RESPIRATION 415 



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

 believing that it belongs to the protein 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 2oC. rises in temperature to 

 37C.; at 6.3 to 29.8C. 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 oxygen is less, the 

 percentage of carbon dioxid greater in the alveolar air than in the trachea 

 and bronchial tubes for the following reasons. If 500 c.c. of air are in- 

 troduced into the respiratory apparatus at each inspiration a portion of 

 it will be retained in the trachea and bronchial tubes to replace that which 

 was discharged at the previous expiration, while another portion will pass 

 downward to replace the air expelled from the alveoli. The first portion is 

 estimated at 140 c.c., this being the estimated capacity of the bronchial 

 tubes, the second portion at 360 c.c. If 360 c.c. of air are expelled from the 

 alveoli at one expiration, they will on passing upward be diluted by the 

 140 c.c. in the bronchial tubes and trachea and therefore the expired air 

 will not represent the true composition of the alveolar air. Inasmuch as 

 the air in the trachea and bronchial tubes undergoes no particular change 

 during the passage of the gases into and out of the blood the space occupied 

 by it has been termed the "dead space." By various forms of apparatus 

 it has been made possible to obtain air from the deeper portions of the 

 lungs, presumably alveolar air, and its composition determined. As shown 

 from the results of analyses the alveolar air contains oxygen 13 to 15 per 

 cent., carbon dioxid 5 to 6 per cent., and nitrogen 79 per cent. 



The determination of the percentage composition and hence the 

 pressure of the oxygen and carbon dioxid in the alveolar air is of im- 

 portance when taken in connection with the pressure of the same gases in 

 the blood, for the following reasons, viz.: It enables one to explain, in 

 accordance with the laws governing the diffusion of gases across a mem- 

 brane, the passage of oxygen across the alveolar-capillary membrane into 

 the blood and the passage of the carbon dioxid from the blood into the 

 alveoli; it affords an explanation for the observed pressure of the carbon 

 dioxid in the arterial blood and its effect on the activity of the inspiratory 

 center. In the pathologic condition known as acidosis the determination of 

 the percentage of CO 2 is of value as a diagnostic factor, as its diminution is 

 taken as an index to which the alkaline reserve of the blood is diminished 



