THE CAUSE OF MOUNTAIN SICKNESS 225 



partial pressure of oxygen in the alveolar air is 96 -2 mm. Hg if 

 the barometric pressure be taken at 760 and the water vapour 

 tension at 47. Under this partial pressure of oxygen the blood 

 is only 79-44 per cent saturated. In other words, under ordinary 

 conditions the blood is only 4 saturated with oxygen. The tension of 

 oxygen in the venous blood = 25 mm. (Strassburg and Wolff berg). 

 The mean driving force therefore for carrying the oxygen through 



the alveolar wall is = 35-5 mm. Zuntz and Durig have 



& 



measured the rate of diffusion of C0 2 , using frog's lungs and dis- 

 tending them with a known volume of this gas. They find that 

 CO 2 diffuses thrice as fast through the lung as through a layer of 

 water of equal thickness, and this is so whether the lung is fresh 

 or killed with alcohol ! 



Now, the diffusion of a gas through water is directly propor- 

 tional to the absorption coefficient of that gas in water and to 

 a constant which is approximately inversely proportional to 

 the square root of its specific weight. Thus from the diffusion 

 velocity of C0 2 (found by experiment) that of oxygen can be 

 calculated. Making this calculation, Zuntz and Durig conclude 

 that a difference of oxygen tension of only 11 mm. would drive 

 sufficient oxygen through our lungs to satisfy our needs even in 

 times of most strenuous exertion such as climbing, while 1 mm. 

 is enough during rest. The diffusion path might be many (25) 

 times longer than normal, as in oedema of the lung, and yet 

 enough 2 pass through. 



At an altitude of 4560 m. the alveolar oxygen tension of 

 Durig fell on one occasion as low as 48-3 mm. Hg, and this makes 

 the saturation of his blood 12 or 13 per cent, less than on the 

 plains. The difference between the oxygen tension in the alveolar 

 air and the venous blood was under these conditions still amply 

 sufficient to maintain an adequate diffusion, and not this but the 

 absorptive power of the blood fell short of the needs of Durig, who 

 suffered from migraine and palpitation. 



The alveolar 2 tension can sink from 113 to 30 mm. without 

 causing an absolute insufficiency, but the brain suffers before the 

 other organs and at a higher tension of 2 . 



Haldane and Lorrain Smith conclude from the study of the 

 relative affinity for CO and 2 of shed blood and blood in r,orpore 



