End tidal water vapor tension of the expirate after nasal and then after oral inspiration was measured at the lips of seven normal subjects using a mass spectrometer. At a mean inspired air temperature of 23°C and Ph.o of 14.7 mmHg, the mean end tidal Ph.o was was 40.57 mmHg after nasal and 37.15 mmHg after oral inspiration. The difference was statistically significant. The evidence for complete saturation of expired gas is discussed. Assuming a 100% saturation of expired gas the average contribution of the nasal mucosa to the water content of the respired air in this experiment was 3 gm/m3 or approximately 10% of the total water contributed by the respiraory tract.
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