56 ORGANISM AND ENVIRONMENT 



as to bring the alveolar oxygen to about the normal at 

 sea level, the difference between arterial and alveolar 

 oxygen pressure almost disappeared. We then deter- 

 mined the arterial oxygen pressure in a newcomer who 

 was still blue, but did not become mountain-sick till 

 some hours later. It was very little above the alveolar 

 oxygen pressure; but three days later when he was 

 acclimatised and well, his arterial oxygen pressure was 

 as high as our own. The mean result was that on 

 Pike's Peak, after acclimatisation, the arterial oxygen 

 pressure was during rest only about 13 mm. lower than 

 at sea level, but was 35 mm. higher than the alveolar 

 oxygen pressure. The complete absence of any blue- 

 ness after acclimatisation was thus easily intelligible. 

 The lungs were actively secreting oxygen into the 

 blood, even during rest. Nevertheless the blueness 

 reappeared temporarily during prolonged muscular 

 exertions, as in a long climb. The lung epithelium 

 could thus apparently be fatigued by the extra work 

 thrown upon it. 



As already seen, the lung epithelium is at all times 

 capable of actively secreting oxygen inwards if the 

 requisite stimulus arising from oxygen want in the 

 tissues is present. But at high altitudes this capacity 

 is greatly increased, and secretion goes on continuously 

 after acclimatisation. The stimulus, moreover, is 

 essentially the same stimulus as produces the changes 

 in the regulation of blood alkalinity and in the haemo- 

 globin of the blood. The stimulus is want of oxygen 

 in some form ; but how does the want of oxygen act ? 

 The haemoglobin of the arterial blood must, after 



