Water content of expired air in man

Abstract

Five determinants of the water content of air expired by man were examined and attempts made to quantify each determinant. Each of the four determinants: ambient temperature, ambient humidity, tidal volume and respiratory frequency was examined at both sea level and altitude. The data collected from 14 men, at 7 ambient temperatures (-35° to +35° C), and 5 minute ventilation rates (which formed a continuum from 4 to 80 L STPD/min.), were corrected and statistically analyzed by computer. The results, presented in graphic, tabular and algebraic form, permit calculation of expired water or net water loss through the respiratory tract in man with corrections for minute volume, ambient temperature, gas density and ambient humidity. An attempt is made to explain the results presented using the notations of thermodynamics. It is assumed that the human respiratory tract is an efficient but limited counter-current heat and mass (water) exchange system. As such, the caloric requirements for equilibrating the respired air to 37°C and approximately 100% relative humidity are contingent upon the five determinants listed. The impact of a change in gas density is completely described in theory, listing changes in molar volume, heat capacity and gas flow characteristics, and how these changes might affect the heat and mass transfer between the respiratory mucosa and the respired air. Several semi-quantitative tables are given which permit a rough estimation of hourly or daily water loss from the respiratory tract for man under various conditions of ambient temperature and oxygen consumption.