THE CHEMISTRY OF RESPIRATION 487 



fluids to exchange their gaseous constituents with the surrounding 

 air. Respiration, therefore, consists of two processes, namely, an 

 interchange between the outside air and the blood and an interchange 

 between the latter and the cellular components of the tissues. The 

 former process is known as external respiration and the latter, as in- 

 ternal respiration. 



The fact that the general metabolism of an animal necessitates 

 an intake of oxygen and an outgo of carbon dioxid may readily be 

 gathered from a comparison of the chemical and physical character- 

 istics of inspired and expired air. Concerning the former, it should 

 chiefly be remembered that the inspired air contains more oxygen 

 and less carbon dioxid than the expired. The figures in volume per 

 cent, generally given arathe following: 



N O CO2 



Inspired air 79.00 20.96 0.04 



Expired air 79.50 16.02 4. 10 



4.94 4.06 



Argon, krypton and neon are not included in this table, because 

 they have not been shown to possess a definite function.^ Besides, 

 it should be remembered that these figures are subject to slight 

 variations, because inasmuch as the composition of the inspiratory 

 air differs somewhat in different localities, the expiratory air must 

 present very similar fluctuations. In addition, the latter exhibits 

 certain minor changes which are caused by periodic variations in the 

 depth of the respiratory movements and intensity of the tissue metab- 

 olism. In general, however, it may be said that the air loses during 

 its sojourn in the lungs 4.94 volumes of oxygen and gains 4.34 volumes 

 of carbon dioxid. Its content in nitrogen remains practically the same. 



These analyses also show that the volume of oxygen retained is 

 larger than the volume of carbon dioxid given off, which fact seems 

 to indicate that a fractional amount of the former gas is excreted as 

 water. In the second place, the constancy of the nitrogen proves that 

 it possesses no respiratory value other than that it serves as the medium 

 in which the diffusion of the other two gases is enacted. It is to be 

 noted, however, that the expired air generally contains a slight quantity 

 of cellular material which on analysis tends to heighten the percentage 

 amount of nitrogen. In round figures this increase is usually esti- 

 mated at 0.4 per cent. The expired air may also contain traces of 

 hydrogen and methane which in all probability find their origin in 

 fermentations in the intestines. 



Regarding the physical characteristics of the respired air, it is noted 

 that the expired air is warmer than the inspired ; but naturally, its actual 

 temperature varies considerably, because the temperature of the in- 

 spired air fluctuates with the time of the year and the conditions under 

 which the animal is living. Besides, much depends upon the rapidity 



1 Regnard and Schloessing, Compt. rend., cxxiv, 1897, 302. 



