How is decompression sickness related to pressure?

Answer 1

Divers breathing compressed gases at depth are themselves under pressure. For each 33 feet/10 meters that a diver descends, they add approximately 1 atmosphere of pressure to their body. When they breathe gases whilst their bodies are under this pressure, the gases dissolve into their bodies tissues.

When they ascend, the pressure is relieved, and gases are released. Provided that diver ascends sufficiently slowly, the gases are released slowly and no problems result. But if the diver ascends too slowly without allowing the gases to expire at a slow enough rate, then the diver will suffer decompression sickness when gas bubbles form in the diver's tissues and blood.