human heart functions are not smooth, and considerable shock waves are created which cause gas bubbles in the blood, and thus, decompression sickness. Unlike cetaceans, humans are not endowed with means to cope with deep diving in the water. Humans have, however, developed some guidelines which, when followed, prevent decompression sickness. These guidelines include mathematical calculations which instruct divers on how long to remain submerged at certain depths and how to avoid absorbing excessive amounts of nitrogen.

Another recent development is the underwater computer which divers wear attached to their gear to provide information on durations of stay at various depths and surfacing procedures. BIBLIOGRAPHY Burton, R. 1980. The life and death of whales. 2 nd ed. New York: Universe Books.

Coffey, D. J. 1977. Dolphins, whales and porpoises: an encyclopedia of sea mammals.

New York: Macmillan Publishing Company, Inc. Gaskin, D. E. 1972. Whales, dolphins and seals: with special reference to the New Zealand region. Auckland, New Zealand: Heinemann Educational Books.

Martin, K. 1988. Giants of the sea. New York: Gallery Books. Martin, R. M.

1977. Mammals of the oceans. New York: G. P.

Putnam's Sons.