The bends (properly called decompression sickness) is a condition experienced by divers who ascend too quickly. When air under high pressure is breathed for any length of time, nitrogen dissolves in the body fluids: release of the outside pressure causes the gas to come back out of solution, and form bubbles in the blood.
The physiologist J.S. Haldane studied this problem in the early 20th century, eventually devising the method of staged, gradual decompression, whereby the pressure on the diver is released slowly enough that the nitrogen comes gradually out of solution without forming bubbles.
Because helium has a far lower solubility in water, deep-sea divers nowadays breathe heliox (helium/oxygen mix).
An alternative name is caisson disease; this name comes from the 19th century, when large engineering excavations (bridges, tunnels) required the work to be done in "caissons" under pressure to keep water from flooding the excavations.
Other ill effects experienced by divers are nitrogen narcosis and oxygen toxicity.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Decompression sickness".