CHANGES ACCOMPANYING MUSCULAR EXERCISE 413 



stimulation of the centers must be due to cerebral impulses independ- 

 ently transmitted to the above centers, since the quickening of the pulse 

 and respirations may be observed to begin before the actual muscular 

 contractions. 



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. 

 Evidence for these statements can readily be supplied in man by analy- 

 sis of the expired air (for carbon dioxide) and of the urine (for lactic 

 acid) before and during muscular work. The real hormone in both cases 

 is believed to be an increase in the H-ion concentration of the blood. 

 There is, however, no direct proof of this assertion that is to say, no 

 one has actually shown that a measurable change in the H-ion concentra- 

 tion of the arterial blood (for of course a change in the venous blood 

 would be of no significance) does occur before the changes believed to 

 be dependent upon acid production make their appearance. The well- 

 known buffer action of the blood (that is, its ability to take up con- 

 siderable quantities of acid or of alkali before any perceptible change 

 occurs in H-ion concentration) furnishes another reason why doubt 

 must be cast upon the H-ion hypothesis. The most delicate means for 

 demonstrating a change in H-ion concentration of the blood consists in 

 finding the dissociation constant for hemoglobin and the results have 

 shown that acidosis develops during exereise at least at high altitudes 

 (Barcroft 1 ). So far as we are aware, however, it has not been possible by 

 direct measurement (page 29) to detect a rise in H-ion concentration. 

 Of course it may well be that the sensitiveness of the various nerve 

 centers and other structures towards the H-ion concentration is very 

 much greater than our most refined and sensitive laboratory methods 

 can reveal. Such is at least commonly believed to be the case for the 

 respiratory center (see page 351), and it may also be so for those of 

 vascular tone and cardiac action. It is nevertheless possible that an 

 increase in the free carbonic acid itself the carbonate anion (-HC0 3 ), 

 in other words is the effective hormone. In the first stages of muscular 

 work, this increase would be due to greater production of C0 2 , .whereas 

 later, especially when the work is strenuous, lactic acid would decom- 

 pose the NaHC0 3 of the blood, liberating -HC0 3 , which would become 

 added to that still being produced by the active muscles, and as the 

 NaHC0 3 (buffer substance) became gradually used up, would cause a 

 relatively greater and greater proportion of -HC0 3 to exist in a free 



