550 



EFFECT OF SUSPENDED RESPIRATION ON COMPOSITION OF ALVEOLAR AIR. 



These figures show that when the breath is held under normal pressure, the 

 alveolar oxygen content at first fell and, during the period of the last three in- 

 spirations, remained at an approximately constant level, having a mean value 

 of 10.31 % or 74.2 mm. Hg. 



When the breath was held in a similar way but under negative pressure the 

 alveolar oxygen content fell as before and, during the period of the last three in- 

 spirations, remained at an approximately constant level having a value of 10.51 % 

 or 75.2 mm. Hg. 



The final values reached are practically the same in these two sets of ex- 

 periments. They are very close to the final value of 11.21 % or 81.0 mm. Hg 

 reached in the author's previous work when the breath was held under normal 

 pressure. The present value for negative pressure, however, is distinctly higher 

 than the value of 9.2 % or 66.5 mm. Hg. reached under negative pressures or 

 during rebreathing in the previous experiments. 



Discussion. 



An essential difference between the present experiments and those carried 

 out before is that, in the present case, the individual periods of holding the breath 

 are so shoi-t that disturbances of the circulation are reduced to a minimum. In 

 the former ease, negative pressure or rebreathing was employed over periods four 

 to six times as long as the present, the forced respirations or negative intrathoracic 

 pressures, in increasing the filling of the right atrium, correspondingly accelerated 

 the circulation although the pulse-rate was hardly affected. 



The present experiments show that when the circulatory disturbances are 

 minimised, an approximately constant composition of alveolar air is reached by 

 holding the breath. This composition is almost the same whether the breath is 

 held under normal atmospheric pressure or under a negative pressure of about 

 30 mm. Hg. 



The existence' of the constant tension of alveolar carbon dioxide indicated in 

 the previous work is thus confirmed. It has been shown that the higher values 

 for carbon dioxide, and the lower values for oxygen obtained under conditions of 

 negative pressure in the former paper are not due to the effect of negative pres- 

 sure directly on the composition, but rather to the acceleration of the circulation, 

 leading to a more rapid accumulation of .carbon dioxide and a more rapid re- 

 moval of oxygen. 



In conclusion I wish to express my thanks to Professor H. G. Chapman, in 

 whose laboratory this work was done. 



Eeferences. 



Henderson and Prince, 1917. — Journ. biol. Chem., 32, 325. 

 Wardlaw, 1916.— Proc. Linn. Soc. N.S.W.. 41, 786. 



