When the dromedary camel gets dehydrated in its hot and arid environment, its nasal surfaces help the animal conserve water using two mechanisms: by cooling exhaled air during the night, and by extracting water vapor from exhaled air.
During the nighttime, outside temperatures are typically lower than the camel’s core body temperature. When the camel inhales, the cool outside air passes through the nasal passages where heat is exchanged: the nasal surfaces are cooled while the incoming air is warmed. Inside the camel’s lungs, air is at body temperature and fully saturated with water (100% relative humidity). When the camel exhales, the warm air inside the lungs passes over the cool nasal surfaces and exchanges heat again. This time, the air is cooled as it’s exhaled, and as it cools, water vapor in the outgoing air condenses onto the nasal surfaces as liquid water. The exhaled air is still at 100% relative humidity, but the lower temperature means that more water exists in liquid form than in vapor form (read an explanation of why this occurs here). Several mammals and birds use this mechanism of cooling exhaled air to conserve water and heat.
But the dromedary camel uses a second mechanism to save even more water: it extracts water vapor from the exhaled air, desaturating it down to 75-80% relative humidity. The dry nasal surfaces of a dehydrated camel are hygroscopic, meaning they can absorb and hold onto water molecules from the surrounding air. The hygroscopic nasal surfaces absorb water from the exhaled air and give off water to inhaled air.
One reason these water recovery mechanisms work so effectively in the dromedary camel is the large total surface area of the turbinate structures in its nasal passages. Turbinates are spongy nasal bones, and the camel’s turbinates are highly scrolled, providing narrow air passageways and a large surface area for water and heat exchange. Measurements suggest that camels have more than 1000 cm2 of nasal surface area, whereas the human nasal cavity may have a total surface area of only 160-180 cm2.
Why does the camel use the first mechanism and exhale cooled air only during the night? During the hot daytime, preventing the brain from overheating is prioritized over conserving water. Exhaling air that’s warm and saturated with water vapor enables the camel to dump excess heat from its body, but this comes at the expense of saving water.
“We have found that camels can reduce the water loss due to evaporation from the respiratory tract in two ways: (1) by decreasing the temperature of the exhaled air and (2) by removal of water vapour from this air, resulting in the exhalation of air at less than 100% relative humidity (r.h.). Camels were kept under desert conditions and deprived of drinking water. In the daytime the exhaled air was at or near body core temperature, while in the cooler night exhaled air was at or near ambient air temperature. In the daytime the exhaled air was fully saturated, but at night its humidity might fall to approximately 75% r.h. The combination of cooling and desaturation can provide a saving of water of 60% relative to exhalation of saturated air at body temperature. The mechanism responsible for cooling of the exhaled air is a simple heat exchange between the respiratory air and the surfaces of the nasal passageways. On inhalation these surfaces are cooled by the air passing over them, and on exhalation heat from the exhaled air is given off to these cooler surfaces. The mechanism responsible for desaturation of the air appears to depend on the hygroscopic properties of the nasal surfaces when the camel is dehydrated. The surfaces give off water vapour during inhalation and take up water from the respiratory air during exhalation. We have used a simple mechanical model to demonstrate the effectiveness of this mechanism.” (Schmidt-Nielsen and others 1981:305)