CH. XXVI.] COLLECTION OF ALVEOLAR AIR 373 



result of the two analyses represents the mean composition of the 

 alveolar air. Since the gaseous interchange between the blood and 

 the alveolar air is going on continuously, it is evident that at the 

 end of inspiration there will be a maximum percentage of oxygen, 

 and a minimum percentage of carbonic acid ; the converse obtains 

 at the end of expiration. These observers proved by other consitfera- 

 tions which it is unnecessary to go into, that the air obtained was 

 really the alveolar or residual air unmixed with any of the air of the 

 " dead space " of the respiratory passages. 



From the analysis of this air, they arrived at the conclusion that 

 the normal oxygen pressure in it is 13 per cent, of an atmosphere 

 (that is approximately equal to 100 mm. of mercury). 



The other important measurement, namely that of the tension of 

 oxygen in the blood, is made by Krogh's tonometer (p. 363), and the 

 experiments show that diffusion is quite sufficient to account 



MOUTHPIECE 



SAMPLING TUBE 



FIG. 298. Apparatus for obtaining alveolar air. 



for the passage of oxygen from the alveoli to the blood. The 

 following experiment may be cited as an example, and will be best 

 understood by the next diagram (fig. 299). 



It shows the relations between the pressures of oxygen and 

 carbonic acid in the air of the lungs and the blood respectively. 

 The pressures of gas are measured vertically, and expressed as 

 percentages of an atmosphere; the horizontal measurements are 

 those of time. It will be seen that the experiment consisted of 

 three periods, separated by the vertical lines A and B. During the 

 first and last periods the animal breathed a mixture consisting of 

 14-7 per cent, oxygen, and the remainder nitrogen; in the middle 

 period ordinary air containing nearly 21 per cent, of oxygen was 

 breathed. It will be seen that the oxygen pressure of the alveolar 

 air (dotted line) is always higher than the oxygen tension of the 

 arterial blood (continuous black line) ; as the former rises the latter 

 rises, and vice versa. The lower part of the diagram shows the 



