BREATHING IN RAREFIED AND COMPRESSED AIR 419 



monary epithelium the simpler process of physical diffusion is sufficient 

 to account for the facts. 



Three mechanisms are known to be responsible for increasing the 

 quantity of oxygen in the blood: (1) increase in the tension of oxygen 

 in the alveolar air; (2) an increased affinity of the hemoglobin for oxygen; 

 (3) increase in the erythrocytes and hemoglobin of the blood. 



The increased alveolar oxygen tension is a result of the increase in 

 pulmonary ventilation. If no adaptation occurred, the oxygen tension 

 at 10,000 feet would be 59 mm. and at 15,000 feet 33.8 mm. Actual 

 observations on men, however, gave at 10,000 feet a tension of 65 mm. 

 and at 15,000 feet 52 mm. 



The increased affinity of the hemoglobin for oxygen is due to a change 

 in the composition of the blood the exact nature of which is as yet un- 

 known. As a result it takes up, when exposed to the alveolar air, about 

 5 per cent more oxygen than would be the case in the absence of this 

 compensation. 



These compensations are not sufficient to restore the blood completely 

 to the condition found at sea level. They bring it at 14,000 feet, only to 

 81 or 91 per cent saturation even in the case of men who have lived for 

 generations at this altitude. As a result cyanosis is prevalent among 

 them, and clubbed fingers, such as may accompany chronic cyanosis in 

 the pathological at sea level, are not infrequent. Yet in spite of the per- 

 sistent unsaturation of the blood, men who are acclimated to these alti- 

 tudes can accomplish prolonged and severe muscular work and take 

 pleasure in such strenuous diversions as dancing, tennis and football. 



The constantly low tension of 2 in the plasma causes the red blood 

 corpuscles and the percentage of hemoglobin to become markedly in- 

 creased after residence for some time in high altitudes. At first this is 

 due to a concentration of the blood by a diminution in plasma, but grad- 

 ually the blood-forming organs become excited and an actual increase 

 in the total amount of hemoglobin occurs. In the light of these facts it 

 is interesting to compare the average number of red corpuscles in the 

 blood of inhabitants of different altitudes. 



(From Starling.) 



