FUNCTIONS OF BLOOD 



101 



duration, the adaptation is not brought about by any shift in 

 the dissociation curve of the blood, which shortly after birth 

 seems to become an individual constant. The mechanism 

 is more indirect and gives room for large personal differences. 



Above a certain altitude the oxygen pressure in the lungs 

 is insufficient to saturate the blood, and this brings about, 

 partly perhaps through the formation of fixed acids, but prob- 

 ably in a more intricate way, an increase in ventilation which 

 is, however, of very different extent in different individuals. 

 Those individuals who promptly respond by a large increase 

 show the greatest resistance 

 in the acute exposure to low 

 oxygen pressures. The 

 breakdown occurs at nearly 

 the same alveolar oxygen 

 pressure in all cases. 



The increased ventilation 

 reduces the C0 2 pressure in 

 the alveoli, and CO2 is grad- 

 ually washed out from the 

 blood and tissues whereby the 

 blood is made more alkaline. 

 In the process of acclimatiza- 

 tion this is compensated by a 

 corresponding decrease in 



the "alkali reserve." Assuming a constant metabolism, the 

 amount of CO2 liberated in the tissues will therefore bring 

 about a larger increase in acidity and thereby raise the actual 

 tension of unloading, a very beneficial adaptation. 



The rate of circulation during rest or corresponding to a 

 well-defined amount of work does not change appreciably, 

 but the oxygen capacity of the blood increases gradually 

 during a couple of months by the formation of new red cor- 

 puscles. The final capacity is closely correlated with the 

 altitude and reaches at 5,300 m an average of 29.2 vols. %, 

 distributed over 7.3 million red corpuscles per cubic milli- 

 meter (Keys, 1938). 



Fig. 57. Oxygen dissociation of the 

 llama and vicuna at body temperature 

 (39°C) and p H 7.1, compared with the 

 range for eight lowlanders including 

 man, horse, and dog. (Hall, Dill and 

 Barron.) 



