RESPIRATION 17 



the lungs ; also that this could be best done on man. The composi- 

 tion of human alveolar air under different conditions had already 

 been calculated by Loewy 2 and Zuntz from the volume occupied 

 by a plaster cast of the respiratory passages in a dead body and 

 the average composition and volume of a breath of expired air. 

 The expired air is evidently a mixture of air from the alveoli with 

 the air which remains in the respiratory tubes at the end of inspira- 

 tion. This air is presumably but little altered by diffusion through 

 the walls of the respiratory tubes, and so far as respiratory ex- 

 change is concerned the volume of the lumen of these tubes must 

 constitute a "dead space" in breathing. The dead space is occupied 

 by alveolar air at the end of expiration, and by more or less pure 

 atmospheric air at the end of inspiration. 



If we know the volume of the dead space, and the volume and 

 composition of the air expired at each breath, we can calculate 

 the average composition of the alveolar air. It is, however, im- 

 possible to estimate directly the volume of the dead space in a 

 particular individual with any accuracy, or to be sure that it 

 remains the same under different physiological conditions. The 

 bronchi and bronchioles are provided with a muscular coat by 

 means of which their lumen is capable of contracting or dilating. 

 Apart from this the air in the alveoli which are nearest to the end 

 of a bronchus will contain purer air during inspiration than during 

 expiration, and this introduces a further complication. 



To get a reliable knowledge of the composition of alveolar air 

 it seemed desirable to make direct determinations. The method 

 introduced by Priestley and myself 3 is simply to make a sharp and 

 deep expiration through a piece of hose pipe about four feet long 

 and one inch in diameter, and provided with a plain glass mouth- 

 piece which is closed by the tongue at the end of the expiration 

 (Figure i). By means of a narrow bore glass tube filled with 

 mercury and introduced air-tight into the hose pipe near the 

 mouthpiece, a sample of the last part of the expired air is then 

 at once taken directly into the gas analysis apparatus as indicated 

 in Figure i, 4 or else into a vacuous sampling tube. 5 If the sample 

 is to be a normal one the breathing must be quite normal before 



2 Loewy, Pfluger's Archiv, LVIII, p. 416, 1894. 



8 Haldane and Priestley, Journ. of Pkysiol., XXXII, p. 225, 1905. 



* For physiological work methods of air analysis which are both accurate and 

 rapid are required. A description of the methods which I introduced with this in 

 view will be found in my book, Methods of Air Analysis, London, Charles Griffin 

 & Co., Third Edition, 1920. 



* If the sample is too large some pure air may be drawn in. 



