Why is the amount of `CO_(2)` less in expired air than that in alveolar air ?

Explanation

The amount of `CO_(2)` in alveolar air is 5.5% whereas the amount of `CO_(2)` in inspired air is 0.04%.
Alveolar air is mixed with inspired air in the air passages to form expired air in which the amount of `CO_(2)` becomes i.e., less than 5.5% `CO_(2)` in alveolar air.