You are correct the body is absorbing higher partial pressures of O2, much of this excess goes into solution in the plasma. The hemoglobin will rapidly become saturated with the maximum amount of O2 they are capable of transporting and much of the excess goes into solution in the fluid portion of the blood or plasma.
This transfer onto the hemoglobin and into solution in the blood stream can be refered to as external respiration.
http://www.breathing.com/articles/hyperbaric-oxygen.htm
Internal respiration occurs at a cellular level and is the transfer process of the individual cells absorbing the O2 needed for metabolism to occur, this is internal respiration.
The body is going to continue to consume oxygen through internal respiration as needed for cellular matabolism to occur. This requires more than just oxygen, in fact the oxygen is only required to allow the cell to "burn" the sugar (glycerin) or fuel as energy is required.
The ratio of fuel to O2 remains constant while the amounts needed are a function of the bodies demand for energy.
http://www.rwc.uc.edu/Koehler/biophys/8e.html
The excess O2 in the system is being utilized to a small extent, area that become vasoconstricted or have a decrease in circulation during the dive may continue receiving adequate amounts of O2 even though the volume of blod is decreased. This is the effect function of hyperbaric O2. Even tissue with a weak blood supply get enough O2 because there is more in solution to be provided.
The remaining amounts of O2 remain in solution in the body until the ambient pressure is reduced...
This can also cause some problems as the divers body is slowing moving toward equilibrium with the partial pressure of O2 being inspired. If that partial pressure becomes to high then symptoms will occur.
http://www.sportdiverhq.com/noaa.htm
Not sure how the topic CO2 came up with this question but since it has it is worth pointing out there are some theories about this excess O2 (NITROX especially) effecting the efficiency of removal of CO2 from the body, this function is carried out by the plasma which absorbs the higher partial pressures produced through internal respiration and carries it to the lungs were the partial pressure of CO2 in the inspired air is lower and external respiration takes place.
I appreciate that logically then you should use a smaller percentage of the oxygen available, but that isn't true either
You are correct the excess O2 is going into solution in the fluid of the body, not being consumed through cellular metabolism...
So the compulsion to breathe comes from CO2 molecules' presence by the alveoli...does that mean that if we could "agitate" the gas in our lungs, we could extract more O2 from a lungful of air, and thereby decrease the pressure to breathe?
Actually the urge to breath is from elevated CO2 levels in the blood, the signal is transfered by the chemoreceptors telling the body to breathe. The elevated levels in the lungs cause the transfer or CO2 from the plasma to the air inside the lung to become less effective as the PP inside the lungs moves closer to equilibrium with the amount in the plasma.
Hope this helps somewhat...
Jeff Lane