CHAPTER XLII 

 THE CONTROL OF RESPIRATION (Cont'd) 



THE EFFECT OF MUSCULAR EXERCISE ON THE 

 RESPIRATION 



During muscular exercise the pulmonic ventilation increases to an 

 extraordinary extent. At rest an average man respires 6 to 8 liters of 

 air per minute, but during walking on the level at the rate of 5 kilometers 

 an hour, this figure may increase to about 20 liters. 



The first investigations as to the cause of the relationship between 

 muscular activity and pulmonic ventilation were made by animal ex- 

 periments in which tetanus of the muscles of the hind limbs was pro- 

 duced by electric stimulation of the spinal cord. The problem was to 

 find out what serves as the means of correlation (nerve reflex or hormone 

 control) between the muscular activity and the respiratory activity. 

 By cutting the spinal cord above the point of stimulation, it was found 

 that the tetanus was still accompanied by as marked a hyperpnea as 

 before. On the other hand, when the spinal cord was left intact but the 

 blood vessels of the limb were ligated, no hyperpnea followed the teta- 

 nus. Evidently therefore the pathway .of communication is the blood. 



The next step was to seek in the blood for the substance or hormone that 

 acted as the respiratory excitant, and naturally the first possibility con- 

 sidered was a change in the gases of the blood, either a deficiency 

 of 2 or an increase in C0 2 . Direct examination of the blood for the 

 quantity of these gases, however, yielded results which were quite con- 

 trary to such an hypothesis. It was found that the percentage of 2 , 

 if anything, was slightly increased, and that of the C0 2 , if anything, 

 diminished. Moreover, when the expired air was analyzed during the 

 hyperpnea, the percentage of C0 2 contained in it was distinctly below 

 the normal average, and the percentage of 2 above it. Evidently, there- 

 fore, the amount of gases in the blood has nothing to do with the excita- 

 tion of the respiratory center, and the conclusion drawn by the earlier 

 investigators was to the effect that the exciting substance carried from 

 the active muscles to the respiratory center must be some unusual meta- 

 bolic product, possibly the lactic acid produced by contraction. 



It was further found, by examination of the respiratory quotient, that 



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