402 TEXT-BOOK OF PHYSIOLOGY. 



cent. This gain is probably of accidental occurrence, due to absorption 

 from the large intestine, in which decomposition of nitrogen-holding com- 

 pounds is taking place. It is generally believed that free nitrogen plays no 

 part in any phenomenon of combination or decomposition within the body. 



The gain in watery vapor will depend on the amount previously present 

 in the air. This is conditioned by the temperature. With a rise in tempera- 

 ture the percentage of water increases; with a fall, it decreases. By breath- 

 ing into a vessel containing pumice stone saturated with sulphuric acid, the 

 vapor may be collected. The difference observed between the weight before 

 and after breathing is an indication of the amount by weight of water exhaled 

 during the time of breathing. It has been calculated that the amount of 

 water exhaled daily varies between 300 and 500 grams. Though invisible 

 at ordinary temperatures, it becomes visible at low temperature as soon as 

 it emerges from the respiratory tract. The loss of heat is followed by a con- 

 densation of the vapor, which appears at once as a cloudy precipitate. 



The gain in organic matter is also variable. The amount present is not 

 sufficient to permit of a thorough chemic analysis, but there are reasons for 

 believing that it belongs to the proteid group of bodies. If it accumulates 

 in the air, especially at high temperatures, it readily undergoes decomposition, 

 with the production of offensive odors. Traces of free ammonia have also 

 been found in the expired air. In addition to these chemic changes, the 

 air experiences physical changes; e.g., a rise in temperature and an increase 

 in volume. The rise in temperature can be shown by breathing through a 

 suitable mouthpiece into a glass tube containing a thermometer. By this 

 means it has been shown that inspired air at 20 C. rises in temperature to 

 37 C.; at 6.3 to 29.8 C. The increase in the temperature will depend upon 

 that of the air inspired and the time it remains in the lungs. If retained a 

 sufficient length of time it will always become that of the body. As a result 

 of the heat absorption the expired air increases in volume about one-ninth 

 of that of the inspired air. When corrected for temperature and pressure 

 and freed from aqueous vapor, the volume of the expired air is less than 

 that of the inspired air by about one two-hundred and fiftieth. 



The Composition of the Alveolar Air. The foregoing statement of the 

 composition of the expired air, derived in part from the upper air-passages, 

 trachea, and bronchi, does not necessarily represent the composition of the 

 alveolar air. It is very probable that the percentage of carbon dioxid is 

 greater, the percentage of oxygen less, in the latter than in the former. This 

 is made evident by collecting in several portions the expired air as it escapes 

 from the respiratory tract and subjecting it to analysis. The last portion 

 always contains a larger amount of carbon dioxid and a smaller amount of 

 oxygen than the first portion. The determination of the composition of the 

 alveolar air is extremely difficult. It has been estimated to contain from 

 5 to 6 per cent, of carbon dioxid and from 14 to 18 per cent, of oxygen. 



Pulmonary Ventilation. It is owing largely to this inequality of 

 volumes and consequently of the "partial pressures" of these two gases in 

 the trachea and alveoli that the degree of ventilation necessary for the ex- 

 change of gases between lungs and air is maintained. Though the respira- 

 tory movements doubtless create currents in the air-passages which carry, 

 on the one hand, a portion of the inspired air directly into the alveoli, and, 



