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The Bends Causes

Nitrogen from a diver's air tank increases in pressure as a diver descends. For every 33 feet in ocean water, the pressure due to nitrogen goes up another 11.6 pounds per square inch, or 600 torr. As the pressure due to nitrogen increases, more nitrogen dissolves into the tissues. The longer a diver remains at depth, the more nitrogen dissolves for any specific depth. Unlike the oxygen in the air tank a diver uses to swim underwater, the nitrogen gas is not utilized by the body and builds up over time in body tissues.

• A clear example to illustrate this process is that of a bottle of carbonated soda. A bottle of carbonated soda is filled with gas (carbon dioxide), which cannot be seen because it is dissolved in solution under pressure. When the bottle is opened, the pressure is released and the gas leaves the solution in the form of bubbles. A diver returning to the surface is similar to opening the bottle of soda. As a diver swims to the surface, the pressure due to nitrogen decreases. The nitrogen, which has dissolved in tissues, wants again to leave, because the body can hold only a certain amount based on that nitrogen pressure.
  
  
  • If a diver surfaces too fast, the excess nitrogen will come out rapidly as gas bubbles. Depending on which organs are involved, these bubbles produce the symptoms of decompression sickness. The process is similar to shaking a can of carbonated soda. If the pressure is high enough, the carbon dioxide in the soft drink will emerge upon opening the can as a gush of bubbles.
    
    
  • The risk of decompression illness is directly related to the depth of the dive, the amount of time under pressure, and the rate of ascent. Dive tables, such as the US Navy Dive Tables, provide general guidelines as to what depths and dive times are less risky for the development of decompression sickness. Much of this data was gleaned from actual human divers in experimental diving units.