Probably the important fact is that pressure * volume is a constant.
So you take 80 cu ft of air at atmospheric pressure (15psi) and squeeze it down into a small cylinder at 3000psi
Then underwater the regulator (the bit in your mouth) delivers air at the same pressure as the water so you can breath with no effort.
With each breath you take about the same volume (1litre) but at a pressure that depends on the depth so the amount of air you use depends on the depth
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