Oxygen in the Atmosphere:

Partial Pressure

Even though the percentage of oxygen doesn’t change with elevation, the pressure of oxygen does.

Air is a mixture of molecules from different gases. The pressure of all these molecules together is the total air pressure.

The pressure of the molecules of a specific gas is its partial pressure. So, the partial pressure for oxygen is the pressure exerted by only the oxygen molecules in the air.

To calculate the partial pressure of a specific gas, multiply the total air pressure by the proportion of the air made up of that gas. For example, air is 21% oxygen. So, the partial pressure of oxygen (written as PO2) is 21% of the total air pressure.

At sea level, where the total air pressure is 760 mmHg, the partial pressure of oxygen is 160 mmHg.

Definition placeholder.
Gases in pressure containers with gauges represent the makeup of total air pressure.
Figure 4. The total pressure is the sum of the partial pressures of each of the gases in a mixture. In this example, the total air pressure is the sum of the partial pressures for nitrogen (N2), oxygen (O2), and other gases.