CHANGES ACCOMPANYING MUSCULAR EXERCISE 431 



The Hormone Factor. We have to consider first the nature of the 

 hormone, and secondly the mode of its action. 



The Nature of the Hormones. The most important hormone is car- 

 bonic acid, but when the exercise is strenuous and continued, or from 

 the very start is of such a nature that it uses up oxygen more quickly 

 than the blood can supply it to the muscles, lactic acid also appears. 

 It is probable also that depression of the tension of oxygen in the blood 

 supplying the medullary respiratory centers is in itself an important cause 

 for their excitation. Evidence for these statements can readily be supplied 

 in man by analysis of the expired air (for carbon dioxide) and of the urine 

 (for lactic acid) before and during muscular work. The carbonic and lactic 

 acid are believed by many to act by causing an increase in the H-ion concen- 

 tration of the blood. There is, however, no direct proof for this belief, al- 

 though it has been shown by determination of the tension of C0 2 in the 

 alveolar air and calculation therefrom of the PH of the blood, that a 

 decrease of 0.02 occurs. (Campbell, Douglas and Hobson 39 ). Barcroft 27 

 by measuring the dissociation constant of his own blood (see page 401) 

 before and after climbing 1,000 feet in half an hour, estimated that P H 

 changed from 7.29 to 7.09. It is possible, however, that these estimations 

 are unreliable since they may not have included all the factors that are 

 involved. 



Another view is that the effective hormone is an increase in the free car- 

 bonic acid itself (see page 368). In the earlier stages of muscular work, 

 the greater production of C0 2 by the active muscles would raise the 

 tension of this gas in the plasma, and later, especially when the work 

 was strenuous, lactic acid would also come into play by decomposing the 

 NaHC0 3 of the blood, and liberating C0 2 . As the NaBC0 3 (buffer sub- 

 stance) became gradually used up, a relatively greater and greater pro- 

 portion of C0 2 would come to exist in a free state in the plasma, so that 

 its stimulating effect became progressively greater. 



One serious difficulty in accepting the free C0 2 as the exciting hormone 

 of the nerve centers during muscular exercise depends on the observation 

 that the alveolar C0 2 after some time is lower than normal. If we ac- 

 cept Haldane's teaching that there is accurate correspondence between 

 the tensions of C0 2 in arterial blood and alveolar air, not only during 

 rest, but also during muscular activity, then obviously we must discard 

 the C0 2 hypothesis. But this assumption is unwarranted, for Leonard 

 Hill and Flack 46 have shown quite clearly both in experimental animals 

 and in man that equilibrium between the blood and alveolar tensions of 

 CO, may fail to occur. When blood with excess of C0 2 is injected into the 

 jugular vein of dogs, the respiratory center is stimulated, as shown by 

 the increased breathing, which indicates that the CO 2 -rich blood must 



