Oxygen is required to sustain life and the body requires a balance of the gas to operate within. With oxygen exposure, too much, too little, or more than average over a long duration can cause serious problems.
In the “Physics” unit, you learned about partial pressure: Air is made up of 21% oxygen and 79% nitrogen at sea level. So, the partial pressure of oxygen at sea level is 0.21. As we descend underwater, the pressure increases, and so do the partial pressures.
The respiratory and circulatory systems will operate within an oxygen partial-pressure limit of 1.6 bar/ata (maximum) and 0.16 bar/ata (minimum). Going above and or below these parameters is dangerous.