Decompression sickness is a condition that occurs when nitrogen that has been dissolved in the tissues and blood by high pressure forms bubbles as the pressure drops.
The primary components of air are oxygen and nitrogen. Since high-pressure air is compressed, each breath obtained at depth encompasses many more molecules than a surface breath. Since the body uses oxygen continuously, extra oxygen molecules inhaled under high pressure usually do not pile up. The extra nitrogen molecules, though, do pile up in the tissues and blood.
When the outside pressure decreases during the upswing from a dive or when passing through a compressed air environment, the accumulated nitrogen that can be released into the air forms bubbles in the blood and tissues. These bubbles can widen and induce tissue damage, or they can clog blood vessels in a wide range of organs, either straight or by provoking small blood clots. Pain and other symptoms are caused by a blood vessel blockage.
