About pressure in scuba diving.?

scuba diving:

1. why is compressed air in the tanks pressurized for lung function?
2. what dangers are associated by changes in air pressure?
3. what us the purpose of a buoyancy compensator and how does it work?

answer all or some of these for best answer. thanks. (:
Update: uhhh i neeed some answerrs here.
gotta pass this projectt.
3 answers 3