628 PHYSIOLOGY OF RESPIRATION. 



air have no distinct physiological effect. The important elements 

 to consider are the oxygen and the carbon dioxid. 



Increased Percentages of Oxygen. The normal pressure of oxygen 

 in the air is 20 per cent, or 152 mms. We may increase this pres- 

 sure either by changing the volume per cent, of the gas or by raising 

 the barometric pressure by compression. The somewhat natural 

 supposition that breathing pure oxygen that is, oxygen at a pres- 

 sure of 760 mm. should have a beneficial effect on the oxidations 

 of the body has found no support in physiological experiments. 

 Atmospheric air supplies us with an excess of oxygen over the needs 

 of the body; a still further increase of this excess has no positive 

 advantage. Paul Bert, in his interesting work on barometric pres- 

 sures,* has called attention to the fact that at a certain pressure 

 oxygen is not only not beneficial, but, on the contrary, is markedly 

 toxic. From experiments made upon a great variety of animals 

 and plants he concluded that all living things are killed when the 

 oxygen pressure is sufficiently high, say, 300 to 400 per cent. 

 Warm-blooded animals die with convulsions when submitted to 

 3 atmospheres of pure oxygen or 15 atmospheres of air. At these 

 high pressures the blood contains about 30 volumes of oxygen to 

 each 100 c.c. of blood instead of the usual 20 volumes. The ad- 

 ditional 10 volumes are contained in solution. Fish also are killed 

 when the oxygen pressure is increased to such a point that the water 

 contains 10 volumes of dissolved oxygen to each 100 c.c. In more 

 recent experiments by Smith, f made upon mice, it was found that 

 oxygen at pressures of 100 per cent, to 130 per cent, proves fatal 

 in a few days, the animals showing inflammatory changes in the 

 lungs. Oxygen at 180 per cent, kills mice and birds within twenty- 

 four hours. Pressures of two atmospheres of air (40 per cent. 0) 

 have no injurious effect. No adequate chemical explanation can 

 be offered at present for this toxic action of oxygen at high tensions. 

 The matter is one of practical importance in connection with caisson 

 and submarine work and the therapeutical use of oxygen. 



Decreased Percentages of Oxygen. Numerous observers (Bert, 

 Zuntz, et al.) have shown that a fall in oxygen pressure has no 

 perceptibly injurious result until it reaches about 10 per cent. At 

 or somewhat below this pressure the hemoglobin is unable to take 

 up its full amount of oxygen, and the body consequently suffers 

 from a real deficiency in its oxygen supply, a condition designated 

 as anoxemia. According to Bert's experimental results, death with 

 convulsions quickly follows a fall of atmospheric pressure to 250 

 mms. (oxygen pressure, 50 mms. or 6 to 7 per cent.). Animals 

 supplied with an atmosphere containing a deficient amount of 



* " La pression baromtrique, " p. 764, Paris. 1878. 

 t "Journal of Physiology/' 24, 19, 1899. 



