The air is only one-fifth oxygen. When you take a breath, your lungs inhale oxygen from the air, and you exhale the rest, which is mostly nitrogen.
But when a diver is in the water and breathing through an aqualung, there’s no way he can get rid of all this nitrogen, and some of it enters his blood. Then, if he rises suddenly from deep water, the change in pressure can produce bubbles of nitrogen gas in his body, and his lungs won’t have time to get rid of them.
The bubbles can pinch a nerve or blood vessel, causing great pain in the chest and the joints, and possible serious injury. This injury is known as decompression sickness, but most people call it the “bends.”
A diver can avoid the bends by rising from deep water slowly and stopping frequently on his way up. This allows his body more time to get rid of the nitrogen.