CHAPTER VII 



The Causes of Anoxaemia. 



In the previous chapter anoxaemia has been defined as the condi- 

 tion in which the partial pressure of oxygen, or, what comes to 

 practically the same thing, the amount of free oxygen, in the 

 systemic capillaries generally, is abnormally low. The causes of 

 this condition must now be examined. 



The first and most important cause of anoxaemia is defective 

 saturation of the arterial haemoglobin with oxygen. This may, as 

 we shall see, arise from several causes; but the most obvious of 

 these is defective partial pressure of oxygen in the alveolar air. 

 It will be shown in Chapter IX that during rest under normal 

 conditions oxygen passes into the blood through the alveolar 

 epithelium by a process of simple diffusion, and that the oxygen 

 pressure in the arterialized blood leaving each alveolus is exactly 

 that of the air in the alveolus. For the purposes of the present 

 discussion we may provisionally assume that this is always the 

 case during rest, so long as the lungs and the inspired air are 

 normal, although modifications in this assumption must be intro- 

 duced later. 



In the light of this assumption and of our knowledge of the 

 dissociation curve of oxyhaemoglobin, it might seem at first that 

 we are justified in assuming that the oxygen pressure of mixed 

 arterial blood is simply that of mixed alveolar air as ordinarily 

 obtained for analysis by the methods already described. In favor 

 of this assumption is the now well-ascertained fact that the 

 breathing is regulated under ordinary conditions in close ac- 

 cordance with the pressure of CO2 in the mixed alveolar air, as 

 explained in Chapter II. Variations in average alveolar CO2 pres- 

 sure are thus a direct measure of variations in the CO2 pressure 

 of the arterial blood ; and it was natural to assume, as was done 

 by myself and others till lately, that variations in alveolar oxygen 

 pressure must also be a measure of variations in the oxygen pres- 

 sure of the arterial blood. One known difficulty in this assumption 

 lay in the fact that the arterial oxygen pressure, as measured in 

 animals by the aerotonometer (Chapter IX) is nearly always 

 lower, and sometimes considerably lower, than the alveolar oxy- 

 gen pressure; but various explanations of this difficulty had been 

 adopted by myself and others. 



