OXYGEN TENSION. 813 



an entirely different principle, tend to corroborate the high results attained 

 by BOHR. 



HALDANE'S method is as follows: The individual experimented upon is allowed 

 to inspire air containing an exactly known but small quantity of carbon monoxide 

 (0.045-0.06 per cent), until no further absorption of carbon monoxide takes place 

 and the percentage saturation of the hemoglobin in the arterial blood with carbon 

 monoxide has become constant, as shown by a special titration method. This 

 percentage saturation is dependent upon the relation between the tension of the 

 oxygen in the blood and the tension of the carbon monoxide, as known from the 

 composition of the inspired air. When this last and the percentage saturation 

 with carbon monoxide and oxygen are known the oxgyen tension in the blood can 

 be easily calculated. 



According to this method HALDANE and SMITH found still higher 

 figures than BOHR for the oxygen tension in the blood, and they calculated 

 the average tension of the oxygen in human arterial blood to be equal 

 to 293 mm. Hg. 



Let us now compare the figures for the oxygen tension of the arterial 

 blood as found by various investigators with the tension of the oxygen 

 in the air of the lungs. 



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 : 



/~i_ Nitrogen Carbon 



Oxygen * 



(and agon) Dioxide 



Atomspheric 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 exphed 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 xesidue 

 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 



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



