Final answer:
Water vapor is less dense than dry air; thus, moist air is also lighter. This difference in density is relevant in both atmospheric and alveolar air, where the humidification from our respiratory system and temperature affect relative humidity and the exchange of gases.
Step-by-step explanation:
The question posed brings to light the fact that water vapor is less dense than dry air, which in turn means that humid air, containing water vapor, is also less dense. This is because the molecular weight of water vapor (H2O) is less than the average molecular weight of air.
Considering that the relative concentration of gases in the atmosphere and alveoli is nitrogen > oxygen > water vapor > carbon dioxide, and that air in the alveoli contains more water vapor and carbon dioxide yet less oxygen than atmospheric air, we can infer that the humidification process in our respiratory system contributes to these differences.
Additionally, hot air is less dense than cold air, since the molecules in hot air move more rapidly. This ties into the concept of relative humidity, which describes the amount of water vapor present in the air compared to what could be present at saturation. The maximum water vapor the air can 'hold' increases with temperature; thus, hotter air can contain more water vapor, leading to lower air density.
It is important to note that the capacity of air to 'hold' water vapor is governed by the vapor pressure of the water and not the properties of the air itself. When the relative humidity reaches 100%, the air has reached its water vapor saturation point, and this can lead to condensation and fog formation. Furthermore, the respiratory system enhances the water vapor content in the alveoli, which impacts the diffusion process of gases during breathing, affecting the exchange of oxygen and carbon dioxide in the blood.