RESPIRATION. 361 



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 approximates 500 grams JJ 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 condensation of the vapor, which appears at once as a cloudy pre- 

 cipitate. 



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 tem- 

 perature to' 37 C.; at 6.3 to 29.8 C. The increase in the tem- 

 perature 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 over 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 state- 

 ment of the composition of the expired air, derived in part from the 

 upper air-passages, trachea, and bronchi, does not necessarily repre- 

 sent 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 

 to exchange of gases between lungs and air is maintained. Though 

 the respiratory movements doubtless create currents in the air-passages 

 which carry, on the one hand, a portion of the inspired air directly 



