Final answer:
Upon entry into the respiratory system, air is saturated with water vapor until the humidity reaches 100%, at which point air cannot hold additional water vapor, facilitating the gas exchange in the lungs.
Step-by-step explanation:
As the air is saturated with water vapor (humidified) upon entry into the conducting pathway, the relative humidity increases. When the air reaches 100% humidity, the partial pressure of the water vapor is equal to the vapor pressure, and the air cannot hold any additional water vapor. This state is important in the human respiratory system as the air that enters through the nasal cavity is warmed up to body temperature and humidified, which protects the system and facilitates gas exchange in the lungs. During this process, the composition of air in the alveoli becomes different from atmospheric air, containing more water vapor and carbon dioxide, but less oxygen, due to the gas exchange that occurs.