402 RELATION OF RESPIRATION TO OTHER PROCESSES [CH. XXVII. 



saturation are obtained by employing the dissociation curve for blood 

 at 40 mm. carbonic acid pressure. The legitimacy of this may be 

 questioned, since the carbonic acid pressure in the blood at an eleva- 

 tion of- 15,000 feet is only about 25 mm.* The curve is, however, 

 true for the blood at all altitudes since the reaction of the blood 

 remains the same (or may shift a trifling amount to the acid side), 

 some other less volatile acid making up for the deficiency in carbonic 

 acid. 



Increase of haemoglobin in the blood. At high altitudes the 

 number of corpuscles per cubic millimetre in human blood is said to 

 be excessive. Some caution is necessary in attributing physiological 

 significance to such statements, for they may be in part illusory. In 

 balloon ascents, for instance, the number of corpuscles in blood may 

 increase very rapidly. This is not due to fresh formation of blood, 

 but to the excessive secretion of lymph, which leads to concentration 

 of the blood. Nevertheless, in animals kept at high altitudes for 

 some days there is an actual increase in the formation of corpuscles, 

 as shown by the appearance of nucleated corpuscles thrust into the 

 blood by the bone-marrow, and an increase in the amount of iron in 

 the blood at the expense of the quantity of that element in the liver. 



Training. We have seen that a factor in the onset of 

 mountain sickness is the amount of oxygen which passes through 

 the lung epithelium. It is a matter of experience that from 

 various causes an individual can do the same amount of muscular 

 work, such as performing the same mountain-climb, with a less 

 degree of metabolism after training. This is in part due to the fact 

 that the trained person has lost unnecessary weight, and in part 

 because he uses his muscles to better advantage. Thus every unit of 

 energy spent in work demanded a total expenditure of energy of 7 

 units in the case of an untrained town-dweller, 5 units in the case 

 of a partially trained tourist, and 3-3 units in the case of an Alpine 

 porter. Therefore, in the same climb, the Alpine carrier would only 

 suffer half the increase of metabolism that the town-dweller would. 

 He would therefore need correspondingly less oxygen, and he could 

 reach a height at which he would have a correspondingly lower 

 alveolar oxygen tension. 



Respiration at High Pressures. 



Prolonged exposure to pressures of oxygen, equal to 1300 to 1400 

 mm. of mercury, induces pneumonia, and death rapidly follows. It is 



* The diminution of carbonic acid tension in alveolar air and blood is termed 

 acapnia, and Thorso attributed mountain sickness to this condition. Although 

 this view is incorrect, it is necessary to remember that carbonic acid is not a mere 

 waste product, but performs certain duties in the body (stimulating respiration 

 and assisting the dissociation of oxy haemoglobin); in certain conditions, as in 

 shock, acapnia is an important factor. 



