RESPIRATION 113 



introduced by the fact that, as Bohr discovered (see Chapter V 

 and Figure 19), deficiency of carbon dioxide causes haemoglobin 

 to hold on more tightly to oxygen. The consequence of this is, that 

 when increased breathing lowers the pressure of CO 2 in the al- 

 veolar air and in the body as a whole, on the one hand the haemo- 

 globin of the blood passing through the lungs is more highly 

 saturated with oxygen than it otherwise would be; on the other 

 hand the blood holds this oxygen so firmly that the oxygen pres- 

 sure in the tissues falls lower than it otherwise would. There may 

 thus be considerable anoxaemia though the blood is almost as red 

 as usual, and the existence of this anoxaemia is only revealed by 

 the immediate physiological effects of raising the alveolar oxygen 

 pressure. 5 



On reducing, in a steel chamber, the atmospheric pressure to 

 half an atmosphere there is a quite appreciable permanent increase 

 in the breathing, and consequent drop in alveolar CO 2 pressure 

 caused by anoxaemia, but, in my own case at any rate, no very 

 striking blueness of the lips, although at the time the alveolar 

 oxygen pressure is only about 34 mm. This pressure would only 

 be sufficient to saturate the oxyhaemoglobin of the blood to the 

 extent of 57 per cent if the pressure of CO 2 were that of normal 

 alveolar air (see Figure 20). Blood with this percentage satura- 

 tion would be very strikingly blue. Owing, however, to the dimin- 

 ished pressure of CO 2 , the saturation is much higher, and this 

 accounts for the color of the lips being nearly normal. The exist- 

 ence of considerable anoxaemia was, however, revealed at once 

 by the effects of adding oxygen to the inspired air : for vision and 

 hearing were at once strikingly improved and the breathing di- 

 minished. The degree of blueness of the lips is thus only a rough 

 index of anoxaemia when anoxaemia is taken in its physiological 

 meaning, as diminution in the oxygen pressure, rather than merely 

 of the oxygen content, of the blood. It is the diminution in the 

 amount of free oxygen, whether or not the amount of reserve oxy- 

 gen combined with the haemoglobin is also diminished, which is 

 functionally important. 



Thus the benefit produced by diminished pressure of CO 2 (as, 

 for example, during forced breathing) in increasing the percent- 

 age saturation of the haemoglobin in the arterial blood is neu- 

 tralized by the disadvantage in the tissues owing to the same cause. 

 The venous blood may, in fact, be as red as usual, although the 

 venous oxygen pressure is abnormally low : for the saturation of 



* Haldane, British Medical Journal, July 19, 1919. 



