CHAPTER XL VIII 



THE ADAPTATIONS OF THE CIRCULATORY AND RESPIRATORY 

 MECHANISMS DURING MUSCULAR EXERCISE (Cont'd) 



THE EFFECT OF MUSCULAR EXERCISE ON THE 

 COMPOSITION OF THE ALVEOLAR AIR 



During muscular exercise the pulmonic ventilation increases ito 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 experiments in which tetanus of the 

 muscles of the hind limbs was produced by electric stimulation of the spinal cord. The 

 problem was to find out what serves as the means of correlation (nerve reflex or hor- 

 mone 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 hyperpnea. On the other hand, when the spinal cord was left intact 

 but the blood vessels of the limb were ligated, no hyperpnea followed the tetanus. 

 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 considered was a change in 

 the gases of the blood,, either a deficiency of O^ or an increase in CO . Direct exami- 

 nation of the blood for the quantity of these gases, however, yielded results which 

 were quite contrary to such an hypothesis. It was found that the percentage of O 2 , 

 if anything, was slightly increased, and. that of the CO 2 , if anything, diminished. 

 Moreover, when the expired air was analyzed during the hyperpnea, the p&rcentage 

 of CO contained in it was distinctly below the normal average, and the percentage of 

 O r above it. Evidently, therefore, the amount of gases in the blood has nothing to do 

 with the excitation 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 metabolic product, possibly 

 the lactic acid produced by contraction. 



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

 CO 2 was being expired during the work and immediately after it, but that this was 

 subsequently followed by a much lower quotient, indicating that CO 2 was being re- 

 truned. Such a result would be in conformity with the view that an acid such as lactic 

 is discharged into the blood, on the carbonates of which it would act as explained on 

 page 372. Breathing in and out of a small rubber bag causes the same alterations 

 in the respiratory quotient (see page 582). 



That lactic acid is actually produced by contracting muscle could not, however, be 

 shown by all investigators, and it was not until some years later that Fletcher and 

 Hopkins29 clearly demonstrated the conditions under which it may appear in active 



435 



