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I know that our bodies use most of the oxygen we breathe in during a dive but, wouldn't our tissues, under pressure, absorb more oxygen than we need? For instance, in normal circumstances, we don't use all the oxygen in each breath and we exhale some (this is what 'rebreathers' are for, right?)

Couldn't any gas we breathe in under pressure cause decompression sickness when we decompress?

Read this, from NASA:

"Astronauts put on their spacesuits several hours before a spacewalk. The suits are pressurized. This means that the suits are filled with oxygen.

Once in their suits, astronauts breathe pure oxygen for a few hours. Breathing only oxygen gets rid of all the nitrogen in an astronaut's body. If they didn't get rid of the nitrogen, the astronauts might get gas bubbles in their body when they walked in space. These gas bubbles can cause astronauts to feel pain in their shoulders, elbows, wrists and knees. This pain is called getting "the bends" because it affects the places where the body bends. Scuba divers can also get "the bends."

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Yes, it is possible to get decompression sickness with other gasses, such as the "oxygen bends" or oxygen decompression. For instance, in this study, they demonstrated that oxygen decompression sickness in mice. Another study demonstrates a more common non-nitrogen decompression sickness - helium bends.

Undoubtedly you will be wondering at this point why breathing pure oxygen would diminish (or, in certain circumstances, completely remove) the risk of bends as compared with nitrogen. The reason is simply that oxygen, unlike nitrogen gas, is constantly being metabolized by the body tissues. Consequently, even if the oxygen levels in your body fluids become elevated, they would have to be way higher than normal levels (like in the first study linked) before you would start to have it accumulating enough to form bubbles.

The body is designed for the shipping and handling of oxygen (think haemoglobin & myoglobin). Nitrogen, on the other hand, is relatively incidentally present - so long as atmospheric pressure remains relatively constant (which it did until we devised technology allowing us to radically change it), it's present in low levels in all the body tissues without causing any trouble.

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