At a depth of 5 atmospheres, nitrogen causes symptoms resembling alcohol intoxication, known as nitrogen narcosis.
Nitrogen narcosis apparently results from a direct effect on the brain of the large amounts of nitrogen dissolved in the blood.
Deep dives are less dangerous if helium is substituted for nitrogen, because under these pressures helium does not exert a similar narcotic effect.
As a scuba diver descends, the pressure of nitrogen in the lungs increases.
Nitrogen then diffuses from the lungs to the blood, and from the blood to body tissues.
The reverse occurs when the diver surfaces ; the nitrogen pressure in the lungs falls and the nitrogen diffuses from the tissues into the blood, and from the blood into the lungs
If the return to the surface is too rapid, nitrogen in the tissues and blood cannot diffuse out rapidly enough and nitrogen bubbles are formed
They can cause severe pains, particularly around the joints.
Another complication may result if the breath is held during ascent.
During ascent from a depth of 10 meters, the volume of air in the lungs will double because the air pressure at the surface is only half of what it was at 10 meters.
This change in volume may cause the lungs to distend and even rupture.
This condition is called air embolism
To avoid this event, a diver must ascend slowly, never at a rate exceeding the rise of the exhaled air bubbles, and must exhale during ascent.
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