Counter-current heat exchange in the respiratory passages: Effect on water and heat balance

Abstract

Temperature measurements were made on exhaled air of various vertebrates at ambient air temperatures from 12 to 30 °C. In birds the temperature of the exhaled air was closer to the ambient air temperature than to body temperature and in small rodents the exhaled air temperature might be even lower than the temperature of the inhaled air. As air is inhaled, it is warmed and humidified in the upper respiratory tract, and the walls of the passageways are thus cooled. Exhaled air passing over these cool surfaces gives up some of its heat and water. The recondensation of water is quantitatively important, e.g.. at 15 °C, 25% r.h., a cactus wren recovered 74% of the water added to the respiratory air on inhalation. In the kangaroo rat, due to the greater degree of cooling of the exhaled air, 83 % of the water is recovered. Of the heat added on inhalation (warming of air plus heat of vaporization) a major fraction is recovered on exhalation. For example, at 15 °C, 25 % r.h., the cactus wren recovers 75% of the heat added on inhalation, and the kangaroo rat recovers 88 %. In terms of the simultaneous metabolic heat production, these amounts constitute a savings of 16.1 % of the total metabolic heat production, heat that would be lost to the exterior if air were exhaled at body temperature and saturated.