Percentage Composition of Inspired and Expired Air and the Reasons for Differences

The air we breathe in, or ‘inspire’, is made up of approximately 78% nitrogen, 21% oxygen, 0.04% carbon dioxide, and a small amount of other trace gases such as argon and helium.
Once this air has been used by the body and is exhaled, or ‘expired’, its composition changes significantly. The percentage of nitrogen remains relatively constant, but the levels of other gases alter.
Expired air typically contains around 78% nitrogen, 16% oxygen, 4% carbon dioxide, and a variety of other trace gases.
The biggest changes between inspired and expired air can be seen in the levels of oxygen and carbon dioxide.
Oxygen levels drop from 21% to about 16% in expired air. This is because the body uses oxygen in the process of respiration, where it is used to release energy from glucose in cells.
Conversely, carbon dioxide levels rise from 0.04% to about 4%. During respiration, carbon dioxide is produced as a waste product and needs to be removed from the body.
The levels of nitrogen remain relatively constant because it is not used in the process of respiration.
The small amount of water vapour in expired air is due to the humid conditions in our lungs and respiratory tract.
Traces of other gases, such as argon and helium, also remain relatively stable as they are not involved in human metabolic processes.
Understanding these compositional changes is key to understanding how the body uses and exchanges gases during respiration.