OXYGEN. 429 



because the composition of the expired air varies according to the depth 

 of the inspiration. In this way average values of 14.6 per cent oxygen 

 and 5.6 carbon dioxide, by volume, have been found for the alveolar air 

 when it leaves the alveoli. During inspiration, its composition tends to 

 approach that of the inspired air. Some idea as to the extent of the 

 changes in the composition of the air in the alveoli, during inspiration, may 

 be obtained by comparing the amount of inspired air with that remaining 

 in the lungs at the end of respiration. In ordinary breathing there remain 

 2800 cubic centimeters of air (1200 cubic centimeters residual and 1600 

 cubic centimeters of reserve air). As the average inspiration amounts 

 to only 500 cubic centimeters, of which about 360 cubic centimeters 

 reach the alveoli, it is evident that the changes in the decomposition of 

 the alveolar air as a result of inspiration are not very great. The necessity 

 of knowing the composition of the alveolar air, in each case, has been 

 realized only as a result of recent investigation. Upon this knowledge 

 depends our judgment as to whether the gas-exchange in the alveoli, 

 between the blood and alveolar air, follows simply the laws of gas absorp- 

 tion, or whether other forces must come into play. It is, to-day, still 

 believed by many that the first explanation of the gas-exchange in the 

 lungs is entirely satisfactory. Blood reaches the lungs, through the pulmon- 

 ary arteries, having a greater carbon dioxide tension than does the alveolar 

 air. An equilibrium must be established between these two gas-pressures, 

 and as a result carbon dioxide diffuses from the blood into the alveoli. 

 Similarly, on account of its greater tension in the alveolar air, oxygen passes, 

 into the blood. The hemoglobin in this way becomes saturated with 

 oxygen, and is ready once more to enter the general circulation. This 

 assumption is supported by the work of Wolffberg 1 and of Nussbaum. 2 

 If, namely, the gas-exchange in the alveoli of the lungs follows exactly 

 the laws of gas diffusion, then, in a lobule, which is cut off by the 

 closing of the bronchial tube leading to it, the alveolar air must be : 

 in equilibrium with the carbon dioxide tension of the blood. Similarly 

 the arterial blood, flowing from this lobule, must have the same carbon 

 dioxide tension as that of the alveolar air. Wolffberg and Nussbaum 

 found, as a matter of fact, that the carbon dioxide tension in the alveoli 

 was the same as that of the venous blood which flows to them. They 

 introduced a double-walled elastic catheter into a branch of the bronchus; 

 of a tracheotomized dog, in which this portion of the lungs was shut off, 

 by inflating a rubber enlargement of the catheter. After a short time 

 a sample of the alveolar air was withdrawn through a tube in the catheter 

 and its chemical composition determined. They found on an average that 

 this isolated alveolar air showed a carbon dioxide tension of 3.84 per cent 



1 Pfliiger's Arch. 4, 465 (1871); 6, 23 (1872) 



2 Ibid. 7. 296 (1873). 



