This affects mainly scuba divers, who are swimming at depths where the water pressure is much higher than near the surface, and they require compressed air to breathe. This presents some problems, because the nitrogen they breathe from this compressed air is not metabolized by the body. The longer the diver is diving, the more nitrogen is absorbed into the tissues of the body.
Then, as the diver returns to the surface, if they ascend too quickly, the highly pressurized nitrogen in their tissues expands, causing very painful symptoms in the tissues and muscles as blood flow is blocked. A diver can notice these symptoms immediately, or they may onset later, but if they are going to get the bends, they will know within 24 - 48 hours at the most. This is why divers have to swim slowly towards the surface, breathing the entire way, to expel some of the nitrogen gas from their system. This is known as decompression, and The Bends are sometimes referred to as Decompression Syndrome.
The bends are a problem faced by people who dive under water and have to stay under high pressure for prolonged durations of time. The bends or decompression sickness is caused when a diver breathes nitrogen or other gasses that are not metabolized by the body under high pressure. As the diver descends the nitrogen in the air contained in a diver’s air tank is dissolve into the tissues of the body. Unlike oxygen, which is used by the body, the nitrogen is not used by the body tissue. This unused nitrogen is released from the tissues if the pressure drops suddenly and bubbles are produced which leads to extreme pains, choking and other similar symptoms.
Therefore to avoid the bends, divers usually use pure oxygen while breathing under water, and the ascent is made slowly, coming up in stages.