This article was researched and written by a student at Texas Tech University participating in the Encyclopedia of Earth's (EoE) Student Science Communication Project. The project encourages students in undergraduate and graduate programs to write about timely scientific issues under close faculty guidance. All articles have been reviewed by internal EoE editors, and by independent experts on each topic.
The nasal passage serves as a major gateway to the respiratory system of many animals. The function of the nasal passage is to allow oxygen-rich air from the atmosphere into the animal and then return oxygen-depleted air from the lungs to the atmosphere. However, oxygen is not the only substance carried through the nasal passages during respiration because air can contain water vapor and hold heat.
In general, the lungs of an animal have higher humidity and are warmer than the surrounding air. Thus, air entering the nasal passages is drier and cooler than air leaving the body. Because exhaled air is saturated with water animals can potentially lose water to the surroundings when they exhale. Many species of desert animals have evolved a counter current heat exchange system in their nasal passages in an attempt to reduce water loss.
How does this work?
Counter current exchange requires that fluids flow in close proximity in opposite directions. In the nose, a single passageway facilitates inhalation of dry, cool air and exhalation of warm, moist air. The two fluids (incoming and outgoing air) are not separated spatially, as the counter current exchange which occurs in the gills of fish. Instead, they are separated temporally.
In general, the ambient air temperature is cooler than the temperature in the lungs of endothermic animals. Also, the desert air is much dryer than air in the lungs. Thus, water from the animal saturates the air as it is inhaled. If this moist air is exhaled to the atmosphere, moisture loss occurs. Warmer air can hold more water than cooler air. The nose is designed so as to maximize water retention, and it does so in several ways.
First, there is a temperature change in the nasal passage. The tip of the nose is the coolest, several degrees below the ambient air temperature. Moving up the nasal passage, the temperature gradually increases until it reaches core body temperature, around 38° C. Thus, during inhalation, the walls of the nose are cooled by the ambient air passing by. During exhalation, as warm air from the lungs passes over the previously cooled surfaces in the nose, the air itself is cooled as the temperature decreases to near that of the ambient air. An important structural feature which allows these exchanges to occur is the very narrow nasal passageways which feature large wall surface area. The nasal passages are so small that air is forced to flow closely over their surfaces, facilitating the maximum exchange of heat between the air and nasal lining. If the passages were wide, heat exchange would be insignificant.
In addition to the pure heat exchange, water is also exchanged in the nose. Water condenses and evaporates in the nasal passage due to the change in temperature. On inhalation, as the dry, cooler air is warmed, water evaporates into the moist nasal mucosa. The now warmer air can hold more water as it enters into the lungs. During exhalation, warm air from the lungs is cooled. This causes the water to condense in the nasal passageway, making the air less saturated and reducing water loss from the respiratory tract.