224 RESPIRATION 



When Bohr's original experiments on the question of secretion 

 by the lungs were published in 189 1, I was just beginning the 

 serious study of mine gases and the physiological effects of vitiated 

 air; and his results interested me greatly. A year or two later 

 Lorrain Smith and I made a visit of several weeks to Copenhagen, 

 and carried out some research work in the laboratory under 

 Bohr's direction, thus learning a great deal which we could not 

 have learned in England about existing methods of blood-ga- 

 investigation, and, far more important, getting into personal 

 touch with Bohr himself. I should like to take this opportunity oi 

 saying how much we, and indirectly other physiologists in Great 

 Britain and America, have owed to Bohr and the Copenhagen 

 School of physiologists. 



The difficulties of the aerotonometer method of determining the 

 oxygen pressure of arterial blood were very evident, and I cast 

 about in my mind for some better method. Soon afterwards I 

 began investigating the action of carbon monoxide in mines, and 

 the results of this investigation, and the colorimetric method of 

 blood examination, which I worked out during the investigation, 

 suggested a new means of attacking the problem which Ludwig 

 had originally suggested. 



The general principle of this method has already been ex- 

 plained in Chapter IV, and depends on the fact that within wide 

 limits the relative proportions in which haemoglobin is shared 

 between oxygen and CO are proportional to the relative partial 

 pressures of the two gases when allowance is made for their rela- 

 tive affinities for the haemoglobin. Hence if the proportions ir 

 which oxygen and CO are shared in the haemoglobin of the 

 blood when equilibrium is established are known, as well as the 

 pressure of CO, the pressure of oxygen can be calculated. To 

 measure the oxygen pressure in the arterial blood it is therefore 

 only necessary to allow a man or animal to breathe a constant small 

 percentage of CO until absorption of CO stops, owing to a balance 

 having been struck between oxygen pressure and CO pressure in 

 the blood passing through the lung alveoli. The percentage satu- 

 ration of the haemoglobin with CO is then determined, and the 

 arterial oxygen calculated from a knowledge of the relative affini- 

 ties of the two gases for haemoglobin, as determined outside the 

 body. 



The method seemed simple in principle, but it turned out to 

 be as full of pitfalls in practice as the use of the blood pump, 

 aerotonometer, or spectrophotometer. What misled us most were : 



