Why is the composition of inhaled air and alveolar air different?

The composition of inhaled air differs from alveolar air primarily due to gas exchange in the lungs. Inhaled air contains a higher concentration of oxygen and lower levels of carbon dioxide compared to alveolar air. Once air reaches the alveoli, oxygen diffuses into the bloodstream while carbon dioxide, a waste product of metabolism, diffuses from the blood into the alveoli to be exhaled. Additionally, alveolar air is also saturated with moisture, which further alters its composition compared to the drier inhaled air.