Water loss during air breathing

You have learnt that water is lost through the respiratory surface. In the terrestrial vertebrates the evaporative loss is reduced, because the respiratory surface in them (the lungs) is internal to the body cavity. Even within the lungs ventilation of the respiratory epithelium by unsaturated air will cause evaporation of the moisture wetting the epithelial surface. In birds and mammals such evaporative loss is enhanced because of the difference between the body temperature and ambient temperature.

Figure:  Temporal countercurrent system

Warmer air can hold more moisture when saturated than cool air. Since the expired air is warmer than the inspired air, water is lost during expiration. In a number of vertebrates the respiratory loss of water is minimised through a mechanism known as temporal countercurrent system. During inspiration cool air entering the lungs via nasal passage gets warmed by the heat of the nasal passage and absorbs moisture from the respiratory epithelium of the lungs. During expiration, the same air loses most of the heat it gained earlier as it warms the cool nasal passage on its way out. As the expired air gives up some of its heat to the tissue of the nasal passage, most-of the moisture acquired from the respiratory epithelium condenses on the cool nasal epithelium. With the next inhalation, this condensed moisture again contributes to the humidification of the inspired air, and the cycle is repeated.