146 RESPIRATION 



orthopnoea, and its causation now seems evident. With a failing 

 respiratory center, and consequent abnormal shallowness of respi- 

 ration, anoxaemia is the natural result of the recumbent position ; 

 and the prevention of this anoxaemia by keeping the patient in a 

 sitting position becomes an important part of treatment unless 

 the same object is attained by oxygen administration. 



Defective distribution of air in the lung alveoli is, of course, 

 only one of the causes of defective oxygenation of the arterial 

 blood; but I have dealt with this cause first, not only because it 

 is of very great importance in medicine, but because an under- 

 standing of it is essential to the understanding of other causes of 

 defective oxygenation. 



A second and hitherto much better known cause of defective 

 oxygenation of the arterial blood is a deficiency in the partial 

 pressure of oxygen in the inspired air, and consequent fall in the 

 alveolar oxygen pressure. As shown in Chapter II, it usually re- 

 quires a fall in oxygen percentage from the normal of 20.9 to 

 about 14 per cent, or a third, before any evident effect on the 

 breathing is produced at the time by the oxygen deficiency. Simi- 

 larly a fall of about a third in barometric pressure (corresponding 

 to about 1 1,500 feet above sea level) is required. Figure 49, from 

 a paper by Boycott and myself,'' shows that until the barometric 

 pressure in a steel chamber falls by about a third, the normal 

 alveolar CO2 pressure is very little disturbed. The alveolar CO2 

 percentage simply goes up as the barometric pressure goes down, 

 but the pressure of CO2 remains almost the same in the alveolar 

 air. In the same investigation we found that even when the bar- 

 ometric pressure was reduced to 300 mm. the alveolar CO2 pres- 

 sure remained the same, provided that any excessive fall in the 

 oxygen pressure of the inspired air was prevented by adding oxy- 

 gen to the air of the chamber. There is thus no trace of foundation 

 for Mosso's contention^ that the diminished mechanical pressure 

 of the air produces by itself a diminished saturation of the blood 

 with CO2. 



Since the alveolar air, with the breathing normal, contains about 

 a third less oxygen than the inspired air, it follows that when the 

 oxygen percentage or partial pressure in the inspired air is reduced 

 by a third the alveolar oxygen percentage will be reduced to about 

 half — i.e., from about 13 per cent of an atmosphere to about 6.5 

 per cent. On comparing this with the dissociation curve of oxy- 

 haemoglobin it will be seen that such a diminution corresponds 



' Boycott and Haldane, Journ. of Physiol., XXXVII, p. 355, 1908. 

 ' Mosso, Life of Man on the High Alps, London, p. 287, 1898. 



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