INFLUENCE OF VARIOUS CONDITIONS ON RESPIRATION. 701 



the increased heart-rate that accompanies even shght muscular 

 activity, and it seems probable that a similar effect is exercised 

 upon the respiratory center.* When muscular activity is main- 

 tained for some time the lactic acid formed in the muscles escapes 

 to some extent into the blood and tends to lower its alkalinity. 

 This factor also should augment the activity of the respiratory 

 center. 



The Effect of Variations in the Composition of the Air 

 Breathed. — Variations in the amount of nitrogen in the inspired 

 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 mm. We may increase 

 this pressure either by changing the volume per cent, of the 

 gas or by compression. The somewhat natural supposition that 

 breathing pure oxygen — that is, oxygen at a pressure of 760 

 mm. — should have a beneficial effect on the oxidations of the 

 body has found no support in physiological experiments. Atmos- 

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

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

 tive advantage. This is true at least for ordinary conditions of 

 rest or moderate activity. In performing heavy muscular work 

 the muscles use more oxygen, and if the work is large in 

 amount and is maintained for some time the quantity of oxygen 

 supplied by the air may be insufficient in spite of the increased 

 respirations and the increased circulation which, as we have seen, 

 accompany muscular activity. In such cases an increase in the 

 percentage of oxygen in the inspired air is helpful in enabhng the 

 muscles to contract more efficiently and to perform more work, 

 owing probably to the fact that the products of metabolism, such 

 as the lactic acid, are more completely oxidized. Experiments 

 indicate that the maximum effect of the oxygen under these con- 

 ditions is obtained when its pressure in the inspired air reaches 

 60 to 70 per cent.f Paul Bert, in his interesting work on baro- 

 metric pressures, t has called attention to the fact that at a certain 

 pressure oxygen is not only not beneficial, but, op 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 sub- 

 mitted to 3 atmospheres of pure oxygen or 15 atmospheres of air. 



* Krogh and Lindhard, "Journal of Physiology," 47, 112, 1914. 

 t Briggs, "Journal of Physiology," 54, 292, 1921. 

 t "La pression barometrique," p. 764, Paris, 1878. 



