CH. XXVII.] MOUNTAIN SICKNESS 401 



shallow breathers. This is illustrated by the following figures 

 obtained from observations on three different men : 



Subject 1 

 2 

 3 . 



Mountain sickness therefore attacks different persons at different 

 altitudes. If no form of adaptation took place, the following would 

 be the approximate pressures of oxygen in the pulmonary alveoli at 

 the following altitudes : 



Sea level . . 100 mm. 



10,000 feet . . 59 mm. 



15,000 feet . . 38 mm. 



It is known lhat at alveolar pressures of 60 mm. or thereabouts 

 the symptoms of oxygen want begin in an artificial chamber. Inspec- 

 tion of the dissociation curve of human blood, p. 370, will show 

 that at 59 mm. the arterial blood might be upwards of 85 per cent, 

 saturated with oxygen, but at 38 mm. it would be only 66 per cent, 

 saturated. This condition of the blood would account for the 

 symptoms. 



Methods of adaptation to high altitudes. Apart from the 

 question of " training " which will be dealt with below, adaptation 

 takes place along two lines, each of which tends to bring more 

 oxygen into a given quantity of blood. This is accomplished (1) by 

 raising the alveolar pressure of oxygen higher than the figures given 

 above ; and (2) by increasing the quantity of haemoglobin in the blood. 



The rise in alveolar oxygen pressure is brought about by more 

 rapid breathing. The more rapid breathing in turn is the result of 

 the respiratory centre being hustled by acid formation if the respira- 

 tion slows down. There is therefore more oxygen and less carbonic 

 acid in the alveolar air and in the blood than there otherwise would 

 be. Actually, the oxygen pressures in the alveolar air were found 

 to be: 



Sea level . .100 mm. 



10,000 feet . . 65 mm. 



15,000 feet . . 52 mm. 



The chief difference is naturally at the greatest altitude. Here, 

 instead of the alveolar oxygen pressure being 38 mm. it is 52 mm. ; 

 the blood could therefore become about 80 per cent, instead of 66 per 

 cent., saturated with oxygen a very material increase. Moreover, 

 since the rate at which the blood becomes oxygenated is directly 

 proportional to the oxygen pressure, the difference between 38 mm. 

 and 52 mm. is most beneficial. The figures for the percentage 



2 C 



