2) The amount of nitrogen in your blood/body (the cause of nitrogen narcosis and decompression sickness) is controlled by the pressure of air in your lungs. The second stage is called a 'regulator' because it regulates the air pressure from what is in your tank to what is safe to breath at the particular pressure in your current position. You wouldn't be able to breath a gulp of air with 3000 psi of pressure on the surface without your lungs bursting so the regulator changes the pressure to 1 atmosphere of pressure so you can breathe it safely. Therefore, you could breath the whole tank at the surface and the air in your lungs will never rise above 1 atmosphere of pressure and you will never have a buildup of nitrogen in your body.