As a diver descends beneath the surface the concentration of nitrogen in his body increases as a result of the high atmospheric pressure. The respiratory gases are compressed and larger amounts dissolve into the body tissues. Ascent after a deep dive must be done slowly as the dissolved gases are released as the external pressure decreases. A rapid decreases in atmospheric pressure leads to a case of the 'bends', or decompression sickness. The diver experiences feelings of dizziness, vomiting and joint pain, and in severe cases, temporary paralysis and other neurological symptoms. These symptoms occur because large bubbles of nitrogen gas block small veins and arteries cutting off oxygen supply. Treatment is with oxygen and prevention involves a very slow ascent to the surface for any deep dives.