During maximal exercise the PaO2 at the level of the mitochondria (tissue):

a. Drops to 0 mmHg because the red blood cell unloads all of the O₂ that it carries in order to meet the metabolic demand of the active skeletal muscle.
b. Is reduced but is higher than the PO₂ in the mitochondria because the red blood cell unloads most, but not all, of the O₂ it carries in order to maintain the driving pressure for diffusion.
c. Always remains ~100 mmHg because the blood is moving so fast through the body that there is always a constant PaO₂.
d. None of the answers are correct.