RESPIRATION 189 



in blood is kept normal ; and no physiological phenomenon is more 

 striking than the constancy of this curve under normal conditions. 

 If the proportion of available alkali is temporarily diminished 

 by acid poured out into the blood, the kidneys help to restore it 

 to normal again ; and similarly with excess of alkali. The action 

 of the kidneys is slow compared with that of the lungs; but is 

 apparently still more delicate. As L. J. Henderson was the first to 

 point out clearly 34 the PH of urine is no measure of the total acid 

 excreted in it, since urine, like blood, contains buffer substances. 

 Among these phosphoric acid plays the main part in acid urine, 

 and carbonic acid in alkaline urine. To measure the acid excreted 

 titration must be employed, and in titrating alkaline urine the 

 combining CO 2 must be allowed to escape. 33 



The part played by the liver is to neutralize as far as possible 

 the disturbing effect of any excess of acid or of alkali introduced 

 into the body through the intestines, or formed in the tissues. By 

 allowing more, or less, ammonia to enter the circulation the 

 liver regulates the reaction of the blood ; and the neutral ammonia 

 salts are afterwards eliminated by the kidneys as being foreign 

 substances. The importance of the part played by the liver under 

 normal conditions is evident enough in view of the fact that in 

 man the ammonia excreted daily would just about suffice to neu- 

 tralize all the sulphuric acid formed daily. Like that of the kid- 

 neys, the action of the liver is slow and delicate as compared with 

 that of the lungs. 



Possibly the intestines also play an active part in regulating 

 the blood reaction. It is known, at any rate, that alkali may be 

 eliminated from them in the form of insoluble alkaline phosphates. 



We have now to consider how this joint regulation behaves 

 when the action of one of the regulators is interfered with; and 

 the case of interference with the lung regulation will be considered 

 first. This regulation may be disturbed in various ways, but per- 

 haps most is known at present as to its disturbance owing to the 

 fact that under abnormal conditions the stimulus of anoxaemia in- 

 creases the breathing, and thus disturbs the normal relation be- 

 tween the lung ventilation and the degree of stimulus of the 

 respiratory center owing to varying reaction of the arterial blood. 

 The history of the development of knowledge on this point is very 

 instructive. 



84 L. J. Henderson, Amer. Journ. of Physiol., 21, p. 427, 1908. 

 86 Davies, J. B. S. Haldane, and Kennaway, Journ. of Physiol., LIV, p. 32, 

 1920. 



