436 LECTURE XVIII. 



The lower limit for oxygen tension in the alveolar air lies a little above 

 30 millimeters. This corresponds to an oxygen content of about 4.5 per 

 cent, assuming a total pressure of 710 millimetres (= 1 atmosphere at the 

 body temperature) . Moreover, this is true only for a period of rest, and 

 not for one of active work. In the latter case an oxygen pressure of 

 30 millimeters is not sufficient. 



Paul Bert, Frankel, and Geppert have shown that the amount of oxygen 

 absorbed by the blood becomes equal to one-half the normal amount, 

 only when the total pressure of the surrounding atmospheric air is less than 

 300 millimeters. This is interesting, because it gives us some conception 

 as to the nature of the behavior of the gas-exchange during passage into a 

 more rarefied atmosphere, i.e., in balloon ascensions, or in mountain climb- 

 ing. In these two examples naturally the requirements upon the blood- 

 gases are quite different. In the former case, there is practically no work 

 to be performed, so that aeronauts reach a much higher altitude than do 

 mountain climbers, before they experience difficulty in breathing. The 

 fact that different individuals are affected differently at one and the same 

 height is explained, first, by the fact that the lung ventilation and amount 

 of air breathed is, as already mentioned, quite different, so that, in one 

 case, the blood has more oxygen at its disposal than in another. It has 

 been found, moreover, that the animal organism possesses an extremely 

 delicate mechanism of regulation, which energetically opposes any defi- 

 ciency of oxygen in the system. To this belongs the increase in the number 

 of red corpuscles, and thereby of hemoglobin, which unquestionably takes 

 place when men and animals pass from a locality into one of higher altitude. 

 The increase disappears as soon as the original level is again reached. 1 

 The object of this is plain. No matter whether we assume that the abso- 

 lute amount of these red corpuscles is increased, or that the increase is 

 merely relative, brought about perhaps by the passing out of plasma, 

 there is in a unit of blood more hemoglobin passing through the lungs in 

 a unit of time than is normally the case. The way this increase in the red 

 corpuscles caused by ascending high mountains is effected, has not been 

 satisfactorily explained. It is remarkable that the change takes place 

 suddenly, and in fact without any indication of there being any new forma- 

 tion of blood (red corpuscles with nuclei, etc.), and that on reaching a low 

 level again, the reverse change takes place without any of the usual indi- 



1 Cf. Paul Bert: loc. cit. Die histiochemischen und physiologischen Arbeiten von 

 Fr. Miescher, Vol. II, p. 328, Leipzig, 1897. Abderhalden: Z. Biol. 43, 125 and 443 

 (1902) ; Medizin, Klinik, No. G (1905); Pfluger's Arch. 110, 195 (1905). von Schrotter 

 and Zuntz: ibid. 92, 479 (1902). van Voornveld: ibid. 92, 1 (1902). Otto Cohnheim: 

 Ergeb. Physiol. (Asher and Spiro) II, 612 (1902). Durig and Zuntz: Arch. Anat. 

 Physiol. Suppl. 1904, p. 417. Jaquet: Ueber die physiologische Wirkung des Hohenkli- 

 mas, Basel, 1904. Zuntz, Loewy, Miiller, and Caspari : Hohenklima und Bergwanderungen 

 in ihrer Wirkung auf den Menschen, Bong et Cie, 1906. 



