i am not that good at explaining stuff but hope u do undrstand!! (and i hope im right)
arterial PaO2 remains normal as it is the oxygen that is dissolved in the blood...and also because they did not reach the muscle yet!!! PaO2 changes only with altitude and pressure changes nd certain respiratory diseases, left to right shunts... but not in anemia or exercise!!!! (in anemia total oxygen content decreases though cuz total o of rbc are less)
the exercising muscles are hungry for oxygen as they r burning themselves out and extract even more O2.....as a part of metabolic process they release CO2....and remb increased CO2 shifts the oxygen dissociation curve to the right!!!!