Oxygen is required to sustain life and the body requires quite a specific amount of oxygen to function properly. Too much, too little, or more than average amounts of oxygen over a long duration can each cause serious health problems.
In the “Physics” unit, you learned about partial pressure. For the sake of scuba-diving calculations, air is considered to be made up of 21% oxygen and 79% nitrogen at sea level. So, the partial pressure of oxygen at sea level is 0.21. As you descend underwater, the pressure increases, and so do the partial pressures of each gas.
In basic terms. your 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 or below these parameters carries very real risks.