CHAPTER XXXVIII. 



THE INFLUENCE OF VARIOUS CONDITIONS UPON 

 THE RESPIRATIONS. 



The Effect of Muscular Work upon the Respiratory Move- 

 ments. — It is a matter of common experience that muscular ex- 

 ercise increases the rate and ampUtude of the respirator}- move- 

 ments. The increase is proportional to the amount of muscular 

 work, so that although there is a very large additional quantit^^ of 

 CO2 given off to the blood in a unit of time, this increase is elimi- 

 nated so completely through the better ventilation of the lungs that 

 only a very small rise takes place in the concentration of CO2 in the 

 alveolar air. The increased respirations not only provide for the 

 removal of the excess of carbon dioxid but also for the added quan- 

 tity of oxygen needed by the working muscles. It is evident that 

 there is a dehcate adjustment between muscular activity and 

 respiratory rate and the nature of the physiological mechanism 

 involved has given rise to much speculation and investigation. 

 From the facts presented in the previous chapter it would seem 

 probable that the regulation or adaptation is effected mainly 

 through the increase in CO2 pressure in the blood. Some older 

 experiments in which the arterial blood was examined before and 

 during exercise show that there was no constant increase in its 

 content in CO2 as a result of the exercise;* but later work indicates 

 that this is evidence rather of the delicacy and completeness of the 

 adaptation, for analyses of the alveolar air show that the concen- 

 tration in CO2 does increase with muscular work, and that a very 

 small rise indeed in the concentration goes along with a large aug- 

 mentation in the respiration. An increase, for example, from 5.6 

 per cent, to 6 per cent, is accompanied by an increase of 200 per 

 cent, in the breathing (Haldane). There is reason to believe that 

 in the beginning of muscular work the respiratory center may be 

 stimulated also by an associated activity of the nerve centers. 

 The nerve impulses coming down from the brain along the vol- 

 untary nerve tracts may overflow into the respiratory center by 

 collateral branches and stimulate the center or increase its irri- 

 tability toward the action of the intrinsic stimulus of the blood. 

 An associated activity of this character is called upon to explain 

 * Geppert and Zuntz, "Pfluger's Archiv," 42, 189, 1888. 

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