CHAPTER XL VII 



THE ADAPTATIONS OF THE CIRCULATORY AND RESPIRATORY 

 SYSTEMS DURING MUSCULAR EXERCISE 



There is probably no field of physiological research in which more 

 important results have been obtained during the past few years than in 

 that pertaining to the effects of muscular exercise on the bodily func- 

 tions. The adaptations in the circulation are particularly important 

 because they can be properly carried out only when this system is in 

 perfect working order, so that a study of them affords us a most valuable 

 method for estimating the reserve power of the heart and the efficiency 

 of the peripheral circulation. We shall first of all consider the adjust- 

 ments of the circulatory and respiratory systems that accompany muscu- 

 lar exercise and then proceed to see how the knowledge may be used in 

 clinical diagnosis. 



The Circulatory Changes Accompanying Muscular Exercise* 



During activity the muscles require many times more blood than dur- 

 ing rest. When the activity is widespread the greater blood supply is 

 provided by increased heart action accompanied by dilatation of the 

 muscular arterioles and capillaries and constriction of those of the 

 splanchnic area so that the entire available blood supply of the body is 

 made to circulate more rapidly. 



If we take as a measure of the extent of muscular activity, the con- 

 sumption of oxygen, it has been found that this runs practically parallel 

 with the output of the heart and with the volume of air breathed. The 

 output of the heart varies between 3 and 6 liters per minute, in the resting 

 individual; during moderate muscular work it becomes 8 or 9 liters, and 

 during very heavy work it may rise to 20 liters or more (Krogh). 



When the activity is confined to a limited group of muscles, the in- 

 creased blood supply is mainly provided by a local dilatation of the 

 blood vessels of the active muscles accompanied by a reciprocal constric- 

 tion of those inactive parts. Under these conditions there may therefore 

 be no quickening of the bloodflow as a whole. In order that this accurate 

 adjustment of blood supply to tissue demands may be promptly and ade- 

 quately brought about, all available types of coordinating mechanism are 



*This chapter is placed here rather than following circulation because of the interdependence of 

 the circulatory and respiratory adjustments. 



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