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

Inspired air refers to the air that you inhale, while expired air is the air that you breathe out.
Typical percentage composition of inspired (inhaled) air includes: 78% nitrogen, 21% oxygen, 0.04% carbon dioxide and small amounts of other gases.
The percentage composition of expired (exhaled) air is different: about 78% nitrogen, 16% oxygen, 4% carbon dioxide and a higher amount of water vapour.
The oxygen concentration is lower, and the carbon dioxide concentration is higher in the expired air because, during respiration, oxygen is used to convert glucose into energy, while carbon dioxide is produced as a waste product.
Even though respiration continuously uses up oxygen and produces carbon dioxide, the air remains fairly constant as fresh air is continuously inhaled and waste-laden air is exhaled.
The role of the respiratory system is effectively to facilitate the exchange of oxygen and carbon dioxide, enabling cells to receive the oxygen they need for respiration while getting rid of the carbon dioxide that builds up in the process.
Despite the conversion of oxygen into carbon dioxide, nitrogen levels remain constant in both inspired and expired air due its role as a relatively inactive gas in the air; it is neither significantly added nor taken away in the process of respiration.
The level of water vapour in expired air is usually higher than in inspired air due to the humidification of air in the respiratory tract and the production of water during the metabolic process of cellular respiration.

Learn to explain these differences by considering the processes of diffusion, ventilation, and cell respiration. Always connect your understanding of the composition of air to the physiology and function of the human respiratory system.