THE ALVEOLAR AIR. 863 



Numerous investigations as to the composition of the inspired atmos- 

 pheric air as well as the expired air are at hand, and it can be said that 

 these two kinds of air at C. and a pressure of 760 mm. Hg have the 

 following average composition in volume per cent. 



f)_ v Nitrogen Carbon 



Oxygen (and argon) Dioxide 



Atmospheric air 20.96 79.02 0.03 



Expired air 16.03 79.59 4.38 



The partial pressure of the oxygen of the atmospheric air corresponds 

 at a normal barometric pressure of 760 mm. to a pressure of 150 mm. 

 Hg. The loss of oxygen which the inspired air suffers in respiration 

 amounts to about 4.93 per cent, while the expired air contains about 

 one hundred times as much carbon dioxide as the inspired air. 



The expired air is therefore a mixture of alveolar air with the residue 

 of inspired air remaining in the air-passages; hence in the study of 

 the gaseous exchange in the lungs the alveolar air must first be con- 

 sidered. There exists no direct determination of the composition of the 

 alveolar air in man, but only approximate calculations. From the 

 average results found by VIERORDT in normal respiration for the carbon 

 dioxide in the expired air, 4.63 per cent, ZUNTZ 1 has calculated the 

 probable quantity of carbon dioxide in the alveolar air as equal to 5.44 

 per cent. If we start from this value, with the assumption that the 

 quantity of nitrogen in the alveolar air does not essentially differ from 

 the expired air, and admit that the quantity of oxygen in the alveolar 

 air is 6 per cent less than the inspired air, it will be seen that the alveolar 

 air contains 15 per cent oxygen. As the total pressure of the air of the 

 lungs after deducting the aqueous tension of about 50 mm. can be cal- 

 culated as about 710 mm. the partial pressure of the oxgyen in man 

 can be put at about 106 mm. and that of the carbon dioxide as about 

 45 mm. 



Based upon several respiration experiments upon different persons, 

 LOEWY has been able to calculate the composition of the alveolar air 

 of human beings almost at the atmospheric pressure, from the com- 

 position of the expired air and the depth of inspiration and expiration, 

 taking into consideration the air in the upper air-passages. He obtained 

 results which varied between 101 and 105 mm. Hg for the oxygen tension 

 and between 32-42 mm. for the carbon dioxide tension. 



The alveolar oxygen tension in dogs can be calculated from the car- 

 bon dioxide content of the alveolar air and is also found to be above 

 100 mm. Hg. 



If the oxygen partial pressure in the alveoli is put at about 105 



1 See Zuntz, 1. c., Hermann's Handbuch, 105 and 106. 



