THE MECHANISM OF RESPIRATION 281 



space " air, and the remainder from the aveoli true alveolar air^ 

 The dead-space air does not undergo a respiratory exchange. 

 In the resting man, 350 c.c. of the total tidal air mixes with 3,000 to 

 4,000 c.c. in the lungs (supplemental and residual air), so that only 

 about one-tenth of the air is changed at each breath; much less in 

 shallow breathing. In deep breathing, the complemental air is added 

 to the tidal, and 1,660 to 2,360 c.c. mixes with the air in the lung 

 about one-half is changed. 



A sample of alveolar air may be obtained by expiring deeply 

 through a piece of rubber tubing of about 1 inch bore and 4 feet length 

 (Fig. 150). The " dead-space " air is blown out of the tube, and the 

 tube is filled with alveolar air. The end of the tube is closed with the 

 tongue or with forceps, and through the T-piece a sample is drawn 

 off into a suitable sampling tube, which has been previously evacuated 



Mouthpiece 



Sampling Tube 



FIG. ' 50. APPARATUS FOR COLLECTION OF SAMPLE OF ALVEOLAR AIB. (Haldane 



and Priestley.) 



by means of a reservoir fitted with a rubber tube. The sample may be 

 taken at the end of a normal expiration following an inspiration, or 

 preferably two samples may be secured, the second immediately after 

 expiration following normal inspiration. The mean of the analyses 

 gives the normal alveolar air. 



Alveolar air contains 4-5 to 6 per cent. C0 2 , 13-5 to 15 per cent. O 2 , 

 79 to 80 per cent. N 2 . The C0 2 percentage in men (5 to 6 per cent.) 

 is generally a little higher than in women and children (4 to 5 per cent.). 



The composition of ordinary inspired air at normal temperature 

 and pressure C< and 760 millimetres may be given as follows: 



Oxygen . . . . . . . . . . 20-92 volumes per cent. 



Carbon dioxide 0-03-0-04 



Nitrogen * . . '. 79-03 



Water vapour . . . . . . . . . . . . Variable 



Temperature . . . . . . . . . . . . Variable 



Expired air: 



Oxygen . . . . . . . . . . . . 16-00 per cent. 



Carbon dioxide . . . . . . . . . . 4-00 



Nitrogen* 80-00 



Water vapour Saturated 



Temperature . . . . . . . . . . . . 37 C. 



* Including 0-94 per c >nt. 



