1108 



PHYSIOLOGY 



Int. Vol. 



In the latter case it seems that the first rise is due to the accumulation 

 of carbon dioxide. The rise, however, under nitrogen when it occurs is 

 extremely abrupt, and the subsequent fall of blood pressure, i.e. the 

 heart failure, is earlier in onset and more rapid than with ordi- 

 nary asphyxia. When excess of carbon dioxide is administered, i.e. 

 5 to 10 per cent., a marked rise of pressure occurs which, like that 



produced by oxygen lack, is 

 almost entirely conditioned 

 by stimulation of the vaso- 

 motor centres and resulting 

 constriction of the peripheral 

 arterioles. If a loop of in- 

 testine be placed in a ple- 

 thysmograph, it will be seen 

 that the rise of pressure co- 

 incides with a shrinkage in 

 volume of the intestine, 

 pointing to a vascular con- 

 striction (Fig. 459). The 

 heart's action, as we have 

 seen, is improved rather 

 than otherwise by moderate 

 increase in the tension of car- 

 bon dioxide in the blood cir- 

 culating through it. Hence 

 the rise of blood- pressure due 

 to the vascular constriction 



off 



CO, 7% 



on 



B.P. 



, may be maintained for a con- 



iG. 459. Tracing of arterial blood pressure and . J 

 of intestinal volume, to show the influence Slderable period, e.g. ten to 



the c 2 tension ***** 



. *** *> 



not get the rapid fall of pres- 

 sure due to failure of the heart that is observed in an ordinary 

 asphyxia tracing. If partial oxygen lack or abnormally increased 

 tension of carbon dioxide be continued for some time, a state of 

 narcosis or paralysis finally ensues which affects not only the higher 

 centres but also those of the medulla, so that death may ensue without 

 convulsions or excessive rise pf blood pressure. 



Is there any common factor in the two conditions of oxygen lack and 

 carbon dioxide excess which may account for the similarity in their 

 effects ? It has been shown that whenever there is a deficiency of 

 oxygen the metabolism of the tissues undergoes alteration, so that as 

 a result of activity, e.g. in muscles, lactic acid is formed instead of 

 carbon dioxide. Lactic acid can therfeore be detected in the blood 

 whenever violent exercise is taken sufficient to produce dyspnoea, or 



