How does my body absorb oxygen?

At sea level or 780 mmHg of atmospheric pressure the air contains 20.8 % of oxygen which translates to 159 mmHg of oxygen pressure available for breathing. During breathing the air entering thru mouth and nose  gets humidified and warmed up to  enter lungs and arterial circulation. All along that path the oxygen pressure is dropping and by the time it reaches the organs, the content of available oxygen for metabolism is almost halved and is in the range of 67 mmHg ±12 mmHg. The units are a measure of partial pressure of oxygen (pO₂) in the tissue.

Situation gets better when breathing 100 % oxygen.  When breathing pure oxygen thru a mask for instance, the level of tissue oxygen will double or even triple and temporarily rise up to 200 mmHg. This is due to extra oxygen bound to hemoglobin in the red blood cells and dissolved in blood plasma. In emergency conditions this extra oxygen is crucial to sustain metabolism or vital organs  such as heart and brain. If we would however breathe pure oxygen under even higher pressure, such as inside the hyperbaric chamber, the amount of oxygen in blood plasma increases 20 times or more. This amount of plasma dissolved oxygen can sustain life even in most complex injuries when circulation is very damaged and can not deliver oxygen.  During usual hyperbaric oxygen treatment at pressures of 2.0 to 2.4 ATA (atmospheres absolute) the tissue available oxygen rises  and up to 2200 mmHg. This high oxygen pressure can penetrate to every cell that is in oxygen crisis to start repair and regeneration. For comparison purposes the inflatable bags, such as “mild hyperbaric” HBO can reach tissue oxygen pressure close to the one when breathing pure oxygen via mask at normobaric conditions.

Picture below describes a situation where the oxygen is breathed at surface (normobaric air) with a mask delivering 100% oxygen and during the hyperbaric oxygen session at 2.0 ATA (1560 mmHg of pO₂).