Friday, October 3, 2008

The Bends

When I teach about hydrostatics from the 4th Edition of Intermediate Physics for Medicine and Biology, I like to make a little digression and discuss a biomedical application of hydrostatic pressure: decompression sickness. Also known as "the bends", this illness occurs after breathing high-pressure air, which causes nitrogen to be dissolved in the blood. If the pressure is then released suddenly the nitrogen can form bubbles that block circulation. This effect is not unlike the formation of foam--made from bubbles of carbon dioxide--when you open a bottle of pop. You can find a nice discussion of the physiological effects of increased fluid pressure in Physics With Illustrative Examples from Medicine and Biology: Mechanics, by George Benedek and Felix Villars.

Often you can teach best by telling a story, and when discussing decompression sickness I like to tell the story of the Eads Bridge. James Eads built the first bridge over the Mississippi River at St Louis. It is a beautiful arch bridge that opened in 1874. When building the supports for the bridge under the river, Eads used "caissons", watertight structures sunk under the river and filled with compressed air. The high air pressure prevented water from filling the caisson, allowing workers to excavate the river bottom. In his book
Engineers of Dreams: Great Bridge Builders and the Spanning of America, Henry Petroski has described decompression sickness experienced by men working in Eads' caissons:
"When the caisson reached a depth of seventy feet, the workmen began to experience some difficulty climbing the stairs to the surface. As the caisson was sunk deeper, men suffered increasing attacks of cramps and paralysis, which were thought to be due to insufficient clothing or poor nutrition. In March 1870, when the caisson had reached ninety-three feet, the air pressure inside it was about four times what it was in the open air, and workmen began dying upon emerging from the caisson, or after being hospitalized for an ailment that came then to be called "caisson disease" but today is known as "the bends." Eads asked his family physician, Dr. Alphonse Jaminet, to look after the workmen, but Jaminet himself became paralyzed one day, having spent time down in the caisson and come up after only a few minutes in the air lock.

Perhaps somewhat to his own surprise, Jaminet recovered, and began to conduct research into these mysterious attacks. He shortly concluded that the major cause was too-rapid decompression in the face of a drastic difference in air pressure between the submerged caisson and the outer air above. Thereupon he placed restrictions on the amount of time the men could work inside the caisson, and on the speed with which the pressure in the air lock could be reduced."
Incidentally, Petroski has written many fascinating books about engineering, including
To Engineer Is Human: The Role of Failure in Successful Design To Engineer Is Human: The Role of Failure in Successful Design and The Evolution of Useful Things. He also has a monthly essay on engineering in American Scientist, the magazine of Sigma Xi, the Scientific Research Society. Although there is not much medicine or biology in Petroski's work, certainly a biomedical engineer studying from Intermediate Physics for Medicine and Biology will find many important lessons about engineering design. I give Petroski's books two thumbs up.

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