Air for a diver comes out of a high pressure tank at - Same- pressure compared to the water around the diver (metered by the regulator).
This means the lungs are inflated with - Highly pressurized- gas.
This does not adversely affect the diver when deep underwater, because the entire environment around the diver is at -Same - pressure.
If the diver suddenly surface, the air in the alveoli in the lungs will still be at - a higher - pressure compared to the air around the diver, which will be at - a lower - pressure.
The gas in the diver's lungs will - expand - and can damage the alveoli.