26 2 RESPIRA TION 



by which the)' were obtained. There is no doubt that in the earlier 

 observations with the aerotonometer (Strassburg) the oxygen of 

 the blood could not have come into equilibrium with the mixture 

 in the gas space, in which the oxygen pressure was at the beginning 

 much lower than that in the blood; the results are therefore too low. 

 The same is true for the oxygen tension of the venous blood, but as 

 this is in any case considerably smaller than that of the arterial 

 blood, the proportional error is not so great. The later experiments 

 (of Herter), given in the second line of the table, yield much higher 

 values, owing to improved technique, but the findings are still to be 

 regarded as minimal and not average results. At the other end of 

 the scale stand the results of Haldane and Smith, who found in man 

 an oxygen tension in the arterial blood of over 200 mm. of mercury 

 equal to more than 26 per cent, of an atmosphere. This exceeds 

 the partial pressure of oxygen in the external air, and is about twice 

 as great as that of the air of the alveoli. In the bird they found 

 an oxygen tensien of between 300 and 400 mm., equal to 45 per 

 cent, of an atmosphere. 



The chief interest of this discussion of the blood-gas tensions lies 

 in their fundamental importance in the problem of the gaseous 

 exchange in the lungs, on the one hand, and between the blood and 

 tissues on the other. In the presence of such results Haldane and 

 Smith necessarily adopted a secretion theory of gaseous exchange 

 in the lungs. For it was manifestly impossible for oxygen to diffuse 

 from the alveoli into the blood against the slope of pressure. 



Their findings, however, differed so vastly from those of all observers 

 who had used tonometric methods, and it seemed so difficult to assign 

 an adequate physiological value to the slight increase in the percentage 

 saturation of the haemoglobin with oxygen (see Figs. 119-121), which 

 could be brought about by even a great excess of oxygen tension above 

 that of atmospheric air, that there was a general disposition to distrust 

 the accuracy of their method. At the same time the ordinary tono- 

 metric technique left so much to be desired that there seemed little 

 hope of bringing the matter to a decisive test. The introduction a 

 few years ago by Krogh, of better aerotonometric methods, and the 

 remarkable series of researches carried out by their aid, have changed 

 the whole aspect of the question. He showed that in the arterial blood 

 of rabbits the oxygen tension was in all cases somewhat (usually 2 to 

 3 per cent, of the atmospheric pressure, or 15 to 23 mm. of mercury) 

 lower than the oxygen tension in air from the bifurcation of the trachea. 

 In animals it is not possible to obtain the actual alveolar air. These 

 results agree very well with those of Fredericq, who found in dogs 

 oxygen tensions of 13 to 14 per cent, of an atmosphere i.e., something 

 like 100 mm. of mercury, when the animals breathed atmospheric air. 

 Krogh proved that the amount of oxygen lost by self-reduction of 

 the blood (see p. 261) in his aerotonometer was negligible, even in 

 rabbits, although it is known that rabbit's blood uses up more oxygen 

 than that of higher mammals. He also pointed our some sources of 

 error in the Haldane-Smith method. About the same time Haldane, 

 after a careful re-examination of the question, came to the con- 

 clusion that his previous results were much too high; and that 





