302 HUMAN PHYSIOLOGY 



If the charge of oxygen borne by a single corpuscle 

 is smaller at the new habitation than it was at the old 

 there will clearly be a gain in adding to the number of 

 the carriers. But we shall probably be wrong if we make 

 the increase in the number of corpuscles the principal 

 feature of the adaptation. 



Students commonly fail to appreciate that the main- 

 tenance of respiration calls for an efficient and economic- 

 ally directed circulation just as urgently as for sound 

 lungs and trained breathing muscles. A person will 

 be breathless if his circulation is not equal to its task, 

 regardless of his success in ventilating the lungs. When 

 exercise is undertaken there must be a coordination be- 

 tween the increase of the breathing and the accelera- 

 tion of the blood-flow. It is hard to see that anything 

 is gained by the first unless it is supported by the second. 

 In becoming acclimated to life at a high altitude one 

 may be supposed to acquire several advantages. The 

 breathing muscles become stronger and their energy 

 is better directed. The heart is trained. The vaso- 

 motor adjustments give the best possible direction to 

 the blood-stream. There are more corpuscles available 

 as noted above. 



Some investigators hold that after giving due weight 

 to all these possibilities it is still necessary to add an- 

 other to the list. This remarkable adaptation, they 

 claim, is a local change in the nature of the cells lining 

 the air-sacs of the lungs. It is asserted that there is 

 evidence to show that these epithelial cells gain a power 

 which they did not have before, the ability to promote 

 the transfer of oxygen from the air to the blood by some 

 application of their own energy. It is hard to tell 

 whether this view will be widely accepted. It is a sound 

 principle to make the most of the simpler explanations 

 that offer themselves before resorting to those that are 

 more complicated. We may find, however, that an 

 active intervention on the part of these cells must some- 

 times be assumed. 



