in theory, the body exposed to the pressure, given the components of the air adjusted for the pressure, would function normally. lemme start from square 1.
the air we breathe now is used at the same component ratio, but pressurized, for people under pressure. the problem is pressurizing the person. safely pressurizing a person in a chamber with the same air theyre breathing would take, in realistic applications, a looong time, as the body would slowly adapt to it. the thing here is the toxic levels of oxygen.
your body takes in a certain mass of oxygen in the air we breathe. if that air is pressurized, the body, taking in that same mass amountor air, is actually taking in more oxygen which, at certain pressures, is at toxic levels.
the key is to pressurize the oxygen with something safe, with a lower oxygen ratio, so the mass of pressurized oxygen inhaled is safe, with the same mass at oxygen at atmospheric pressure. nitrogen at high levels is narcotic, so helium is used. you step into a chamber, which is pressurized with helium to keep the same amount of oxygen available in every breath, so your body can safely adapt to the pressure.
imprroperly pressurizing someone can lead to oxygen toxicity. decompression work by reversing the pressurization.exposing someone to lower pressure makes the oxygenn in your blood expand, which the body will slowly adapt to. rapid depressurization will make your blood cells, and at certain levels the whole body, explode.
may have been a little off topic, but i hope it helps