RESPIRATION 



245 



the existing alveolar oxygen pressure will increase more and more 

 if diffusion alone determines the saturation of the blood in the 

 lungs, and will tend in the same direction even if active secretion 

 assists diffusion. We can thus easily explain why some of the 

 persons who ascended Pike's Peak were very blue in the face, and 

 why fainting or partial loss of consciousness were common occur- 

 rences. We can also understand why some persons become more 

 or less unwell at first on going to an altitude of only four or five 

 thousand feet, and why in all persons there is a distinct physio- 

 logical reaction to anoxaemia, as shown by lowering of the al- 

 veolar CO2 pressure and rise in the haemoglobin percentage. This 

 physiological reaction would be difficult to understand if there 

 was uniform saturation of the haemoglobin in all the alveoli. We 

 tnust conclude that whether or not a person is acclimatized to a 

 low barometric pressure the percentage saturation of the mixed 

 arterial haemoglobin with oxygen is distinctly diminished, though 

 the amount of the diminution is not indicated by the carbon mon- 

 oxide method. 



In the process of oxygenation of the blood in the lungs, the 

 oxygen has to pass from the alveolar air through a thin layer of 

 living tissue into the blood and into the corpuscles. This process 

 must take some time. To the genius of Christian Bohr we owe the 

 principle of a method by which the time may be estimated, in so 

 far as the process is one of diffusion. In connection with the ab- 

 sorption of oxygen by the lungs it is not possible to measure the 

 rate at which, with a given diffusion pressure, oxygen passes 

 inwards, because we do not know the mean diffusion pressure. 

 We can, as will be shown later, measure the oxygen pressure of 

 the venous blood, as well as that of the alveolar air and arterial 

 Tjlood ; but we do not know how quickly the blood becomes satu- 

 rated in its passage along the alveolar capillaries. Hence we can- 

 not estimate the mean difference in oxygen pressure required for 

 the diffusion inwards of a given quantity of oxygen in a given 

 time. In the case of absorption of CO present in the air in a low 

 proportion the conditions are quite different, however: for we 

 can determine the percentage of CO in the alveolar air, and the 

 rate at which the gas is absorbed, while, for short experiments, 

 the difference in CO pressure between the alveolar air and the 

 blood is constant. In this way we can tell how much CO is ab- 

 sorbed per minute with a given pressure difference; and from 

 this, allowing for the greater solubility and slightly lower dif- 

 fusibility of oxygen, we can calculate the rate at which oxygen 

 diffuses in with the same pressure difference. 



