What changes as a diver rises
At depth, water pressure compresses the air a diver breathes. More nitrogen moves from the lungs into the blood and then into body tissues. How much accumulates depends partly on how deep and how long the diver stays down; tissues do not all absorb or release it at the same speed.
During ascent, the surrounding pressure falls. Nitrogen that was dissolved under higher pressure must leave the body through the lungs. If pressure drops faster than the gas can be carried away, the blood and tissues become temporarily oversaturated—holding more nitrogen than they can keep dissolved.
From dissolved gas to injury
That excess nitrogen can collect into bubbles. Small bubbles may be filtered or cleared without causing noticeable harm, but bubbles that grow or form in the wrong places can obstruct blood flow and irritate tissue. The resulting injury is called decompression sickness, sometimes known as “the bends.” It can affect joints, skin, the nervous system, or breathing, depending on where bubbles and tissue damage occur.
The soda comparison captures the pressure change, but the body is more complicated than a bottle: nitrogen moves through different tissues over time, so a safe ascent is not simply a matter of reaching the surface slowly once. Divers use planned ascent rates and, when needed, pauses at depth to give nitrogen time to leave the body. Those pauses turn an invisible change in dissolved gas into time for the lungs to carry it away before it can cause trouble.
