THE RESPIRATORY SYSTEM. 251 



currents of air in the room. They may be avoided by constant renewal 

 of the air in rooms by adequate ventilation. 



The expired air comes partly from the lungs, partly also from the 

 respiratory passages, namely the bronchi, trachea, pharynx, and nose. 

 Since the air in these passages undergoes very little change in composi- 

 tion during respiration, they are known as the "dead space," the 

 capacity of which varies from 130 to 150 c.c. and is very constant for the 

 same individual. The air contained in the dead space is expelled during 

 the first part of expiration, the air expelled during the latter part of 

 expiration, particularly if this is forcible, coming directly from the 

 alveoli of the lungs. Since the interchange of oxygen and carbonic 

 acid between the blood and the air in the lungs takes place solely in 

 the alveoli, it is of great importance to ascertain the composition of the 

 alveolar air. 



Haldane has devised a simple apparatus for collecting samples of 

 alveolar air in man. It consists of a piece of rubber tubing about 



1 inch in diameter, 3 or 4 

 feet long, and provided - 

 with a mouthpiece. About 



2 inches from the mouth- 



r(/S 



piece the tube is connected 



with a sampling tube 



(fig. 95), which has pre- FIG. 95. 



viously been made vacuous. 



The subject breathes normally for a few moments, and then, at the end 



of a normal inspiration, he expires deeply through the mouthpiece and 



instantly closes it with his tongue. The upper tap of the receiver is 



at once opened, and air rushes into it from the tube ; the tap is then 



closed, and the sample can be analysed. A second similar experiment 



is made, in which the subject expires deeply at the end of a normal 



expiration, and another sample of air is obtained. The mean of the 



analyses of the two samples gives the average composition of the 



alveolar air. 



The reason for taking two samples is that at the end of inspiration 

 the lungs contain a maximum percentage of oxygen, whereas at the 

 end of expiration they contain a maximum percentage of carbonic acid. 

 The amount of carbonic acid in alveolar air obtained by this method 

 varies from 5 to 6 per cent, in different individuals, but is remarkably 

 constant in the same individual ; the amount of oxygen is usually 

 1-3 to 14 per cent. Alveolar air thus contains considerably less oxygen 

 and more carbonic acid than ordinary expired air, the reason being that, 

 in the expired air, the alveolar air is mixed with tlie contents of the 



