TOU( 



RESPIRATORY INTERCHANGE UNDER DIFFERENT CONDITIONS 517 



of the atmospheric air must finally lead to a serious disturbance of the normal 

 metabolism. The conditions to be dealt with at this time are a decreased and 

 increased supply of oxygen and an increased supply of carbon dioxid. In all these 

 cases we are concerned with changes in the volume per cent, of the gases. 



A diminution in the partial pressure of the oxygen of the air must necessarily 

 induce a similar change in the pressure of this gas in the alveoli. But inasmuch as 

 the intensity of the pulmonary ventilation and the magnitude of the oxygen con- 

 sumption vary almost from moment to moment, a direct relationship cannot 

 exist between these factors, and hence, it is more correct to speak of the tension of 

 this gas in the alveoli than of that in the surrounding air. While the lower limit 

 of the former, which may be endured without danger to life, differs somewhat in 

 different persons, it may be adjudged at 30-35 mm. Hg. This value, of course, 

 holds true only under a normal atmospheric pressure and corresponds to an oxygen 

 content of the alveolar air of 4.5 per cent. Consequently, provided that 500 c.c. 

 of air are respired 17 times in a minute, an altitude of 5000 m. may be attained 

 before the oxygen tension in the alveoli reaches this low level. ^ Any difficulty 

 arising therefrom, may be remedied immediately by increasing the amplitude of 

 the respiratory movements. This change augments the alveolar tension and en- 

 ables the individual safely to ascend even to somewhat higher altitudes if he reduces 

 his muscular activity to a minimum. Paul Bert^ places the lower limit of the 

 oxygen pressure of the outside air at 50 mm. Hg, which corresponds to an oxygen 

 content of 6 to 7 per cent. At this time, the total atmospheric pressure amounts 

 to 250 mm. Hg. An animal which is exposed to still lower tensions develops symp- 

 toms of dyspnea and convulsions which generally terminate fatally. An oxygen 

 content of 12 per cent, is usually endured without changing the quality of the 

 respiratory movements, although the deficiency in oxygen may be quite apparent 

 from the bluish color of the face. 



In explaining this phenomenon it is commonly believed that an alveolar tension 

 of the oxygen of about 30 mm. Hg causes the oxyhemoglobin to be dissociated 

 so rapidly that the blood is no longer in a position to aerate the tissues properly. 

 Hence, we are dealing here with a real deficiency in the oxygen supply which is 

 commonly designated as anoxemia. This explanation may also be expressed as 

 follows: We know that for physical reasons the system cannot absorb the oxygen 

 under a lower tension than the one just given. Consequently, the 350 c.c. of 

 oxygen which each kilogram of substance requires in an hour can only be obtained 

 with a differential pressure of 29 mm. Hg or more. If the pressure falls below this 

 value, the driving force behind the atoms of oxygen becomes so slight that they 

 cannot traverse the alveolar lining to enter the blood. 



An increase in the partial pressure of the oxygen in the alveoli can be attained 

 either by the inhalation of a mixture of gases which is rich in oxygen, or by the 

 inhalation of pure oxygen under atmospheric pressure. But even if this gas is 

 supplied in a pure form, so that its pressure is increased five times, namely, from 

 152 mm. to 760 mm. Hg, no considerable variation in the consumption of oxygen 

 and the output of carbon dioxid results ; provided, of course, that the experiment is 

 not continued for an unusually long time. This fact tends to show that the oxida- 

 tions in our tissues cannot be affected in a favorable manner by this means so long 

 as physiological conditions prevail. It also proves that the atmospheric air 

 contains an amount of oxygen which is more than sufficient to satisfy our needs. 

 Any variation in our requirements is immediately adjusted by changing the respira- 

 tory rate and amplitude. But while ordinarily no advantage can be derived 

 from breathing pure oxygen, this procedure may prove beneficial in those diseases 

 which are associated with pulmonary infiltrations and a poor aeration of the tissues. 

 In accordance with the foregoing discussion, it must be clear that all we can hope to 

 accomphsh by this means is to increase the driving force behind the atoms of oxygen, 



^Loewy, Respiration und Zirkulation bei Ander. des Sauerstoffgehalts der 

 Luft, Berlin, 1895. 



2 La pression barometrique, Paris, 1878. 



