Normal humidity in the airways
With nose breathing at rest, inspired gases become heated to 36°C and are about 80% to 90% saturated with water vapour by the time they reach the carina, largely due to heat transfer in the nose. Mouth breathing reduces this to 60-70% relative humidity. Heat and moisture content falls from carina to nares, so that the nose is typically at 30°C. A countercurrent mechanism of heat and moisture exhange in the aiways maximises efficiency, with nasal cooling on inspiration and warming on exhalation. Tracheal temperature and humidity fall with increased ventilation, particularly when the inspired gases are cold and dry.
Heat and water loss
If totally dry gases were inspired and fully saturated gases exhaled, the total water loss from ventilation at rest would be about 300 ml/day in the average adult. Normally, about half is retained due to the efficiency of the nose and the humidity of inspired room air. Bypassing the nose with an endotracheal tube and not humidifying gases causes maximal losses.
Non-respiratory water losses are typically 300-600 ml/day but are increased if warm, moist surfaces are exposed (i.e. burns, open abdomen), particularly if the operating theatre is cold and has high-flow air conditioning.
Heat losses are the result of four primary processes: