a percentage of the gas we breathe (air) is always dissolved in our blood, like the gas dissolved in a carbonated drink bottle with the lid on. if a person moves to a higher ambient pressure, then the gas inhaled is at a higher pressure, so more of it dissolves in the blood and diffuses into body tissues (henry's and fick's gas laws). if they slowly move back to a lower pressure, then the extra gas comes out slowly until they are back to their normal amount of dissolved gas. but if they move quickly to a lower ambient pressure, then the gas comes out of our blood and tissues violently, in large bubbles, in the same way that quickly removing the cap from a bottle of soft drink produces far more bubbles than slowly opening the bottle.