CTQ #1. Harry does not know, or has forgotten blood that it is not customary for serum oxygen content to decrease by more than 40%. Under normal circumstances, if Po2 falls , there is a resultant decrease in blood oxygen content below 60% , Pco2 will rise. Under normal circumstances, the increase in blood CO2 will trigger breathing -- automatically and unavoidably. This is a protective life-saving mechanism.
Work Step by Step
The problem with Harry is that he could, in his attempts to load up with reserve oxygen, also lower his Pco2 to the extent of creating an abnormal level of hypocapnia. If the Pco2 gets low enough ( below 32 mmHg), he may experience a short period of apnea while CO2 accumulates to a high enough level to trigger breathing again. During the period of apnea, Po2 will continue to fall. If the swimmers Po2 gets close to 50 mmHg he may lose consciousness (black out) and drown.